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Weed Biology Part IV: Summer Annuals vs. Winter Annuals

Summer Annuals vs. Winter Annuals

Last week we talked about the different weed life cycles. Annuals are the most common type of weed we see in turf and ornamental weed management. But in addition to being able to identify a weed as an annual, biennial, or perennial, we need to know if an annual weed is a summer annual or a winter annual. Why? Because pre-emergent herbicides only work on seeds that are newly germinated and haven’t emerged from the soil yet. Once a seedling is able to reach the soil surface and start the process of photosynthesis it will no longer be controlled by a pre-emergent herbicide. Understanding the difference between summer annuals and winter annuals will help you determine when to apply pre-emergent herbicides to your turf and ornamental areas.

prostrate spurge weed

Summer Annuals

Summer annuals are the weeds that are most prolific during the summertime. Their development follows the calendar year. They’ll germinate in the spring, flower during the summer, set seed in the fall, and then die in the winter. The seeds will overwinter on the soil surface and then germinate in the spring starting the whole cycle over again. Pre-emergent will need to be applied fairly early in the spring in order to be effective against summer annual weeds. Prostrate spurge (pictured) and common lambsquarters are both summer annuals.

henbit weed

Winter Annuals

Winter annuals on the other hand, split the calendar year. Winter annuals will germinate in the late summer or early fall. Usually after the hottest weather has passed. Winter annuals will overwinter in a vegetative phase and be present in the spring when the air and soil warms. They’ll flower in the early spring, set seed in early summer, and then die in the hottest part of the summer. Once the temperatures start to cool in late summer the new seeds will germinate, and the cycle will start over again. Another application of pre-emergent herbicides may be necessary in the summer months if winter annuals are prolific. Henbit (pictured) and annual bluegrass are both winter annuals.

This installment marks the end of our weed biology series. I hope you’ve enjoyed it. Check out these courses if you’d like more information on weed biology or weed management in turf and ornamental settings.




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