Specimen Spotlight: Annual Bluegrass

By Melinda Heigel, EMGV

It’s early February, spring is on the way, and annual bluegrass (Poa annua) flower heads are emerging in the Triangle area. (Image credit: M. Heigel)

There’s a popular saying that weeds are just plants in the wrong place. And to be sure some plants we think of as unsightly weeds serve important roles in the environment. One that immediately comes to mind is the much-maligned dandelion, whose yellow flowers are early-season food sources for pollinators. But one weed that plagues many home gardeners’ lawns is starting to make its yearly appearance: annual bluegrass (Poa annua).

Annual bluegrass–not to be confused with Kentucky bluegrass (Poa pratensis), a perennial grass which is the second most widely grown cool-season grass in NC lawns– is a winter weed, bright green-to-chartreuse in color, finely textured, and with a clumping habit. The blade of this grass is identifiable by what many turf experts call its “boat-shaped” and pointed tip. When annual bluegrass infests lawns, it shows up prominently in a stand of fescue due to its color. And if you have warm season grass like zoysia or Bermuda, it’s even more evident because this time of year, it’s a burst of color in what otherwise is now a sea of brown. This annual weed is considered one of the most aggressive weeds around. It thrives in all sorts of soils and conditions, but it can often be an indicator of compacted and poorly-drained soil.

(Left to right) Detail of the folded boat-like tip of the weed’s blade. Annual bluegrass evident in a fescue lawn. Note the difference in color, density, and texture. (Image credit: Michigan State University and M. Heigel)

In terms of its overall life cycle, annual bluegrass geminates in North Carolina in the fall, typically in September through October, although some additional germination may occur in the spring. If your lawn is made up of cool-season grass, it’s still green and growing in the fall, so you may not notice this small weed as it emerges. However, in the late winter and early spring, annual bluegrass gets more sunlight as days lengthen, grows rapidly, flowers, disperses seeds and dies just as the warm weather arrives beginning in May. By summer, this weed seems to simply disappear. But wait…. Annual bluegrass is prolific; as each plant flowers in the spring, its flower heads can produce upwards of several hundred weed seeds in one season–remember that’s per plant. Those seeds can lay dormant in the ground for years making this one of the toughest annual weeds to control.

While it sounds dire, rest assured that there are means that gardeners can employ to control annual bluegrass in their lawns, and using these measures in concert with one another (a practice called Integrated Pest Management), will often lead to the best results. 1

Manual practices–Gardeners may have mixed results by pulling or digging up this weed. If you catch annual bluegrass early in its life cycle, especially when there are solitary plants, manual extraction can be helpful. Gardeners must weed frequently and diligently. But if seed production and sewing has begun by the time gardeners realize this plant is colonizing, it might be too late to make a huge impact by hand weeding as your main method of control.2 Note that in some cases, annual bluegrass can also take root in landscape beds. Here, it’s often easier to spot and hand pull early before a larger problem develops. Mulching in landscape beds to cover bare soil can also aid in weed suppression.

Cultural practices–As mentioned above, some conditions in the lawn and landscape are more favorable to annual bluegrass than others, namely compacted soil and poor drainage. Two ways to combat these conditions and potentially make you lawn less desirable for this weed are to regularly aerate your lawn and address any areas where you may have drainage problems.

The most important tool gardeners have in their toolbox to combat annual bluegrass infestation is to keep existing turf grass as dense and healthy as possible. If this sounds too simple, consider one of the major biological needs of Poa annua or any weedy plant that might try to set up housekeeping in a lawn: sunlight. According to NC State Extension Specialist Dr. Fred Yelverton, “Soil shading is the best defense against annual bluegrass.” 3 Put another way, the fuller your lawn is with healthy turf grass, the less likely it is for sunlight to penetrate the turf grass cover and reach germinating weed seeds.

Here is where the fundamentals of lawn maintenance really come into play: proper selection of turf for your site, proper fertilization and amendments based on a recent soil test,4 proper mowing frequency and height, pest control, seeding when appropriate, irrigation, and core aeration are all essential to controlling unwanted weed infestations. Home gardeners may take for granted these steps, but it might be a good time for a review of science-based information on how to maintain a lawn. One of the best guides for both cool-season grasses and warm-season turf is North Carolina State University’s online resource “Carolina Lawns: A Guide to Maintaining Quality Turf in the Landscape.” Find it online here: https://content.ces.ncsu.edu/carolina-lawns.

