Cheatgrass seedling. (Photo: Colorado State University Extension).

While this may not seem to be the best time of year for weed management, it is a good time to start dealing with winter annual and biennial weeds and planning your strategy for the 2021 growing season.

Winter annual weeds can germinate in the late summer, fall or winter, over winter and start actively growing again in the spring. Winter annual weeds finish growing and produce seed by early summer. Biennial weeds start growing in the summer or fall, over winter and produce seeds the following summer. Since both weed types only reproduce by seed, keeping them from going to seed is the key management method.

For all weed types, the best management approach to follow is Integrated Pest Management. This consists of the following methods – cultural (keeping desired vegetation healthy and not introducing weed seeds via contaminated seed or on vehicles or clothing – seeds caught in your socks), mechanical (hoeing, hand pulling, mowing), biological (livestock, insects, fungi and bacteria) and chemical (organic and synthetic herbicides). Utilizing all of these methods is more successful than just relying on one method.

Some biennial and winter annual weeds are already growing so you can manage them over the winter. Depending on how many you have, you can be out when the weather is good, temperatures are warm and the soil is not frozen, undercutting the rosettes or hoeing young seedlings. Biennial thistles, knapweeds and other biennials are easily managed by removing the rosette by undercutting it (cutting the root below ground and removing the above ground plant) before it has a chance to flower and produce seed next year. Winter annuals such as prickly lettuce, redstem filaree, cheatgrass and mustards (flixweed, blue mustard and tumble mustard) can be hoed or pulled when the soil is not frozen.

Herbicide use is limited in the winter. Read the herbicide label for the required application temperature and weather conditions. On warm winter days, you may be able to use an organic herbicide, those based off acetic or other acids, ammonium nonanoate or citrus oils. The organic herbicides are non-selective meaning if you have desired plants with green leaves on them next to your weed, you need to be careful to not spray plants other than the weed. If the plant is a perennial and is well established, it will likely suffer some burned leaves, but recover in the spring. To prevent over spraying, you can cover the desired plant with a bucket, plastic or cardboard or cut the bottom out of a bucket and use it to go over the weed and then spray the weed while it is contained in the bucket. You need to be as careful with an organic herbicide as with any other herbicide. They are all meant to kill or damage something.

Even if you don’t do any mechanical or chemical management over winter, you can at a minimum be identifying what weeds you have, where they are, how many there are and planning what methods you can use to manage them (mechanical, biological or chemical).

For perennial weeds, there are limited options that you can do in the winter. Perennials are not actively growing at this time so there’s not much you can do other than plan and perhaps line up a contractor. You can identify what you have and what management methods you can use at appropriate times in 2021. If you need help with identifying what weeds you have or what management methods are available, please contact the Extension Office. While dead plant identification is tough, it can be done on some weeds/plants.

By Sharon Bokan, Colorado State University Extension Boulder County. Sharon is the Small Acreage Coordinator at Colorado State University Extension Boulder County. For more information call 303.678.6176, e-mail sbokan@bouldercounty.org or visit ext.colostate.edu/boulder.