Crop & Pest Report


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Weeds Germinate and Emerge Temporally During Spring and Summer (05/10/18)

nnual weeds germinate in the spring, grow during the summer and set seeds in the fall. Sounds basic, doesn’t it?

Weeds Germinate and Emerge Temporally During Spring and Summer

Annual weeds germinate in the spring, grow during the summer and set seeds in the fall. Sounds basic, doesn’t it? Unfortunately, that is where the simplicity ends. Weed management would be a straight forward task if all weeds germinated and emerged at the same time and weeds probably would not be an annual challenge if we could take this approach. Instead, weeds germinate at various times, allowing them to escape control, produce seed and contribute to seed banks that may persist for multiple years.

Weed emergence can be predicted by tracking air or soil temperature and by calculating accumulated growing degree days (GDDs) using base temperature (Tbase = 48 F) staring on January 1 of each year. Accumulated GDDs suggest weeds germinate in clusters including early-emerging, middle-emerging and late-emerging species. The earliest emergers including kochia, common lambsquarters, and common and giant ragweed emerge at <150 GDD. Research indicates kochia will emerge with as few as 5 accumulated GDD. Middle-emerging species include foxtail species, Venice mallow and common sunflower and emerge at between 150 and 300 GDD. Late emerging-weeds include redroot pigweed and waterhemp and emerge at > 300 GDDs.

Knowing when weeds begin to emerge can direct scouting activities and improve overall weed management strategies. For example, early emergers, especially those with short emergence duration, can be managed after most seedlings have emerged using postemergence herbicides or with tillage prior to planting. In comparison, middle and late emergers may need to be controlled with preemergence or postemergence herbicides. Some weeds like lambsquarters, redroot pigweed or waterhemp have extended emergence patterns and may require multiple control strategies including layer application of soil-residual herbicides. NDSU research indicates cultural practices, such as use nurse crops appear to delay emergence and suppress development of certain middle and late emerging weeds.

The emergence sequence of different weeds is consistent from year to year although the initial emergence date for weeds varies from year to year. Weeds emerge over a prolonged time period, so weeds from earlier clusters may still be emerging when a later cluster begin to emerge. Modeling temperature is a reasonable way to predict weed emergence. However, emergence is influenced by several other factors than air temperature, including cloud cover, soil type and moisture, and crop residue.


Tom Peters

 Extension Sugarbeet Agronomist

NDSU & U of MN


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