Re-seeding or sodding bare patches with desired turf varieties as well as over-seeding for lawn renovation are excellent cultural controls for weed encroachment. Warm-season grasses need attention when they are actively growing and cool-season turf rejuvenation is most successful in the fall once heat stress from hot summer temperatures has abated.

Chemical controls–Pre-and-post emergent herbicides can also offer a measure of control for knocking down annual bluegrass infestations and as part of a multi-pronged approach to management. However, it’s important to note that the repeated and widespread use of these products as the sole means of control has lead to herbicide resistance. This resistance has even been shown with non-selective glyphosate in some trials. According to Auburn University’s Department of Crop, Soil and Environmental Sciences, annual bluegrass ranks third among all weeds for its strong herbicide resistance.5 Moreover, some herbicides are not safe to use on all varieties of turf grass. If home gardeners wish to use chemical controls, growing herbicide resistance means that they should do thorough homework and consult up-to-date credible sources like NCSU’s Turf Files for the most recent science on currently effective pre-and-post emergent agents for annual bluegrass. Likewise, home gardeners should consult the NC Agricultural Chemicals Manual (see the online version at https://content.ces.ncsu.edu/north-carolina-agricultural-chemicals-manual) and thoroughly read all labels on all products they may use.

If looking to both employ chemical controls and rejuvenate the lawn to combat this weed, note that timing is paramount. Pre-emergent agents are not selective in nature, so if you are looking to re-seed any cool-season grasses like tall fescue or a Kentucky bluegrass mix in the fall, these herbicides can prevent your desired seed from growing. Some experts advise gardeners wait up to six months before applying pre-emergents to new grass.

Lawn Alternatives

To be sure, turf grass has environmental benefits, including carbon sequestration, erosion control, and even a cooling effect in the time of rising temperatures. It’s important for recreation around your home and, if gardeners live in a community with a home owners’ association, maintaining certain lawn standards may be required. But with the example of managing an annual bluegrass infestation, lawns can be expensive, require the use of chemicals, demand water resources, and require a lot of upkeep. Here are some thoughts on ways to simplify your lawn routine and potentially keep annual bluegrass under control.

  • Extend your tree, shrub, and flower garden beds and simply reduce the size of your lawn. This can be helpful in areas where you find turf grass maintenance hard such as under trees, in tight spaces, or narrow strips.
  • Consider turning your turf into a pollinator lawn, which is a mixture of traditional turf grass and other flowering plants like Dutch clover (Trifolium repens), dandelion (Taraxacum officinale), and lanceleaf tickweed (Corespsis lanceolata). Blending tall fescue with low-growing mini and micro clovers is also a trend. This diversity in your lawn helps nourish pollinators while providing a similar-looking backdrop as a monoculture of turf grass.
  • In shady spots in the lawn with bare patches, try planting a “green” ground cover that can compete with and suppress weeds like annual bluegrass. Some shade-loving ground covers include dwarf mondo grass (Ophiopogon japonicus), foam flowers (Tiarella cordifolia), green and gold (Chrysogonum virginianum), and shade-tolerant sedges (Carex spp.)
  • Chronically wet areas in the lawn are especially susceptible to annual bluegrass. In place of grass, other options include rain lilies (Zephyranthes candida), common rush (Juncus effusus), or sweet flag (Acorus spp.)

Don’t get the blues when you see annual bluegrass flowering. Take an integrated and educated approach with the information above to control this hard-to-control weed.

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Notes

1–Keep in mind that the goal of eliminating all weeds, especially in your lawn is not realistic. Managing weed populations in your lawn and landscape is an obtainable goal.

2–Many experts classify hand weeding as a temporary means of weed control. When weeding, try and leave the soil as undisturbed as possible. By turning up soil, gardeners are likely bringing dormant weed seeds to the surface and giving them a chance to germinate. Hand pulling when the ground is moist makes for easier work and less soil disturbance.

