By Melinda Heigel, EMGV
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.
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.
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.
4– For information on soil testing, see https://gardening.ces.ncsu.edu/soils/soil-testing/
Resources and Additional Information
North Carolina State University has several online resources on annual bluegrass. Below are links to learn more.
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://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)
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