When to Apply Preemergent Herbicides for Turfgrasses

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When to Apply Preemergent Herbicides for Turfgrass in North Carolina

It is well documented that our winter temperatures are rising in NC, as well as the rest of the US. As winter temperatures rise, so do soil temperatures. As a reminder, crabgrass will germinate when 24-hour mean soil temperatures average approximately 55 degrees for 3 or 4 consecutive days.

Starting about 2020, I have recommended that preemergent herbicides be applied earlier because crabgrass is germinating earlier. In 4 of the last 5 years, I have found crabgrass germination in NC in February. In 2023, I found crabgrass germination in February, 2023 in Wake County (Raleigh).

Lets take a look at where we are in 2024. The two charts below show 24-hour average soil temperatures at our Turfgrass Field Lab in Raleigh. Note that as of today, the soil temperatures are 41 degrees. This is well below where crabgrass will germinate. However, note that in late January, soil temperatures were above 55 degrees for 3 days. I do not have any evidence that crabgrass germinated during this time. However, it is worth noting that soil temperatures can rise rapidly when air temperatures rise as they likely will in February.

 The graph shows the variation in average soil temperature at a 10 cm depth recorded by the Lake Wheeler Rd Field Lab over a period from January 8 to February 5, with temperatures fluctuating between approximately 35°F and 60°F.

Another useful tool is to examine growing degree days (GDD). This can be useful to see how our current weather conditions compare with last year and a 30-year average. Note that currently, our GDD are above the 30-year average but below last year.

The graph tracks accumulated growing degree days (GDD) over time, comparing data from 2023, a 30-year average, and a forecast for 2024, indicating how much heat accumulation (a measure used to predict plant development rates) has occurred compared to historical averages.

So what does all of this mean? It appears the winter of 2024 is tracking similarly to the past several years. In summary, we are ahead of the 30 year average for GDD. Also, soil temperatures can rise quickly with rising air temperatures. Therefore, it is my recommendation that preemergent herbicides should be applied earlier due to the danger of early crabgrass germination. This is consistent with similar recommendations over the last few years. My recommendation is preemergent herbicides be applied in central NC by the end of February. East of Raleigh, application should be made earlier. Mid-February would be a good target date for that are of NC. As you move west in the state, applications can be made slightly later but I would suggest very early March would be a good target date in the central and western Piedmont regrion of the state. Of course, higher elevations in the mountains would be later.