3–https://turfpathology.ces.ncsu.edu/2019/04/annual-bluegrass-poa-annua-spring-invasion/

4– For information on soil testing, see https://gardening.ces.ncsu.edu/soils/soil-testing/

5–https://ocm.auburn.edu/newsroom/news_articles/2018/12/140932-poa-turfgrass-weed.php

Resources and Additional Information

North Carolina State University has several online resources on annual bluegrass. Below are links to learn more.

https://plants.ces.ncsu.edu/plants/poa-annua/

https://www.turffiles.ncsu.edu/2019/04/annual-bluegrass-poa-annua-spring-invasion/

https://www.turffiles.ncsu.edu/weeds-in-turf/annual-bluegrass/

For more information on reducing, diversifying, or replacing your lawn, Iowa State University Extension and NC Cooperative Extension have some informative articles full of additional research-based online resources.

https://hortnews.extension.iastate.edu/lawn-alternatives-turfgrass (with a link to how to create a pollinator-friendly lawn)

https://forsyth.ces.ncsu.edu/2022/02/lawn-alternatives-2/

https://chatham.ces.ncsu.edu/wp-content/uploads/2019/04/Sustainable-Lawns-and-Lawn-Alternatives-EG-2019-short.pdf?fwd=no ( a great powerpoint presentation on sustainable lawns and lawn alternatives)

Article Short Link https://wp.me/p2nIr1-3by

February To Do in the Garden

By Gary Crispell, EMGV

Wasn’t January fun?  A little rain (well, maybe more than a little) every two or three days whether we needed it or not, and sometimes a day of sunshine in between.  We had some nights in the low 20’s and some days in the low 70’s.  Fortunately, no more 10’s and no snow.  I grade it out as a net plus.

Heads up! You may see some grape hyacinths (Muscari armeniancum pictured here) thinking it’s already spring. (Image credit: Bff CC BY-SA 4.0)

The ACG (that’s short for the Accidental Cottage Garden) is asleep except for a lone grape hyacinth (Muscari sps.), Family Asparagacea!?, with a single open bloom.  I would have missed it except our nine-year-old grandson who doesn’t miss anything pointed it out.  The most exciting event here lately was the great horned owl (Buo virginianus) that landed in the driveway a couple of evenings ago.  It just sat there until I closed the garage door.  Then it employed some of its mystical wisdom and removed itself with a silent flapping of its great wings.  What a magnificent bird.

So, are you ready to get your hands dirty (mud caked, frozen)?  We may have to wait for the winter monsoons to abate along with some of the excess soil moisture.  Should that happen, here are some tasks you may endeavor to undertake.

LAWN CARE

Break out the spreader if you are a caretaker of cool season grasses (tall fescue and non-banjo associated bluegrass).  Follow the recommendations from the SOIL TEST you had done last fall and apply a slow-release product.  (What?!?  No soil tests?  Follow the general instruction on the fertilizer bag and get a soil test done in the fall.  They are free April through November.) For more information on soil testing see https://durham.ces.ncsu.edu/2020/05/nows-the-perfect-time-to-test-your-soil/.

Later this month or early March before the dogwoods (Cornus florida) bloom, apply an accurately calibrated amount of crabgrass preventer.  Too little will not provide the control you want and too much may damage the turf.

FERTILIZING

See LAWN CARE above and PLANTING below

PLANTING

Woo, woo!!  Plants and seeds going in the ground!  Oh, the anticipation.  Warning; Unless you are planting rice (Oryza sativa) or cattails (Typha latifolia) you might want to wait just a bit.  As mentioned above, the soil is a bit damp as yet.  Most garden plants except the aforementioned genera and bald cypress (Taxodium distichum)–doesn’t everybody plant bald cypress in their garden?–find saturated soils less than conducive for optimal performance.  That said, when the soil moisture reaches an acceptable level you can plant a spring vegetable garden with any (or all) of these:  cabbages, carrots, leaf lettuces, onions, potatoes, radishes, rutabagas, spinaches and turnips.  It is assumed (with all the danger implied in that word) by this writer that lime was applied according to NCDOA recommendations from your fall soil test.  Now is the time to follow the recommendations for NPK (nitrogen, phosphorus, potassium) fertilizer applications.

PRUNING

Prune fruit trees and bunch grapes ASAP to prevent excessive “bleeding” when the sap begins to rise.

