Category: polyculture

From Beloved to Invasive: How does it happen?

On By durhammastergardenersIn benign non-native plant, bird friendly habitat, bugleweed, conservation, crape myrtle, Durham NC, ecological sustanability, ecology, gardening, Gardening information, gardening NC, heavenly bamboo, hellebore, how plants become invasive, Invasive Atlas of the United States, invasive plants, Japanese stiltgrass, lenten rose, lesser threat invasive plant list, liriope, monkey grass, nandina, native ecology, native ecologynative plant ecology, native plants, Natives, NC biodiversity, NC conservation, NC Native Plants, New Hope Bird Alliance, non-native plants, ox-eye daisy, perennials, polyculture, privet, protecting native plants, Rose of Sharon, Russian olive shrub, sustainable garden design, sustainable gardening, sustanability, Uncategorized, urban garden, urban gardening, watch list for invasive plants

By Jeannie Arnts , North Carolina Extension Master GardenerSM volunteer of Durham County

I have volunteered as a recorder for the New Hope Bird Alliance (formerly Audubon) Bird Friendly Habitat (BFH) team since 2018. The BFH team, consisting of a plant expert and a recorder, visits homeowners’ yards to identify native and invasive plants and make recommendations for improvements to achieve an ecologically beneficial habitat for birds, insects, and other wildlife. Since I have joined the group, plants previously recognized as non-native, non-invasive plants have been moved onto the “Watch List” or “Lesser Threat Invasive” list. Other plants previously considered lower threat have been moved to a higher threat category. The question arises, “What is it that changes a benign non-native into an invasive?

When is a Plant Determined to be Invasive?

Plants are categorized as invasive once they disrupt the ecology of intact, functioning eco-systems, leading to loss of biodiversity and habitat degradation. This results in huge economic damage valued in the billions of dollars to agriculture, forestry, and personal property. Invasive plants compete with our natives for critical and often limited resources like sunlight, water, nutrients, soil, and space. Anyone who has ridden the train at the Museum of Life and Science in Durham and seen the Russian olive shrubs (Elaeagnus sp) overtaking the landscape along the tracks or been on a hiking trail in many Triangle forests and seen the stiltgrass (Microstegium vimineum) engulfing the entire ground cover of the forest floor can easily see the impact of invasive species.

Russian olive shrubs (left) out-compete native plants and dominate riparian areas primarily in central and western USA. Japanese stiltgrass (right) was accidentally introduced into Tennessee around 1919 as a result of being used as packing material for porcelain from China.  it has invaded many woodlands in North Carolina. (Image credit: NC State Cooperative Extension)

What Factors Influence a Plant to Become Invasive?

Biologists have been “vexed for decades” by the question of what causes some plants, once thought to be benign, non-invasive exotics, to become a risk to our eco-systems due to their invasive characteristics. The ecologist, Charles Elton, wrote the book, The Ecology of Invasions by Plants and Animals, in 1958 and, more than 100 years earlier, Charles Darwin pondered this question during his voyage on the Beagle (1831-1836). Johnny Randall, former Director of Conservation Programs at the NC Botanical Garden, noted that one reason plants become invasive is that in their new environment, they no longer have natural controls, such as pests, pathogens, and herbivores. This gives them an advantage over native plants that have been part of a region’s ecology for millennia; consequently, the invasives often out compete the natives.1

In addition, a plant’s propensity to become invasive is a function of the plant’s own biology, including a relatively short generational period, large seed production, its ability to reproduce asexually (without the need for fertilization), and having large fleshy fruits, such as those produced by Autumn or Russian olive trees (Elaeagnus sp), privet (Ligustrum sp.) and heavenly bamboo (Nandina sp.) With this latter trait, birds will eat the berries and deposit the seeds far from the original location of the plant.

A non-native, is especially prone to invasion when it sets its roots in a “matched habitat” similar from which it came, i.e., their native ranges tend to have similar temperature extremes, precipitation levels, and seasonal cycles. Stiltgrass (Microstegium vimineum) is an example of a plant that meets most of the criteria for becoming invasive. It is an annual that produces up to 1000 seeds per year, has no natural controls, reproduces asexually, and its seeds are distributed by the wind. In addition, the seed bank may take up to five years to germinate. Many gardeners who think they have eliminated stiltgrass on their property one year are befuddled to find a number of plants from the seed bank sprouting the next summer. The good news is that if you continue to manage the stiltgrass, you will have fewer plants each year.

