The traditional question for spring lawn management – when is the best time to put on a preemergence herbicide? Also called “crabgrass preventer”, timing is important, but is not the only consideration. In addition to timing, there are at least 5 others.
Target temperature – Since the weeds that are targeted with these products are summer annuals such as foxtail, prostrate spurge and crabgrass, it’s recommended that they are applied just before the soil temperature conditions that encourage germination are conducive. For these weeds, it’s when the soil temperature is 55 degrees F for 3 days in a row. As of today, April 9, 2020, the soil temperature at the 4 inch depth is 49 degrees F. At the 1-2 inch depth level, it’s usually a few degrees higher. Considering these data, we’re close, but not quite there yet. If weather predictions are accurate for the coming 6-7 days, it’s reasonable to expect a slow, slight decrease in soil temperature, perhaps to about 45 – 48 degrees or so.
Determining soil temperature – Up to the minute soil temperature and other weather data can be obtained at http://www.mgextensionwx.com/.
Other Weeds – Goosegrass, sandbur and windmill grass require a slightly higher temperature to germinate, in the 60 to 65 degree range. If these are the target weeds, consider delaying the application until soil temperatures are more conducive.
Part of the landscape – Every part of the landscape is different; soil temperatures are likely to be slightly higher in the full sun exposures, especially in thinner parts of the lawn and cooler in the shadier, thicker stand locations.
Thickness of the turf – Newer, thinner lawn areas usually have less density, and allow greater sunlight exposure, which encourages greater temperatures and more germination. If the lawn is older, mature and uniformly dense, consider not applying a preemergence product, especially if the weed population has been minimal.
Split application – Recent UNL research has indicated that applying half of the recommended rate in mid-April, followed by the other half in early June provides slightly better results than one application in spring.
Also has fertilizer – Most, if not all crabgrass preventer products available to homeowners also include a full rate of fertilizer. The rationale for this type of formulation is that it is more convenient and time saving for the applicator. The trouble with this approach is that more fertilizer gets applied than is recommended by UNL research. The most desirable situation would be for manufacturers to produce 2 separate products – preemergence herbicide and fertilizer – and provide more flexibility for the consumer, but at this time, that isn’t the case.
Lawn care company already put it down? If you or your lawn care company have already put the application on, is it “too early” or a waste? No. It will still work well. The only real down side to this earlier application is that the product may stop working a week or two before mid to late April applications. Lawn care companies have more options available to them than homeowners and can adjust their applications to apply only preemergence herbicides or a full or split rate of herbicide and a light rate of fertilizer.
All in all, the decision to apply, not apply, apply half now, half later or to hire a lawn care company to apply it is not an easy one. If you have questions, please send me an email – jfech1@unl.edu – or call 402 444 7804.