Summer flowering shrubs and trees are ready to be shaped and thinned.  Plants in this category include rose of Sharon (Hibiscus serriatcus), butterfly bush (Buddleia davidii), hydrangeas that bloom on new wood (H. arborescens, H. panniculata) and crape myrtles (Lagerstroemia sps.). 1

A word about pruning crape myrtles.  Excessive pruning of crape myrtles is referred to in the green industry as “crape murder.” That in itself should tell you something.  There are now crape myrtle cultivars available in a wide variety of sizes and a multitude of colors within those sizes.  It might be better to remove the plant that doesn’t fit its allotted space and replace it with a cultivar more suited to that space.  The plant esthetics will be more pleasing, and you won’t have to prune other than to take out unwanted stems.  Win, win.

SPRAYING

Peaches and nectarines should be sprayed with a fungicide to prevent leaf curl this summer. 

After you prune the rest of the fruit trees break out the sprayer and apply a horticultural dormant oil.  (This is a horticultural oil applied to dormant plants, not oil applied to dormant horticulturists in preparation for a massage.)

FEBRUARY DIRT DIGGING DEEDS DECICEDLY DETRIMENTAL TO DELICATE DIGITS

This is a good time to divide perennials that multiplied last summer.  (See, your third-grade teacher was right.  Math is everywhere.) 

While not technically a digging exercise, propagating plants from hardwood cuttings can be a fun experiment this month.  Try cuttings from crape myrtles (Lagerstroemia sps.), flowering quince (Chaenomeles sps.), junipers (Juniperus sps.) spiraea (Spiraea sps.) and weigelia (Weigelia sps).  Dip the cuttings in a rooting hormone, stick them in a very loose planting medium (pine bark, shredded cypress bark) water frequently and see what happens.  For more detailed information on all things propagation check with the Durham County Extension Master Gardener Propagation Team.  

Finally, but by no means least important for this edition of the calendar, there is a widely celebrated holiday mid-month that seems to require the exchange of brightly colored plant material (or rocks formed deep in the ground under extreme heat, but we’re not about rocks here) between people who care about each other.  You know, significant others or just good friends.  One does not have to identify with a specific gender to either give or receive said brightly colored plant material.

And let us not neglect the more roisterous celebration exactly one week later.  Laissez bon temps rouler!!

And eat pancakes.  I know a place where they will be free (to eat, not run around).

Rosa sps. are red.

Some Viola sps. are blue.

Just to be clear,

Centurea cyanus are, too.

Fabulous February, y’all!

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Notes

1–Some hydrangeas such as big leaf hydrangeas, also known as French or mophead hydrangeas (H. macrophylla), and oakleaf hydrangeas (H. quercifola) bloom on last year’s wood. If you prune them now prior to blooming, you will have no spring blooms. Prune these plants after they flower, but as a good rule of thumb, no later than August 1.

Resources and Additional Information

For lawn care for cool-season grasses, check out North Carolina State University’s TurfFiles, where you will find information on lawn establishment and maintenance. Two of the most popular cool-season grasses in North Carolina are tall fescue and Kentucky bluegrass. Find links to their maintenance calendars below.

https://content.ces.ncsu.edu/tall-fescue-lawn-maintenance-calendar

https://www.turffiles.ncsu.edu/grasses/kentucky-bluegrass/

Explore the world of plant propagation through the North Carolina Extension Gardener Handbook in the online link below. Also, as mentioned above the Durham Country Extension Master Gardener Volunteer Plant Propagation Team holds workshops throughout the year, so check our calendar of events on our website for future dates.

https://content.ces.ncsu.edu/extension-gardener-handbook/13-propagation

Avoid the improper pruning known as “crepe murder” by employing these simple tips from Clemson University’s Home and Garden Information Center’s factsheet which has great photos of properly and improperly pruned trees.

Article Short Link https://wp.me/p2nIr1-39H

Fruits of Labor: Upcoming Scion Exchange Event

Annual Scion Exchange

Saturday, February 11, 2023, from 11:00 am – 12 noon

NC Cooperative Extension, Durham County Center, 721 Foster Street, Durham, NC 27701

No Registration Required. (Please note that the preceding grafting workshop is sold out, but the scion exchange is open to everyone regardless of workshop participation.)

Do you grow tree fruits or nuts and are interested in growing additional varieties? February is the perfect time for winter pruning of fruit and nut trees. Instead of composting the discarded branches (scions), exchange them with other orchardists and pick up some new-to-you varieties. Bring your scions to the annual Scion Exchange hosted by the Extension Master Gardener VolunteersSM of Durham County.

