Biennial weeds, such as this musk thistle, are easier to control in the fall.

Fall weed control: Don’t wait for biennials to flower in the spring

The issue: As you drive around Bonneville County, the skeletons of dead musk thistle plants are a common site. Like musk thistle, many of our weeds have a biennial life cycle, meaning it typically takes two growing seasons to complete their life cycle. The first year they germinate and form a rosette, which is a low-growing mat of leaves. The rosette tolerates cold winter temperatures. The following spring or summer they send up a flower stalk. Some biennial plants flower in early spring, while others flower a little later in the season. Regardless, once the plant produces seeds, it dies. Biennials only reproduce by seed, so the key to good control is to keep it from going to seed.

Integrated Control Options:

· Mechanical: Tilling, digging, or hoeing biennial rosettes in the fall works very well. Be sure to cut the root off a couple of inches below the rosette.

· Cultural: Competition from desirable plants can help reduce the ability of many biennial plants to get established.

· Biological: Biological control options are geared for specific plants or families of plants. Specific biological controls are better talked about for individual plants.

· Chemical: Fall is the best time to spray biennial rosettes. Spraying in the spring may not guarantee the plants don’t produce seed while the plant is dying. Be sure the target plant is listed on the label of your herbicide.

Combine them: Fall weed control sure makes spring gardening a lot nicer. Biennial weed control should be in our minds throughout the year, but especially in the fall. Combine different control options to help improve the effectiveness of your efforts. Always read and follow herbicide label directions.

For more information, contact Ron Patterson, University of Idaho Extension horticulture/agriculture educator in Bonneville County.