University of Maryland Extension

Herbaceous Invasives Control

*******View Key to Control Methods that are referenced here*******

Canada thistle and bull thistle (Cirsium arvense, C. vulgare) These European thistles, perennial and biennial respectively, are designated noxious weeds in Maryland. They prefer full sun and quickly form dense patches, possibly aided by root toxins. A threat to agriculture as well as natural ecosystems, they overtake any non-forested area from dry to seasonally wet. The perennial’s root system holds huge energy reserves. Tap roots send out side roots as deep as three feet and can regenerate from a one-inch-piece. Hand pulling is only possible with the youngest seedlings in moist pliable soil. Pink to purple flowers produce fluffy windborne seeds that remain viable for up to 20 years. P/D (youngest seedling), F/S,C/M/reC, PostE

Garlic mustard (Alliaria petiolata, A. officinalis)
This Eurasian herb brought by settlers for food and medicine is a biennial in the mustard family. The first year, it is a low rosette. The second year, stems reach 3-4 feet. White flowers produce skinny seed capsules, which propel seeds. Seed remains viable up to five years; thus controls must continue for five years. In sun or shade, it invades woods, stream banks, and roadsides, where its garlicky leaves are ignored by deer, and it crowds out native plants. It is especially damaging to spring wildflowers and wildlife dependent on them as early season sources of nectar, pollen, and foliage. It threatens to cause extinction of the rare West Virginia white butterfly, which feeds on the foliage of wildflowers in the mustard family. Garlic mustard replaces these native wildflowers and, when the butterfly lays its eggs on garlic mustard foliage, a leaf toxin prevents egg hatch. It can re-sprout from root fragments. P/D, F/S (before seed ripens), C/M/reC, PostE

Japanese knotweed/Mexican bamboo (Polygonum cuspidatum)
The hollow shoots of this Asian native have somewhat bamboo-like joints. Once valued for its white flowers and erosion control, its aggressive roots penetrate two to three feet deep, enabling it to form exclusive thickets 6- to 8-feet tall. It grows in sun or shade and dry or wet, even salty, soils. Threatens riparian areas where it is able to withstand severe floods. Root fragments spread via fill dirt, and seed travels in shoe treads. P/D (young), F/S, SM (black plastic), PostE, C&P/S

Lesser celandine/celandine buttercup (Ranunculus ficaria)
The glossy leaves and eight-petal yellow flowers of this European perennial arrive in March and April before most spring ephemeral wildflowers. A rapid spreader, its leaves create an impenetrable mat that blocks emerging wildflowers. Small bulblets at the base are easily dislodged by digging humans or animals. The bulblets and tuberous roots are often carried downstream by high waters from the wet areas it favors. Digging to remove it is possible only when the entire plant plus soil can be lifted. Caution: Distinguish carefully from our native Marsh marigold which has similar leaves, carpet-forming habit, and yellow flowers. Marsh marigold blooms atop eight-inch stems does not form bulblets or tubers, and its flowers have five to nine petals. Lesser celandine is still sold, though of limited ornamental value because it goes dormant by early June. P/D, PostE

Mints (Mentha spp.)
Mints include several lawn weeds such as creeping Charlie, but also herb mints. These grow rampantly in natural areas, crowding out natives. They spread by seed as well as roots and runners and will resprout from root fragments. Grow herbal mints in containers or root-confined areas. P/D, F/S, PostE

Purple loosestrife (Lythrum salicaria, L. virgatum)
Flower stalks of the perennial dubbed the “Purple Plague” can reach up to 10 feet in ideal conditions. Mature plants sprout 30 to 50 stems that flower from June to September and can produce two million to three million seeds a year. Underground stems extend about a foot a year to form new plants. Invading any wetland, from tidal and non-tidal marshes to bay, pond, river edge, and ditch, it has monopolized millions of acres, forcing out diverse native plants critical to wildlife nutrition and habitat, and threatening some plants with extinction. Plants sold as sterile and apparently safe to use in the Chesapeake watershed are not. Insects introduced for biological control are showing results in the Midwest. P/D, F/S, PostE

Spotted knapweed/yellow star thistle (Centaurea. maculosa, C. biebersteinii)
These asters originated in Eurasia and Africa. With strong taproots and abundant seed, they invade sunny fields, meadows, or wood edges and form dense communities that out-compete natives. Yellow star thistle is not extensive in Maryland yet. It causes “chewing disease” in horses that ingest it, attacking the nervous system and usually causing death. It likes summer droughts. Livestock should not be grazed in infested areas. Use certified weed-free hay. Clean seed from clothes, shoes, and tires before leaving infested areas. Insects, i.e., two seed-head attacking flies, have been introduced elsewhere to provide biological control for both plants. P/D, PostE, C/M/reC (below leaf area)

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