Visit https://trianglefruitandnutgrowers.weebly.com/collect-scions.html for photos and detailed instructions on how to collect and prepare your scions for the event or email KatCauseyEMGV@gmail.com.

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Resources and Additional Information

Want to know more about propagation techniques and growing tree fruit and nuts in your home garden? Check out these two chapters from the North Carolina Extension Gardener Handbook.

https://content.ces.ncsu.edu/extension-gardener-handbook/13-propagation

https://content.ces.ncsu.edu/extension-gardener-handbook/15-tree-fruit-and-nuts

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Gardening Ground Truths: Revisiting “Finding Credible Sources”

This is the time of year gardeners begin to dream about spring and summer and the possibilities for their landscapes, vegetable gardens, and containers. The old adage “fail to plan, plan to fail,” rings true in gardening. In addition to keeping detailed records annually about your garden, doing your homework both upfront and along the way is key. But not all research–especially on the internet–is created equal. As you begin planning your 2023 garden, it seems like an excellent time to revisit Ann Barnes’s 2017 article on conducting quality scientific-based research and best practices for your own experimentation. In keeping with Ann’s suggestion to look for the latest information, we’ve included updated links and resources.

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(Image credit: M. Heigel; North Carolina State University)

By Ann Barnes, EMGV

When looking for answers to gardening questions, the internet is a fast and convenient place to search. It is important to remember that anyone can post content online, and there is no review process to ensure information on every website is correct. Extension Master Gardeners recommend unbiased, research-based information. There are many websites that claim to provide gardening information. Not all of them can be considered credible.

The “Who, What, Where, When, Why, and How” of Finding Trustworthy Information

Who are the authors, and for whom are they writing? Are they experts in a scientific field related to gardening, affiliated with a university, or trained Master Gardener Volunteers? Do the authors cite sources and/or provide links for further reading, and are those links to research-based sources? Does the author (or the entire website) seem to have an agenda or are they presenting unbiased information? Are the authors even listed, or does the article appear to be shared from some unnamed source? If you can’t determine who wrote the article (individual or organization), how can you know if the author is a credible source?

If a light switch in my house wasn’t working, I would call an electrician. Advice from my pharmacist or from a blogger who posts “home hacks” would not give me as much confidence as advice from an expert. The same should hold true for gardening – consult an expert in agriculture for the most reliable answers to questions about growing plants.

What: There is a science to growing great plants, so look for answers on sites that are research-based. There should be links to studies that support recommendations. When applicable, both organic and chemical options will be discussed in an unbiased manner. Keep in mind that natural doesn’t always mean safer or more effective, and not everything presented as a fact on the internet is true. Always check sources, particularly when you see a “scientific fact” shared as a meme or a link on social media.1

Credible sources don’t promise miracles or promote home remedies over conventional growing practices. Credible sources aren’t trying to make a sale or to criticize an existing product. Credible sources typically won’t promise that “your jaw will drop.”

Where: Check the URL of sites in your search results. Sites that end in .edu or .org (education or organization sites) are more likely to contain unbiased and research-based information. Master Gardener Volunteers are affiliated with Cooperative Extension, whose purpose is to share information obtained from research conducted at land grant universities, such as North Carolina State University (NCSU) and North Carolina Agricultural and Technical State University (NC A&T). Therefore, we recommend searching extension websites. Sometimes answers can be found at any state’s land grant university, but keep in mind your USDA Plant Hardiness Zone when searching for information about specific growing questions. For example, the average last frost date in the Triangle is much earlier than the average last frost date in Wisconsin, so you would need to look closer to home when searching for when to plant your tomatoes.

Here’s ONE WEIRD TRICK for finding all the garden answers you need:

Bookmark this link, and you can search like a Master Gardener: https://search.extension.org/

This will take you to a custom search engine that will only search Cooperative Extension sites. If you’d like to broaden your search beyond land grant universities but still search only for sites associated with academia, use your favorite search engine and follow your search terms with “site:.edu”. For example, when researching how to grow tomatoes in containers, type: Tomatoes in containers site:.edu in your browser’s search bar.

When: Does the article have a publication date? Science is an ongoing process and recommendations sometimes change due to new research, so it is best to use fairly recent sources. If there are links, are they current? Broken links may be a clue that an article is older.