Beloved Plants that Become Invasive

I think we can all agree that we want stiltgrass eliminated from our property, but there are non-native plants that have been beloved by gardeners for generations that are being added to the invasive lists, much to the consternation of gardeners. One example is the lenten rose (Helleborus orientalis).

Some hellebores  have started to escape into woodland areas and are able to stifle our favorite native understory flowers. They prevent seedlings of other plants from getting established as dense mats of their offspring grow. (Image credit: NC Extension Toolbox, Bob Gutowski CC-BY-NC-SA-2.0; Jim Robbins CC-BY-NC-4.0).

There are many good reasons that hellebores are such a beloved plant: they bloom over the winter months when very few other plants are blooming; they require little care; they are deer and rabbit resistant; they tolerate a wide range of light conditions; including light shade; and they survive even deep frosts. Unfortunately, they meet many of the conditions that place them at risk to become invasive in our region: they have no natural pests or pathogens; they can successfully fertilize themselves (although bees will seek nectar and pollen from the plants); and they produce a multitude of fertile seeds. While there is not a perfect native substitute for hellebores, the gardener may consider planting a matrix of sedges (Carex species), coral bells (Heuchera americana), and ferns, many of which are host plants, to provide winter interest .

While not yet on the invasive lists in N.C, another beloved plant that is of concern is the crape myrtle (Lagerstroemai indica). Its spread is being tracked in Southern states by the Invasive Atlas of the United States and the plant’s spread is being watched with caution by conservationists in N.C. Each flower of the crape myrtle produces a prodigious number of seeds that are carried or blown far from the mother plant. I have had a crape myrtle in my yard for probably the past 40 years and it has only been recently that I have seen it sprouting up elsewhere in my yard and ¼ acre woods.


Crape myrtles are seen along roadways and in gardens in North Carolina but conservationists have raised concerns because it outcompetes many native plants (Image credit: NC Plant Toolbox, skdavidson)

Liriope (Liriope muscari and L. spicata), bugleweed (Ajuga reptens), Rose of Sharon (Hibiscus syriacus), and ox-eye daisy (Leucanthemum vulgare) are all common ornamental plants that have been added to the NC Invasive Plant list as “Low Threat” species. While they have the potential to harm our local ecology, they are not yet doing so.

Liriope (Liriope muscari), bugleweed (Ajuga reptens), Rose of Sharon (Hibiscus syriacus), and ox-eye daisy (Leucanthemum vulgare) are plants that many of us have in our garden. They are considered to be “Low Threat” to native plants of North Carolina. (Image credit: North Carolina Toobox, Manuel-CC-BY-2.0; Jim Robbins-CC-BY-NC-ND; Frank Mayfield, CC-BY-SA-2.0)

As gardeners, we can control the spread of potentially invasive plants in our own yards or we can choose to remove them and plant a native that supports our native ecology. A follow-up blog will suggest options to consider for replacing invasive plants with natives that power our ecosystems.

Notes

  1. Personal communications Peter Schubert, NC Invasive Plant Council, October, 2024 and Johnny Randall, former Director of Conservation Programs, NC Botanical Garden, October 2024.

Resources and Additional Information

NC Invasive Plants List adopted by NC-IPC, November 16, 2023NC Invasive Plant Council
https://nc-ipc.weebly.com/uploads/6/8/4/6/6846349/invasive_plant_list_-_ranked_-_2023-11-16.pdf

Grzędzicka, E., Assessment of Habitat Selection by Invasive Plants and Conditions with the Best Performance of Invasiveness Traits, February, 2023. https://www.mdpi.com/1424-2818/15/3/333

Invasive plant Atlas of the United States University of Georgia-Center for Invasive Species and Ecological Health, October 2018 https://www.invasiveplantatlas.org/index.cfm

