As early as 1895, Minnesota has had laws to prevent invasive weeds from taking over our yards, parks and forests. In 2009, an advisory board was put together to help the Minnesota Department of Agriculture update the list of noxious weeds and devise ways to categorize them. The goal is to stop the spread of these aggressive weeds that threaten our wildlife, native plants and, in some cases, even our health. I asked the advisory board which weeds were the worst of the worst. They came back with a list that included weeds we've known -- and hated -- for years, and weeds that are just starting to appear in the state. Some are so new to Minnesota that the Ag Department is still trying to figure out how widespread they are and what kind of damage they may do. Read it and weed:

1. Wild parsnip

This perennial weed has many small yellow flowers that appear in a flat cluster at the end of a flower stalk that looks a bit like Queen Anne's lace. It's easiest to identify when its flowers start appearing, usually around June.

This invader from Europe and Asia has gotten a strong foothold in Minnesota, growing in fields, prairies and along roadsides. In addition to displacing native plants, it can cause burns if the sap comes in contact with your skin.

2. Yellow starthistle

An annual with stems covered in distinct woolly hair, starthistle is topped by yellow flowers that look like spiky dandelions. It thrives in sunny locations, such as fields and grasslands, and typically grows to 1 to 2 feet tall.

Although it hasn't been found in Minnesota yet, it's expected to displace many of our native plants if it arrives. If you see this plant, report it to the Agriculture Department's Arrest the Pest hot line, 1-888-545-6684.

3. Japanese knotweed

A shrub-like perennial, knotweed has hollow stems and small white flowers that bloom in clusters in August and September.

This tall invasive (it can reach 6 to 8 feet in height) is at home in sunny areas and along riverbanks. It can shade out native vegetation and invade the habitats of local fish and wildlife.

4. Oriental bittersweet

Similar to American bittersweet, Oriental bittersweet is a vine that climbs trees and other structures. It's easiest to identify in the fall, when yellow fruit capsules cover the stem.

It was first reported in Minnesota in 2010. Gardeners are encouraged to help stop it from getting established here.

5. Narrowleaf bittercress

This low-growing weed lives for two years. The first year it has larger leaves shaped like a catcher's mitt. The second year it produces narrow leaves and four-petaled white flowers. A single plant can produce up to 5,500 seeds.

First reported in Minnesota in 2008, this European import displaces native plants in forests and is beginning to invade rivers and forestland near the St. Croix and Mississippi rivers.

For more information on these and other noxious or potentially noxious weeds and other pests, go to

Jeff Gillman, an associate professor of horticulture at the University of Minnesota, has written several gardening books.