Why: What is the purpose of the site? Is it providing unbiased, research-based information? If it isn’t a university website, is it selling a product? Does the site have bias towards or against a certain way of gardening? A site promoting a product has a goal of making sales, so there will be a bias towards their product. Similarly, articles published by authors with a general mistrust of agricultural chemicals will show a bias towards organic-only solutions. While biased sites may provide information that is correct, they may not be showing you all your options or the data behind these options.

Finally, there are the click bait articles, those amazing home remedies that are supposedly better than commercially available products, full of “facts” that sound miraculous but often aren’t true. The same information, word-for-word, may be posted on multiple websites. Click bait sites are designed to get people to click on them; it’s as simple as that. They don’t have to cite sources, use research methodology, or even be scientifically accurate. There is a lot of bogus science out there, and some of it looks pretty believable if you don’t check for source material.

Debunking some internet myths about gardening:

https://durhammastergardeners.wordpress.com/tag/baking-soda/

https://durhammastergardeners.wordpress.com/2013/04/07/pinterest-myth-busters

How: Good science is based on experiments that follow the scientific method, in which a scientist tests his or her hypothesis in a series of experiments. If I wanted to test Grandma’s Homemade Weed Killer to see if it was as effective as a leading Big Agricultural Product, I would need to conduct tests using each product on similar weeds in similar conditions, then compare results to see if there is a statistically significant difference in the two.

If I simply sprayed weeds with the Grandma’s Homemade Weed Killer, found some dead weeds, and announced that this remedy was safer and more effective than the Big Agricultural Product (without comparing the two in well designed experiments), my claim would be invalid, since I didn’t test for safety, nor did I compare the two weed control products. Claims need to be backed up with experiments.

How to design an experiment:

https://explorable.com/conduct-science-experiments/

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Notes

1–Many botanical gardens, cooperative extension offices, and master gardener volunteer programs now have a social media presence on apps like Instagram and Facebook where they present videos and links to solid information. Make sure you are following the organizations’ official accounts. You can follow Durham County Extension Master Gardeners on social media at @durhamncmastergardeners on Instagram and NC State Extension Master Gardener Volunteers, Durham County on Facebook.

Resources and Additional Information

Three great examples of university-based sites with searchable gardening information appropriate for our Plant Hardiness Zone are North Carolina State’s Plant Toolbox, NC Cooperative Extension, and Clemson University’s Home and Information Garden Center. See links to these amazing resources below.

https://plants.ces.ncsu.edu/

https://www.ces.ncsu.edu/categories/lawn-garden/

https://hgic@clemson.edu

Botanical gardens, arboretums, and gardening non-profit organizations such as state native plant societies (.org or .edu sites) can be excellent and trusted resources for gardening information. The American Horticultural Society website, while not a complete listing, has geographic search features for master gardener programs, plant societies, and over 345 North American botanic gardens, including links to their websites. Some great North Carolina-based garden sites that have a plethora of plant information on their sites include the J C Raulston Arboretum, North Carolina Botanical Garden, and the Sarah P. Duke Gardens.

https://jcra.ncsu.edu/index.php

https://ncbg.unc.edu

https://gardens.duke.edu/

Look for Extension Master Gardener training materials online. For example, North Carolina State University and NC Cooperative Extension provide full online access to their entire gardener handbook, the foundation for all Master Gardener Volunteer instruction.

https://extensiongardener.ces.ncsu.edu/extension-gardener-handbook/

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Calling Plant 911: Understanding Cold Injuries and Caring for Plants After Extreme Weather Hits

by Melinda Heigel, EMGV

Discoloration on an evergreen fern (Crytomium spp.) began to appear a week following the extreme cold in our area. (Image credit: M. Heigel)

Regardless of where you live, if you’re in the US, you know what unusually prolonged cold much of the country experienced in late December 2022 as a bombogenesis (“bomb cyclone”) made an unwelcome appearance. Here in Durham, NC, December nighttime temperatures typically range in the low 40s. But during the “freeze out” of December 2022, our nighttime lows plummeted quickly ushered in by strong sustained winds. Beginning on December 24th, temps averaged a mere 14.3 degrees over 3 nights and averaged a frigid 20 degrees over 6 consecutive nights. While this is not arctic by a long shot, it is for our plants, trees, and shrubs, many of which are neither hardy nor acclimatized to these constant low temperatures. With such weather extremes, even zone-appropriate plants can come under great stress.1 Needless to say, many gardeners had some nail-biting nights. While this extreme weather event has passed, you may just be seeing its effects in your landscape. Identifying the damage and understanding how to treat cold injuries will likely come in handy about now. In a later blog post, we’ll address how to protect your plants best when cold temperatures hit, including those late-season frosts that always tend to zap blooming hydrangeas in late April and early May. Right now, though, let’s get down to brass tacks about cold injuries and how to evaluate and tend your plants.

