Using Plant Competition to our Advantage | Connecticut Invasive Plant Working Group

Are There Vigorous Native Plants that Exclude Unwanted Plants?

By Kathy Connolly

Updated 7/1/25

People often ask, “Is there a list of plants that ‘hold out the weeds’? The question is more complex than it appears at first glance. Below, find some ideas and questions for consideration.

Which plants are you trying to exclude? If the answer is one of the “super-weeds,” the answer may be no. Phragmites, knotweed, mugwort, black swallowwort, barberry, burning bush, and a host of non-native invasive vines (bittersweet and porcelainberry) don’t seem to have effective competitors among regional native plants.

Are there desirable plants that exclude other plants? Maybe! But it depends on what plant you’re trying to exclude and the site. Good site preparation and careful plant establishment are key to excluding invaders. But plant selection is just as important. How can we identify plants—particularly groundcovers—that will be competitive in the selected space?

How do plants compete for their space? Like all living creatures, plants have competitive strategies. They try to “edge out” other plants for light, water, nutrients, germination opportunities, canopy growth space, pollinators, and soil resources, including microbes. Below, see seven competitive strategies plants use to establish themselves and thrive. We may be able to exploit one of these in favor of a native plant or use it against an invasive plant.

1. Allelopathy: A plant can exude toxic substances that discourage germination and seedling development in other plants. Allelopathy is a common phenomenon among plants worldwide, but the amount of exudate and its seasonal presence vary widely. Allelopathy most frequently occurs through roots, but can come from leaves and decaying plant parts.

Both native and non-native plants use allelopathy, but it appears to be more widely studied in non-natives (1). Agricultural studies note that some native sunﬂowers (Helianthus spp.), milkweeds (Asclepias spp.), and goldenrods (Solidago spp.) are allelopathic to ﬁeld crops (2, 3). In another study, pussytoes (Antennaria spp.) and ragweed (Ambrosia artemisiifolia) exhibited allelopathy. (4)

Members of the mint family are often allelopathic in garden settings. Native wild ginger (Asarum canadense) grows densely by rhizomes in shady spaces, and some sources have identiﬁed allelopathy in its root exudates. White snake root (Ageratina altissima) is both allelopathic and deer-resistant, but it is also toxic to pets and humans. In one study, it appeared to have a greater allelopathic effect than Japanese stiltgrass. (5)

Use this search engine “hack” to identify plants with allelopathic tendencies, place a botanical name in a search engine window and follow the name with the word “allelopathy,” excluding the quotes. This approach will produce citations if they exist.

Note that evidence of allelopathy in lab tests may not predict the presence or amount of allelopathy in ﬁeld conditions. Allelopathy seems to be expressed where and when it is needed by plants.

2. Competition for light: Plants with the biggest leaves and/or tallest stature win the most sunlight. For instance, the broad leaves of native hay-scented ferns effectively “shade out” many competitors in woodlands and around woodland edges. The plant has fronds that cross one another and hover densely over the ground. This plant’s tendency to dominate both shady and sunny sites throughout rhizomes can be troubling in some circumstances. (6)

Mugwort (Artemisia vulgaris) is a non-native invasive plant that is both tall and dense, as well as allelopathic.

Yet even mugwort cannot resist mowers and weed whackers, and I know of one weed warrior fighting this plant with a mowing experiment. He cuts the mugwort and allows the very vigorous native panicled aster (Symphyotrichum lanceolatum) to grow through and around the mugwort. He is testing the ability of an aggressive native to vanquish aggressive mugwort with strategic mowing and cutting on the unwanted mugwort.

3. More light competition. Ground-hugging leaves represent another competitive strategy. Many of these plants have tight basal rosettes, such as some hawkweeds (Hieracium spp.) Pussytoes (Antennaria plantaginifolia and A. neglecta) are native examples of plants that claim space with ground-hugging leaves. These plants tend to form small, dense patches.

Some other low-growers have are looser in form but a bit taller, including wild strawberries (Fragaria virginiana), woodland strawberries (Fragaria vesca) and barren strawberries (Geum fragarioides, also known as Waldsteinia fragarioides). Both wild strawberries (true strawberries) and barren strawberries (not true strawberries) are capable of covering extensive spaces.

