No. 4 Article 10/April 15, 2005

Predicting Weed Emergence

Knowing when weed species begin to emerge can vastly improve your weed management program if you practice timely scouting and subsequent control tactics. Weeds can be unpredictable; the time of their emergence can vary from year to year, and we cannot track their movement via traps like we do with insects. The duration of weed emergence can also add to the problem of timely management. Some weeds, such as woolly cupgrass, emerge over a short period (2 to 3 weeks), while others, like waterhemp, emerge over 3 to 5 months.

Entomologists have used growing degree-days (GDDs) to predict insect presence and development for quite some time (see "Scouting for alfalfa Weevils" in this issue). We can use GDDs (base 48) information as an estimate of heat units required to reach 10% of the total emergence for a weed species. Heat units are not the only factor that influences weed emergence; others include soil type, soil moisture, crop residue, and nitrogen.

To help in determining when to scout and what weeds to scout for, along with the implementation of control tactics, midwestern weed scientists developed a factsheet to provide some direction related to timing and duration of weed emergence. It can be downloaded as a PDF file from the Web, or you may contact us if you would like to order an 8.5" x 11" or 24" x 36" full-color poster.--Dawn Nordby and Aaron Hager

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