Red leaves evidence chilling injury on this princess flower (Tibouchina urvileana). (Image credit: UCCE Master Gardeners of Sacramento County)

Understanding and Identifying Cold Injuries

There are several types of cold injuries plants can experience. A chilling injury is damage to plant parts like leaves, flowers, or fruit that happens when the temperature is above the the freezing point. This generally occurs between 32 -55 degrees depending on the cold-hardiness of the plant. This kind of injury is often found in tropical or subtropical specimens. This damage often shows up quickly after the exposure. Signs of chilling injuries can appear as leaves that turn red, purple or black in color, leaves that wilt, and flowers that die. This damage may stress plants, lead to some dieback, or slow future development, but it isn’t always fatal.

Freeze injury to a pansy in container (Viola x wittrockiana). Notice that the leaves look both wet and wilted in appearance. Containerized plants are especially susceptible since their roots don’t have underground protection. (Image credit: M. Heigel)

Frost and freeze injuries are more in line with what most of us experienced in the recent deep freeze. When the conditions are like those we experienced in December, we have what is known as an advective freeze.2 Frost and freeze injuries are very similar and tend to affect plants in the same way. With these injuries, water inside the plant cells basically crystallizes with rapid temperature fluctuations. As these ice crystals expand, they cut and burst cell membranes, and fluids leak from the cells leading to plant death. Leaf and stem tissues are most at risk with this type of fast freeze.

The timing for identifying frost and freeze damage to plants can be tricky. Sometimes, as in the case of the photo of the potted pansy, the evidence of injury appears quickly. This photo is after only one night of severe cold. However, in other cases, frost and freeze injuries can take days, weeks, or even months to show up. Immature plants are more susceptible than mature ones. Damage often starts with the softest, actively-growing plant parts–like the basal leaves of the pansy. Some signs of damage may show up after a week or two. For example at my house we have mature yucca plants (Yucca spp.) and after the 6 nights of extreme cold, they appeared to have weathered fairly well. One week later, they began showing signs of distress. And today, nearly one month later, the extent of the injuries is becoming more and more evident as the photos below demonstrate. Overall, damage to flower buds, vegetative buds, stems, and entire plants may remain hidden until active growth begins again many weeks later in the springtime. When it comes to evergreen shoots, the extent of impact is usually evident within 2 weeks.

(Left to right) Freeze damage may take time to materialize. This once-evergreen mature yucca began to show discoloration and necrosis about a week after the deep freeze. It continues to evolve nearly one month later as what appears to be significant dieback is now occurring in the apex of the plant. (Image credit: M. Heigel)

Frost and freeze injuries can impact all of the parts of a plant and show up in myriad ways. Leaves can appear wilted, black or otherwise discolored, have a mosaic-like appearance due to damaged or dead cells, and even look wet or waterlogged from the bursting of plant cells. Later signs might be misshaped or curled leaves. In evergreens, coloration changes can occur. Needled evergreens like junipers (Juniperus spp.) may turn uniformly brown to bronze; broadleaf evergreens such as boxwoods (Buxus spp.) and hollies (Ilex spp.) may show leaf burn or scorching at their margins from cold damage. In terms of vegetables, many cool-weather crops like kale, collards, and Brussels sprouts and can handle temperatures below 26 degrees briefly, but damage can materialize in the form of affected foliage and potentially reduced yield.