Jones, M., Gaster, R. Invasive Species, NC Cooperative Extension, Lee County, July 30, 2019 https://lee.ces.ncsu.edu/2019/07/invasive-species/

Moony, H.A., Cleland, E.E., The Evolutionary Impact of Invasive Species, Proceedings of the National Academy of Sciences. May 2001, 98(10) 5446-5451 https://www.pnas.org/doi/full/10.1073/pnas.091093398

Moorman, C, Megalos, M, Douglas, K. Invasive Plants and Your Forests, NCSU Extension Publication. Revised September 11, 2024. https://content.ces.ncsu.edu/invasive-plants-and-your-forests

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Bringing Plants of the Piedmont back to Downtown Durham

On By durhammastergardenersIn bees, beneficial insects, birds, cover crops, CPTED, crime prevention through environmental design, Downtown Durham, NC, drought tolerant plants, Durham NC, garden design, gardening, Gardening calendar, Gardening information, gardening NC, green mulch, ground covers, ground covers for living mulch, how to improve soil, insects, landscaping for wildlife, living mulch, native plant list, native plant list for Piedmont NC, native plants, Natives, NC Native Plants, Perennial, phenology calendar, Piedmont Prairie Garden at Rotary Plaza, pollinator gardening, polyculture, renewable gardening, soil, soil health, sustainability, sustainable garden design, sustainable watering, sustanability, Uncategorized, urban challenges in landscaping, urban garden, urban gardening, urban heat islands, urban planning, urban prairie garden, urban wildlife, uuban service benefits in landscapes, water wise plants, wildlife

By Eric Wiebe, NC State Extension Master GardenerSM volunteer of Durham County

A once neglected site, Rotary Plaza in downtown Durham comes alive in July 2021 with sunflowers that produce seeds for wildlife. Millet, a grain used here in the design and novel to many urban dwellers, also serves as a great food source for gold finches and other birds. (Image credit: Ben Bergmann)

Have you noticed changes to the plantings in downtown Durham over the past few years? If not, you need to head on down, walk around, and appreciate the transformation of the many of the city-owned green spaces. This transformation, and the strategies behind it, point to a new way of thinking about our relationship to the landscape, both in urban settings like downtown, but also in our own yards. 

I had the pleasure of both walking around downtown Durham with and then sitting down and talking to Ben Bergmann, the architect of this transformation. Ben was hired by City of Durham in September 2020 and charged with re-envisioning and elevating horticulture in downtown Durham, including creating a Downtown Horticulture Crew within Landscape Services. Ben brought a widely varied background to this position, trained with BS, MS, and PhD degrees in plant sciences, and working as everything from small business owner and farmer to tropical agroecologist and research scientist. One thing all this work has in common is plants and his great love of them. Ben feels this broad range of experiences has helped prepare him for a job that has him applying both the art and science horticulture to a municipal government position with its many constraints and opportunities and multitude of stakeholders. 

“When I started, I realized I needed a mission statement to clearly and concisely communicate to my supervisors and the public what I and my unit in government is doing.” He stated that his landscape design and management are guided by the goal of enhancing

  • Environmental sustainability
  • Programmatic stability
  • Aesthetic appeal
  • Service benefits

He notes that it is important to recognize that these categories are not mutually exclusive, but each has a role to play in his horticultural work supporting the living landscape in downtown Durham. 

Designing Sustainably

Ben explains that a crucial re-orientation is to start planning a landscape by thinking about how it can best align with the ecosystem, and that means starting with native plants that can be used. “For urban contexts some people would say that there is nothing native to these downtown spaces, and they will use this as an ‘out’ to plant whatever they want. I don’t embrace this–something was native, originally, in these spots. Many times these native plants can and will grow and thrive in these spaces.”

When Ben thinks about which native species to use, he first looks for plants native to the North Carolina Piedmont and secondarily to the larger Piedmont of eastern U.S. He notes, however, that it is very important to select plants with a long-range vision. That is, when accounting for global climate change, plants that are going to be here in 20 years may very well be in an environment that looks more like the southern Piedmont, our coastal plains, or South Carolina.

