The 10 Worst Invasive Plants to Watch Out for in Your Region

When you see a plant for sale that grows quickly, blooms like crazy, and tolerates pests and diseases, it might sound like the perfect choice to add to your garden. But that vigorousness could be a red flag that the plant may be more trouble than it's worth, even to the point of being considered invasive. According to the USDA National Invasive Species Center, a plant earns the label "invasive" when it's non-native to a region and causes, or may cause, environmental, economic, or health issues. Unfortunately, many invasive plants have escaped from gardens into natural ecosystems, such as woodlands, prairies, and mountain wilderness. There, they spread and push out native plants and wildlife. Invasive plants also can also become a huge headache in your own garden.

A Measure of Invasiveness

A plant's invasiveness can vary from one region of the country to the next. For example, a plant that's invasive in a warm climate might not be problematic in a colder region, since its seeds die in freezing temperatures, making it impossible to multiply out of control. To help you zero in on the worst invasive plants for your region, public gardens in six major regions across the United States offer their top ten species to avoid. Although some of these plants are banned for sale in certain areas, you might encounter others at your local garden center or online. In recent years, plant breeders have developed better-behaved versions of some popular ornamental plants that have invasive tendencies. When in doubt, though, don’t plant a potentially invasive species.

The Northeast

Horticulturists at the New York Botanical Garden (NYBG) curate a global plant collection, so they're hyper-aware of how non-native species can impact the wildlands right outside the garden. Of the top 10 worst species they've identified for the Northeast, “all but knotweed and mugwort are reported to have been intentionally introduced to the United States as ornamental species during the 19th century," says Eliot Nagele, director of the Thain Family Forest at NYBG. "This emphasizes the need for gardeners to research the plants they're using to ensure the continued health of their gardens and the surrounding natural environment."

1. Amur cork-tree (Phellodendron amurense)
2. Asian bittersweet (Celastrus orbiculatus)
3. Field bindweed (Convolvulus arvensis)
4. Japanese angelica tree (Aralia elata)
5. Japanese honeysuckle (Lonicera japonica)
6. Knotweed (Reynoutria spp.)
7. Lesser celandine (Ficaria verna)
8. Mugwort (Artemisia vulgaris)
9. Multiflora rose (Rosa multiflora)
10. Porcelain berry (Ampelopsis brevipedunculata)

The Midwest

The worst invasive plants in the Midwest are wreaking havoc in native woodlands and grasslands. Kayri Havens, the Medard and Elizabeth Welch director of plant science and conservation at the Chicago Botanic Garden, warns gardeners to be vigilant about aggressive non-native plants that have adapted a little too well to the Midwestern climate. In addition to the top 10 list below, new problems continue to pop up on the botanic garden's radar. For example, she points out, “Kudzu, the vine that ate the South, is marching north and may be coming to your neighborhood soon. Persistent, reproducing populations have been found as far north as Chicago.”

1. Barberry (Berberis thunbergia)
2. Buckthorn (Rhamnus carthartica)
3. Burning bush (Euonymus alatus)
4. Bush honeysuckle (Lonicera morrowii, maackii, and tatarica)
5. Callery or Bradford pear (Pyrus calleryana)
6. Common reed (Phragmites australis)
7. Japanese honeysuckle (Lonicera japonica)
8. Japanese knotweed (Polygonum cuspidatum)
9. Leafy spurge (Euphorbia virgata)
10. Purple loosestrife (Lythrum salicaria)

Southeast

The warm, moisture-rich climate of the Southeast encourages all kinds of plants to thrive, so invasive species can seem to grow before your very eyes. Amanda Bennett, vice president of horticulture and collections at the Atlanta Botanical Garden, says that rampant seeding and accelerated growth are two major challenges when it comes to invasive plants in this region. “Eradication of invasive plants is difficult, at best, due to the strenuous and relentless attention it demands,” she says. Bypass the extra work and don’t plant these invaders.

1. Burning bush (Euonymus alatus)
2. Chinese privet (Ligustrum sinense)
3. Chinese wisteria (Wisteria sinesis and W. floribunda)
4. English ivy (Hedera helix)
5. Arundo grass, also called giant reed (Arundo donax)
6. Large periwinkle (Vinca major)
7. Pink silk tree (Albizia julibrissin)
8. Silvergrass, also known as maiden grass (Miscanthus sinensis)
9. Spanish bluebell (Hyacinthoides hispanica)
10. Virginia knotweed (Persicaria virginiana) *native but exceptionally tough to control and hard to get rid of

Mountain West and High Plains

The semiarid climate of the Mountain West and High Plains brings plenty of challenges when it comes to invasive species. Mike Kintgen, curator of alpine collections at Denver Botanic Gardens, says that several of the top 10 worst invasive plants in this region may not be problematic in other regions. He's also seen the opposite, when plants invasive in other regions don't spread as aggressively in the harsher climate of the mountains and plains. “This all means we have to create our own lists of invasive plants from observations as data,” he says.

1. Buckthorn (Rhamnus carthartica)
2. Butter and eggs (Linaria vulgaris)
3. Dame's rocket (Hesperis matronalis)
4. Myrtle spurge (Euphorbia myrsinites)
5. Purple loosestrife (Lythrum salicaria)
6. Rover bellflower (Campanula rapunculoides)
7. Russian olive (Elaeagnus angustifolia)
8. Tamarisk (Tamarix chinensis and T. parviflora)
9. Tree of heaven (Ailanthus altissima)
10. Yellow iris (Iris pseudacorus)

Desert Southwest

Invasive and aggressive plants in the Southwest not only threaten the native ecosystem, in some cases, they also contribute kindling to this wildfire-prone region. Kimberlie McCue, senior director of desert horticulture and conservation at Desert Botanical Garden in Phoenix, Arizona advises gardeners to avoid planting any of the species on her top 10 invasive plants list, and if you already have any of them, to remove these troublesome plants from your landscape. In some cases, it will take several passes to get the upper hand, but the effort is always worth it.

1. African daisies (Osteospermum spp.)
2. African sumac (Searsia lancea/Rhus lancea)
3. Bermuda grass (Cynodon dactylon)
4. Buffelgrass (Pennisetum ciliare)
5. Fountaingrass (Cenchrus setaceus/Pennisetum setaceum)
6. Oleander (Nerium oleander)
7. Oxeye daisy (Leucanthemum vulgare)
8. Red brome (Bromus rubens)
9. Stinknet (Oncosiphon piluliferum)
10. Periwinkle (Vinca major, V. minor)

Pacific Northwest

In the Pacific Northwest, differing growing conditions on the east and west sides of the Cascade Range impact invasive species in the region. “The east side is hotter and drier in the summer and colder in the winter, and the west side is influenced by the moderating temperatures of the Pacific Ocean and the increased winter rain,” says Raymond Larson, associate director and curator of the University of Washington Botanic Gardens. These differing conditions demand gardeners to be exceptionally vigilant when selecting plants. The following 10 are ones Larson recommends gardeners avoid and remove.

1. Butterfly bush (Buddleja davidii)
2. English and Atlantic ivy (Hedera helix, H. hiberinica)
3. English holly (Ilex aquifolium)
4. English laurel (Prunus laurocerasus)
5. Himalayan blackberry (Rubus armeniacus/R. bifrons)
6. Herb-Robert (Geranium robertianum)
7. Knotweed (Reynoutria spp.)
8. Shiny geranium (Geranium lucidum)
9. Tree-of-heaven (Ailanthus altissima)
10. Yellow archangel (Lamiastrum galeobdolon)