Your Fall Weed Challenge: Winter Annuals

Your weed problems for next spring may already be starting in your fields. When soils are moist in the late summer and fall, the seeds of winter annual weeds are signaled to come out of dormancy and germinate.

What are winter annual weeds? They are weeds with a growth cycle that begins in the fall. They grow from seed into a tiny rosette that lays flat on the soil. During the fall, they grow a small taproot and a few leaves before becoming dormant for the winter. Some stay green under the snow, ready for springtime.

When spring brings warm temperatures, winter annuals will bolt a flower stalk early in the season. They flower and set seed in early summer, often shedding their seeds before the heat of the summer. These seeds are now lying on or below the soil surface waiting for fall rains to germinate.

Some weeds can be both summer annuals and winter annuals, like common chickweed and groundsel.

Why should we be concerned about winter annuals? Winter annual weeds are very successful, often causing problems each spring and throughout the early summer. Their growth cycle is ideally suited to our crops for several reasons. Their growth periods coincide with periods of good soil moisture. They are growing when crops are not using moisture or nutrients. Their small size at the rosette stage makes them inconspicuous, and they may be less susceptible to traditional weed control tools and timings. And they often shed large numbers of seeds to help them survive the summer.

Winter annual weeds do well in perennial cropping systems, such as strawberries, asparagus, nurseries, vineyards and orchards. They are also successful where reduced tillage is practiced in the fall or the spring. Recently we have noticed winter annual weeds like Canada fleabane, garlic mustard, buttercups, shepherd's purse and pepper grass becoming more of a problem.

How can we control winter annuals? Early fall gives us a window to prevent the establishment of winter annuals. In strawberries, apply Sinbar, Dacthal or Devrinol around Labour Day. In early harvested crops, use light tillage or glyphosate burndowns. In orchards, apply postharvest applications of 2,4-D. Mow field edges before seeds shed to prevent winter annuals from moving into your fields. Scout your fields several times throughout the fall so to know what to expect next spring.

| Top of Page |