Watch out for poison hemlock and other noxious weeds
As we come into the summer gardening season, some plants fall into the “not wanted” category. Do we despise weeds because they are ugly, or that they dare invade our perfectly planned gardens?
But there are some plants that are so aggressive and persistent they should be removed for the health of the environment. The Pennsylvania State Department of Agriculture has researched and determined noxious weeds that are widely established in Pennsylvania. While most can’t feasibly be eliminated, they can be controlled.
Poison hemlock leads the list and is increasingly common in our region. It is blooming now and is a dangerous plant that requires extreme caution when eradicating it. Worst of all, it is closely resembles Queen Anne’s lace ( Daucus carota), a trendy wildflower for indoor arrangements. It is imperative that you distinguish between the two if you see a plant with lacy leaves and white umbels. If you’re not sure, leave it alone and source Queen Anne’s lace from a florist.
Poison hemlock ( Conium maculatum) is the plant used to kill Socrates. It is a biennial. First-year plants are flat clumps of feathery leaves; in its second year, it stands tall, between 3 and 7 feet, and is well branched. The stems are hairless with purple steaks or spots. The flat lacy flowers are used by pollinators and beneficial wasps. Poison hemlock is so toxic that even a small amount ingested or absorbed from the eyes or cuts in the skin can be fatal.
If you identify poison hemlock on your property, it is best to call a landscape professional licensed in commercial pesticide application to eradicate it. However, if you decide to tackle the job yourself, wear proCarol tective clothing, gloves and eyewear. Be
sure to shower afterward and launder your clothes. Further information on the plant can be found at extension.psu.edu.
Here are some other noxious weeds to watch for:
• Garlic mustard ( Alliaria petiolate) pops up in early spring and is a wildly invasive biennial with scalloped leaves alternating on a smooth stem, topped by a cluster of small white fourpetaled flowers. The first year it produces a 4-inch basal rosette. The second year it flowers and sets seeds. The 2-inch-long seed pods explode and scatter small black seeds. Most animals, including deer, won’t eat the plant because of the taste.
If you are unsure what it is, pick a leaf and crush it. It will smell like garlic with a base note of broccoli. Its one redeeming quality is that it is edible. It can be added to salads or substituted for garlic in pesto recipes.
Garlic mustard produces chemicals that suppress the growth of mycorrhizal fungi that live on the roots of other plants. The chemical also suppresses the growth of nearby native wildflowers, disrupting the ecological niche of those desirable plants.
The preferred method of control is to dig it out manually. Plants do not regenerate from root fragments. Do this before the seeds ripen. Previous years’ seeds can stay viable in the ground for up to seven years, so eradicating garlic mustard is a long-term but doable project.
• Canada thistle ( Cirsium arvense) was one of the first plants to be placed on the noxious list. Despite having
Canada in its name, this thistle is a European plant. Canada thistle, a perennial that emerges in spring, is a prickly plant with late summer purple flowers. After flowering, fluffy white seedheads form.
Control requires persistence. Wear gloves and pull every stalk you see. Do this every week or as soon as you see emerging leaves. The root will grow back from any missed fragment, but continual pulling will starve the plant. Every time you pull the top, the root has to expend energy to grow a new shoot. Eventually it will die. Once you have made a dent in the colony, a pre-emergent herbicide can be used to prevent seeds from germinating.
• Mile-a-minute vine ( Persicaria perfoliate) is a fastgrowing annual vine with triangular leaves that are barbed on the underside.
Vines produce green fruits that turn metallic blue. Hand-pull vines, wearing gloves, and dispose in the trash if plants have berries. For large tangled masses, herbicides are effective. To enhance their performance use a horticultural surfactant to penetrate the leaf waxy coating and spray plants in summer before fruiting.
For more on these plants and other noxious weeds found in our state, go to www.agriculture.pa.gov.
Dianne Machesney is a Penn State Master Gardener. This volunteer program supports the outreach mission of Penn State Extension and provides research-based information on best practices in sustainable horticulture and environmental stewardship. Information: alleghenymg@psu.edu or 412-4823476.