(Left to right) By selecting plants that can cope with a warming environment, Bergmann installed dwarf palmetto (Sabal minor), a plant typically associated with climates in the native ranges east and south of Durham, NC, and shown in green on the USDA’s zone map. (Image credit: Ben Bergmann; https://plants.usda.gov)

While native species take the lead when planning bed renovations, Ben states that non-native plants can still play a role. Returning to his guiding principles, non-natives can help fill functional gaps that might be hard to resolve in the short run with native species. Similarly, there are times that a spot planting of a non-native will help achieve a particular aesthetic goal by providing highlight color, texture, or architecture.

(Left to right) Before, a non-imaginative flag pole bed at Durham Parks and Recreation building included a highly invasive and exotic species: Nandina domestica. After a re-imagining, the same bed demonstrates there is a place for non-native plants alongside natives, especially during transition phases. (Image credit: Ben Bergmann1

Because the legacy plantings downtown were almost 100% non-native, it means that during a bed’s transition period, some non-natives may be left to help visually and functionally stabilize the bed until new, native plants can get established and then take over. Among other things, the non-native plants retained just for the transition can provide shelter from the sun and wind during establishment.

(Left) The process of establishing an urban prairie at the Rotary Plaza began in September 2020 with beds filled with weeds and remnants of long past garden attempts. (Middle) In March 2020, the the beds are full of cover crops: crimson clover, a nitrogen fixer that is great for pollinators in early spring, and winter rye, which has high biomass production and deeply penetrating roots. The intricate planting pattern reflects intentionality and provides visual interest. (Right) By July 2022 the Piedmont Prairie Garden at Rotary Plaza is realized and chock-full of sustainable native plants. (Image credit: Ben Bergmann)2

Even given how much he has achieved is such a short time, Ben still takes the long view when planning and planting. For example, cover crops have been used in very purposeful ways in areas such as the Rotary Garden. During the initial transition of this space, cover crops were planted and maintained in ways that not only helped build the soil, but created visual interest. Over time, the cover crops matured and eventually replaced with native perennials, creating an evolving horticultural narrative in this space. As Ben said, “Some [landscape] designers shy away from transitions–they want to ‘put a tarp over it’ until it has reached its final phase. I don’t shy away from transitions; they are an important part of my plans.”

Ben notes that using select non-native plants, cover crops, and other annuals allows him to more easily use seeds, plugs, and small plants in the beds. In the case of herbaceous perennials and woody plants, besides being less expensive, younger, smaller plants will typically develop more robust root systems and quickly outgrow their bigger counterparts.

Addressing Urban Challenges

Ben was quick to remind me of the many contextual limitations and challenges the Downtown Horticulture Crew faces daily that home suburban gardeners rarely have to contend with. For example, CPTED (Crime Prevention Through Environmental Design), is an important set of guiding heuristics that often preempts other design considerations. In addition to making sure plantings do not screen illicit activities, it also disallows designs that interfere with security cameras or impair pedestrian safety, including driver site lines at street crossings. 

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The center of the Chapel Hill Street parking lot is a perfect example of an urban heat island with even hotter microclimates where landscapes are needed. (1) In January of 2022, the lot was cleared of weeds and other unhealthy, dated, and non-native landscape plants. (2-5) The view in 2024, after Bergmann and his team revitalized the area with an assortment of heat-tolerant natives. (Image credit: Ben Bergmann)3

Most of the beds Ben and his crew manage are surrounded by heat absorbing asphalt and concrete and buildings with glass that intensify solar radiation by reflecting light onto them. This creates heat islands much hotter than in surrounding communities. In addition, tall buildings funnel wind through these spaces and block rain, further desiccating plants. 

Humans also are unhelpful, by walking on, driving over, or otherwise physically challenging plantings. Some soils are more like landfill or construction sites, and many have the negative impact of overuse by dogs on their walks. Even with these challenges, Ben has identified quite a few native species that are up for environmental extremes while still fulfilling functional and aesthetic goals. Not surprisingly, he is currently exploring the possibility of a demonstration xeriscape-inspired bed. This one may very well include some non-native plants more typically found in desert environments.