Wild ginger (Asarum canadense) and Allegheny spurge (Pachysandra procumbens) are slightly taller than the ground-hugger category, but capable of forming a dense canopy. Another native in this category is Lance-leaved heal-all (Prunella vulgaris ssp. lanceolata), a plant that fans out its leaves in a circle about 18” wide. (The eastern native of Prunella vulgaris is a subspecies lanceolata.)

The mint family offers some great spreaders, most notably mountain mint (Pycnanthemum muticum).

When creating a planting plan, look for plants with leaves that can “steal” light from unwanted competitors and quickly fill space.

4. Herbaceous density. Plants that spread by stolons and rhizomes and make good ground-covering candidates, yet it may take time to achieve the desired density. We can interplant species that grow as singles or in small patches among the spreaders. Note the root structures of these plants, which don’t tend to migrate by roots.

Anise hyssop, Agastache foeniculum (ﬁbrous roots)
Penstemon/Beardtongue: Penstemon digitalis, P. virginiana, and P. hirsutus (Fibrous roots)
Nodding onion, Allium cernuum (bulb and ﬁbrous roots)
Butterﬂy weed, Asclepias tuberosa (tuberous root)
Swamp milkweed, Asclepia incarnata (tuberous root)
Downy phlox, Phlox pillosa (tuberous root)
Blue false indigo, Baptisia australis (tuberous root)

5. Opportunism: Some plants can become well established where few others can survive. Look for evidence of unique adaptations while doing plant research.

Moss is a great example; it attaches to surfaces with “rhizoids.” Moss has no roots. Mosses compete well in places unfriendly to rooted plants, such as soil with very low or high pH, deep shade, areas under mature trees, or areas with little or no soil depth, and areas with high humidity.

Other plants are adapted to very speciﬁc environments. For instance, rock polypody ferns live on bare rock and shady cliffs. Low-bush blueberries cover rocky “balds” where almost nothing else but moss and lichen grow. American beach grass can thrive in 100 percent sand. Sedges and ferns often grow well under mature trees, where little else will grow.

6. Resistance to herbivory. Plants that resist deer, rabbits, and other plant-eaters have an obvious advantage. Herbivore-resistant plants often have one or more of the following characteristics:

Fuzzy leaves
Bad-tasting substances in leaves and branches. (These substances may also be involved with a plant’s allelopathy.) For instance, white snakeroot (Ageratina altissima) contains poison and can thrive in areas with heavy deer pressure.
Scents unwelcome by animals.
Other mechanical deterrents, such as thorns and briars.

7. Microbial populations: Plants control microbial populations in their locations with root exudates. (This is separate from allelopathy.) They can favor “friendly” microbial populations or disfavor microbes needed by competitor plants.

8. Pollinators: Plants can make themselves easier to discover in a location, thus attracting pollinators and robbing nearby plants of pollinator visits.

Competitive Native Plants

Low-growing—Under 1’

Sedges: Healthy fine-leaf sedges such as Pennsylvania, Appalachian, White-tinged, and Rosy sedges create a ground-level canopy where few other plants seem to grow. Sedges prefer part to full shade. Carex pensylvanica, C. Appalachica, C. albicans, and C. rosea.

Barren strawberry: Geum fragarioides (a.k.a. Waldsteinia) is a ground-hugging shade lover that seems to defy unwanted ground-huggers such as creeping Charlie

Wild strawberry: Fragaria virginiana seems adaptable to a range of conditions, from moist to dry and from part shade to full sun. Once established, it covers the ground densely. Light foot traffic and occasional mowing don’t seem to hurt it. Its cousin, the woodland strawberry (Fragaria vesca) is very shade tolerant.

Canada Wild Ginger: The plant thrives in moist shade or part sun, where it appears to crowd out ground-level competitors. I have found it slow to establish. Asarum canadense shows up on some lists as possibly allelopathic.

Allegheny spurge: Pachysandra procumbens is a lovely replacement for Japanese pachysandra in all but one way: it is not evergreen.

Medium height 1’ – 3’

Hayscented fern: Dennstaedtia punctilobula produces groundcover so dense it deters germination of other desirable native plants. https://pubmed.ncbi.nlm.nih.gov/24248724/

Pussytoes: The leaves of both Antennaria plantaginea and A. neglecta form dense, ground-hugging leaf covers that seem to eliminate competition. At least one source refers to pussytoes as possibly allelopathic (4).

