Written by Dave Gardner, Ph. D., The Ohio State University
This year has been a particularly difficult one for turfgrass managers who have problems with warm season weeds such as crabgrass (Digitaria) and nutsedge (Cyperus). At the Ohio Turfgrass Foundation Research and Education Center our crabgrass research area has been completely overrun by crabgrass, regardless of product or application timing that has been tested. Many individuals have observed that preemergence herbicides that are traditionally very reliable are not providing control that we have seen historically. Additionally, postemergence herbicides have been relatively less effective this year because crabgrass continued to germinate later into the season and the new seedlings replaced any plants that were controlled.
Much of what we are observing with weed control issues this year may be attributable to the weather patterns we have observed this growing season. April was the coolest of the last 25 years in Columbus, with an average monthly temperature of 47.7 degrees. By comparison, this is cooler than the months of March 2012 and March 2016. Because of this unusually cool weather to start the season, cool season turfgrasses, such as Kentucky bluegrass, did not have favorable conditions in which to begin growing.
On May 1 the high temperature in Columbus was 81 degrees. What then followed was the warmest May on record. The average temperature was 71.5 degrees, 9.1 degrees above normal. In 5 years since the turn of the century, the month of June has been cooler than what we observed in the month of May.
In a typical year cool season turfgrass has the months of April and May to break from winter dormancy and begin growing. The denser the turf, the fewer problems we tend to have with annual weeds such as crabgrass because dense turf shades the soil so weed seeds are not as likely to germinate. Denser turf is also more competitive with crabgrass seedlings. Crabgrass typically germinates in April but does not really begin growing aggressively until the second week of June. In 2018 we had less than ideal conditions for cool season turfgrass growth in April followed immediately by conditions that were more favorable to crabgrass and other warm season grasses. Additionally, precipitation has been at or above normal and very timely.
Because of the extended warm and wet conditions, preemergence herbicides that would normally provide near season long control likely began to break down before crabgrass stopped germinating in July. This combined with the less than competitive cool season grasses at the start of May and an extended season favoring warm season grass germination and growth has resulted in the crabgrass problems we are seeing now.
On a related note, this has been an exceptional year for sedges. They are perennial species that are favored by warm and wet conditions. Again because of the weather we have observed and the lack of cool season grass competition, nutsedge has been more prevalent this year.This deviation in weather patterns will, of course, hopefully not be observed in 2019. No changes in your current herbicide program are probably necessary or warranted at this point except to make sure that you are using recommended products. Some key activities for this fall will be to map out areas with crabgrass so that preemergence herbicides can be applied next spring. Also, a good fertility and broadleaf weed control program this autumn in order to try to recover as much density in the cool season turfgrass will hopefully help with weed control next season.