Educating the Community

Ben understands that an important part of his job is educating not only the workers and residents of downtown Durham but all city residents and visitors as to a new way of envisioning landscapes. Landscapes that, while they exist in a human-built environment, much more clearly reflect real ecosystems of the NC Piedmont. This means not only planting predominantly native species but planting them in soil nourished by natural mulches created by the plants themselves rather than imported, dyed wood chips. He also allows the plants to go through full cycles, creating visual landscapes that sometimes predominate in browns, grays, and blacks, as much as greens, yellows, and reds. 

(Left to right) The Piedmont Prairie Garden at Rotary Plaza in early January with the last vibrant colors and in late February, when the remaining plants still create an almost black-and-white picture. (Image credit: Ben Bergmann)4 (Below) By designing a phenology calendar for his plantings, Bergmann pays careful attention to nature’s timing. This tool helps gardeners know when to plant, specific bloom times, and when colors, textures, and other elements are present throughout the seasons. For a closer view, click on the link for an electronic version of the calendar in our Resources and Additional Information section below.

He is heartened that there is a growing critical mass of professionals helping him in this educational work. The City of Raleigh and Wake County has been hard at work creating plantings that reflect this same vision. He also has strong allies at Duke Gardens, where Annabel Renwick, has provided both inspiration and plants for some of the bed designs. Ben recognizes that it will take a while both for his beds to reach the final stages of their design vision and for Durham residents to embrace this new look, but he has already received lots of positive feedback for his renovated beds. It will also be up to Durham Master Gardeners, and committed gardeners everywhere, to help continue to educate the public and support this work.

Plant lists for featured Downtown Durham Sites

1–The planting design at the Parks and Recreation administration site include a mix of natives (*) and non-native plants: threadleaf coreopsis* (Coreopsis verticillata), creeping phlox* (Phlox subulata), eastern ninebark* (Physocarpus opulifolius), panicle hydrangea (Hydrangea paniculata), lantana (Lantana camara), coleus (Solenostemon scutellarioides).

2–Plants in the Piedmont Prairie Garden at Rotary Plaza include swamp milkweed (Asclepias incarnata), blue mistflower (Conoclinium coelestinum), purple coneflower (Echinacea purpurea), Joe-Pye weed (Eutrochium fistulosum), Spotted Joe-Pye weed (Eutrochium maculatum), common sunflower (Helianthus annuus), scarlet rosemallow (Hibiscus coccineus), larger blue flag iris (Iris versicolor), cardinal flower (Lobelia cardinalis), switch grass (Panicum virgatum), garden phlox (Phlox paniculata), hoary mountain mint (Pycnanthemum incanum), brown-eyed Susan (Rudbeckia triloba).

3–Center of Chapel Hill Street parking lot showcases the following native plants: sedge (Carex bicknellii), purple love grass (Eragrostis spectabilis), Elliott’s lovegrass (Eragrostis elliottii), scarlet rosemallow (Hibiscus coccineus), swamp rosemallow (Hibiscus moscheutos), Virginia sweetspire (Itea virginica), tulip poplar (Liriodendron tulipifera), black-eyed Susan (Rudbeckia hirta).

4–The Piedmont Prairie Garden at Rotary Plaza in late February still offers color, texture, and winter interest. They include climbing aster (Ampleaster carolinianus), scarlet rosemallow (Hibiscus coccineus), switch grass (Panicum virgatum), hoary mountain mint (Pycnanthemum incanum), white goldenrod (Solidago bicolor), showy goldenrod (Solidago speciosa)–with touches of hopeful green, beardtounge (Penstemon digitalis), eastern beardtounge (Penstemon laevigatus), stoke’s aster (Stokesia laevis), adam’s needle yucca (Yucca filamentosa).

5– See below for Bergmann’s list of natives that are tried-and-true performers for Durham and central North Carolina.

Resources and Additional Information

Piedmont Prairie Garden at Rotary Plaza Bloom Time / Color Calendar (electronic version)

https://go.ncsu.edu/piedmontprairiecalendar

NC State Extension on landscaping for wildlife with native plants in urban environments

https://content.ces.ncsu.edu/landscaping-for-wildlife-with-native-plants

North Carolina Native Plant Society’s list of recommended native species

Recommended Native Species

NC State Extension on all things native plants

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