Lanceleaf self-heal: The native self-heal, Prunella vulgaris var. lanceolata, is a low-care vigorous northeastern native that tolerates many conditions. (Note the eastern native is a variety.) Its flowers are about 12” tall, but the leaves form a dense, low-growing canopy that appears to deter ground-level competitors. It can be used in low-mow “bee lawns” because it can take occasional mowing, according to University of Minnesota Bee Lab. (https://beelab.umn.edu/bee-lawn)

Taller, 3’ +

Bergamot, Monarda fistulosa, grows to 5’ tall and forms dense stands that exclude other plants. A strong spreader.

Mountain mint, Pycnanthemum muticum, forms dense stands up to 4’ tall that seem to defy invasion by other plants.

Ostrich fern: Matteuccia struthiopteris is an aggressive fern in both shade and sun. Very few plants appear to compete with it, except the native Virginia creeper. I have also observed non-native invasive porcelainberry vines among ostrich fern.

Rough goldenrod, Solidago rugosa, forms dense colonies about 4-5’ tall in sunny, dry spaces. The ‘Fireworks’ cultivar is particularly dense when established, and it is shorter and less aggressive than the Canada goldenrod.

Bergamot, Monarda fistulosa, grows to 5’ tall and forms dense stands that exclude other plants. A strong spreader.

Mountain mint, Pycnanthemum muticum, forms dense stands up to 4’ tall that seem to defy invasion by other plants.

Shrubs

Densely planted shrubs of many sorts can be very effective against seedlings of invasive plants. Two common junipers, however, are worth noting for their wide range of tolerances and attractive appearance

Common juniper ‘Blueberry Delight’, Juniperus communis, seems to thrive on neglect. It grows about 4’ wide and 18” tall in part to full sun. After the initial establishment period, water sparingly. Keep it out of range from irrigation systems. Common juniper, ‘Repanda’, Juniperus communis, is a reliable selection that grows densely about 4’ wide and 18” tall.

Trees

Some trees appear in lists of plants with allelopathic or other exclusionary tendencies:

American elm: (Ulmus americana)
Black cherry: (Prunus serotina)
Cottonwood or Eastern Poplar: (Populus deltoides)
Hackberries: (Celtis occidentalis)
Maples: (Acer spp.)
Pines: (Pinus spp.)
Red oak: (Quercus rubra)
Sassafras: (Sassafras albidum)
Sumacs: (Rhus spp.) Fragrant sumac shrub (R. aromatica) is noted by one study as allelopathic.
Sycamore: (Platanus occidentalis)

References:

Kalisz, Susan & Kivlin, Stephanie & Bialic-Murphy, Lalasia. (2021). Allelopathy is Pervasive in Invasive Plants. Biological Invasions. 23. 10.1007/s10530-020-02383- https://www.researchgate.net/publication/344953865_Allelopathy_is_Pervasive_in_Invasive_Plants
Bhowmik, Prasanta. (1994). Biology and control of common milkweed (Asclepias syriaca). Reviews of Weed Science. 6. 227-250. https://www.researchgate.net/publication/256486001_Biology_and_control_of_common_milkweed_Asclepias_syriaca
Wang C, Wu B, Jiang K. Allelopathic effects of Canada goldenrod leaf extracts on the seed germination and seedling growth of lettuce reinforced under salt stress. Ecotoxicology. 2019 Jan;28(1):103-116. doi: 10.1007/s10646-018-2004-7. Epub 2018 Dec 13. PMID: 30547327. https://pubmed.ncbi.nlm.nih.gov/30547327/
Coal Mine Reclamation with Native Allelopathic Plants. https://warnercnr.colostate.edu/frs/coal-mine-reclamation-native-allelopathic-plant
Corbett, Brian K., Morrison, Janet A. The Allelopathic Potentials of the Non-Native Invasive Plant Microstegium vimineumand the Native Ageratina altissima: Two Dominant Species of the Eastern Forest Herb Layer Northeastern Naturalist, 19(2):297-312 (2012). https://doi.org/10.1656/045.019.0211
Horsley SB. Hay scented fern: Role of allelopathy in hay-scented fern interference with black cherry regeneration. J Chem Ecol. 1993 Nov;19(11):2737-55. doi: 10.1007/BF00980704. PMID: 24248724. https://pubmed.ncbi.nlm.nih.gov/24248724/