Boxwood (Buxus spp.) following December’s freeze exhibiting damage with marginal leaf burn and possible desiccation.3 (Image credit: M. Heigel)

The bark of woody plants and trees can also be impacted by rapidly freezing temperatures. Long cracks that are deep and narrow called frost cracks can occur in trees that experience wide-ranging temperature changes. The crack can expand and contract after the initial injury and callus over in the summer. Frost cracks can reoccur. In other woody plants such as azaleas, rapid temperature changes can cause the bark and/or stems to literally split near the plant’s base. Unfortunately, this can be a tough injury for plants to overcome. Damage to roots, while harder to see, can definitely occur. This is especially true if plants are in containers where their roots aren’t protected below ground. Likewise roots of ball-and-burlap plants are also at higher risk. According to Clemson University’s Home and Information Garden Center, even zone-hardy plants planted above ground in planters with “unprotected roots” can experience lethal root injuries at temperatures beginning at 28 degrees.

Caring for Affected Plants

Pruning –Once you look for and identify signs of frost/freeze damage, exercise restraint. Some plants may appear dead, but they aren’t. While you might not like the looks of your plants, think twice about reaching for the clippers. Dead foliage might protect the plant’s crown and roots from further damage by providing insulation, especially with evergreens and perennials. As mentioned, damage is not always apparent and can take months to surface. Any pruning or cutting back now may mean you would be leaving potentially unhealthy plant tissue behind. Wait until you can fully determine the extent of damage before taking action like cutting out dead wood and plant material. This could mean waiting until the spring. One exception to this would be herbaceous plants (plants that have no woody stems above ground) like the pansy above whose cells have essentially collapsed. Removing this specific kind of dead plant material is recommended as there is little hope for revival.

Fertilizing–While it might seem tempting to help nurse your ailing plants with added nutrition, don’t apply fertilizers following these types of events. Your actions may indeed encourage the plant to put out some new growth, but if we are in the dead of winter or in late fall or early spring when frosts happen, you might be harming your plant further. Tender new growth is especially prone to cold injury. Wait until the any threat of additional frost/freeze has passed before your fertilize.

Watering–Pay close attention to moisture. After an extreme event like a hard freeze where the ground is potentially frozen around the plant, its roots are not able to take up adequate moisture. Likewise, very windy conditions like we experienced with the December cold snap also contribute to plants’ inability to capture and use moisture when under stress. Water your plants at the soil line after harsh cold-weather events to ensure proper hydration. Likewise, make sure the plant receives adequate water if nature doesn’t provide it throughout the upcoming growing season. The cold pressure plants experience in these freeze events means that gardeners need to pay special attention to keeping them healthy in the months that follow.

Mulching–Some experts suggest that keeping the base of plants well mulched is a way to help protect roots of plants, moderate soil temperatures by lessening heat loss, and hold moisture–all things especially critical during extreme freeze events.

If you haven’t surveyed your landscape thoroughly after the December weather–or even if you have, go out and assess what you see today using these descriptions of cold injuries as a guide. Be patient and remember that spring is on the way!

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Notes

1–Knowing your area’s USDA Plant Hardiness Zone is an important tool in plant selection and success year round in outdoor growing. However, it’s not a bulletproof guide. These zones are based on the average minimum temperature your area has historically experienced. In extreme weather, how quickly and low temperatures fall can negatively impact even zone-appropriate plants.

2–An advective freeze is a wind-borne event that happens when an air mass with below-freezing temperatures moves into an area and displaces warmer air. With this type of freeze, windy conditions tend to persist throughout the event. This influences the temperature of the plants and makes conditions right for ice crystals to form within the plants’ tissues.

3–Desiccation can also be a type of cold-related injury to plants, and evergreens, both narrow and broad-leafed varieties alike, are especially susceptible Our December 2022 weather event presented the perfect example of conditions that easily lead to this type of damage. During very windy conditions, and especially when they are exacerbated by frozen ground, plants can lose water faster than they can take it in. This often results in damage that appears like leaf burn on scorch on leaves. This damage pattern can also come from extremely low temperatures. Root injury can also result.

Resources and Additional Information

To determine your USDA Plant Hardiness Zone, plug in your location on USDA’s interactive map.

https://planthardiness.ars.usda.gov/

NC Cooperative Extension offices have serval sites with articles featuring cold-related plant injuries and possible methods for protecting plants during extreme weather.

https://lenoir.ces.ncsu.edu/2018/01/plants-and-cold-temperatures/

https://lee.ces.ncsu.edu/2018/04/identify-frost-damage/

https://forsyth.ces.ncsu.edu/2021/04/protect-your-plants-from-frost/

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