Cold Soils and Weeds

Kat Caswell, Washington and Weld Agronomist

The spring season of 2023 is off to a chilly start. Low temperatures and late snows in the spring have kept soil temperatures in northern Colorado lower than we’ve experienced the last few planting seasons. The cooler temperatures are having a noticeable effect on what weeds have started to emerge in the northeast part of Colorado.

Most weed species have a specific time window in which they emerge each year. Kochia is one of the earliest weeds, emerging in late March to early April. By the middle of May, 90% of the kochia will have emerged for that year. As of late April, kochia around the region was only beginning to emerge and form seedling mats. Cheatgrass, another early emerging weed, has been behind the kochia emergence this spring. Weed seed germination is heavily influenced by soil temperature and soil moisture. While soil temperatures remain lower, summer annual weed seed germination will be delayed compared to the normal emergence window.

Despite being delayed, some of the early emerging weed species will still have time to germinate and become a challenge to manage. For example, palmer amaranth can emerge, flower, and produce viable seed through the entire growing season. Weed control options will to be adjusted to match weed emergence in years such as this. Soil applied herbicides are only able to kill weeds that are actively growing. If a soil applied herbicide with a three-week residual activity is applied but weed seeds do not germinate until the last week the residual is effective, there will be a population of weeds that escape control. An additional herbicide application will need to be done later, allowing the crop to establish during the weed free critical period, but adding an additional expense for equipment and chemicals.

In years with varying weather conditions, active scouting and adaptive management will be a necessity for maintaining weed control. Palmer amaranth was identified to have resistance to six different sites of action in March of 2023 in Kansas. Due to the limited efficacy of herbicides during hot and dry conditions, accurate timing of herbicide applications will be both challenging and crucial.