Goosegrass is a summer annual weed of cool-season turfgrass that has become more problematic in recent years. My conversations with turfgrass managers suggest that goosegrass is firmly entrenched as a top 5 turfgrass weed issue of highly managed systems in the Northeast region. Crabgrass is more prevalent than goosegrass, but is less problematic in highly managed systems because it is effectively controlled with pre-emergence herbicide programs.
Goosegrass and crabgrass are both summer annual grassy weeds. So why do pre-emergence herbicides typically used in cool-season turfgrass (dithiopyr, prodiamine, and pendimethalin) often provide inconsistent goosegrass control? It should be noted that the pre-emergence herbicide oxadiazon often provides excellent goosegrass control but is not widely used in cool-season turfgrass.
One reason is that goosegrass seedlings are thought to emerge 2 to 3 weeks later than crabgrass and continue germinating later in the summer. Indeed, the research we conducted this summer to evaluate seasonal goosegrass germination patterns found that the first goosegrass seedlings emerged in mid May, about 3-4 weeks after the first crabgrass seedlings. Interestingly, when we calculated the time it took 90% of the plants to emerge, there was a big difference between our two locations. Ninety percent of the goosegrass seedlings had emerged by early July at the golf course location, but at the research farm location just a few miles away (soil temperatures were similar) 90% emergence occurred in mid to late August. But I digress…
If goosegrass germinates later than crabgrass, making split applications of pre-emergence herbicides should provide more consistent control than single applications. However, my conversations with many in the turfgrass industry suggest is that goosegrass has become more problematic only in the last 5 to 15 years regardless of application strategy and mitotic-inhibiting pre-emergence herbicides have been used widely for over 25 years. So this would suggest some other factors are in play.
On the same golf course where we found 90% of the total goosegrass seedlings had emerged by early July, we evaluated several pre-emergence herbicide programs for goosegrass control in 2017. Treatments were applied in early May. By mid June we noticed that plots treated mitotic-inhibiting herbicides had as much goosegrass as non-treated plots. Meanwhile, plots treated with oxadiazon (a different mode of action) had almost no goosegrass. For reasons mentioned earlier, I would not expect these mitotic-inhibiting herbicides to provide excellent goosegrass control, but they should be providing some control, especially in mid-June.
We collected seed from the site in December 2016 and we compared this seed to a population collected from our turfgrass research farm using a controlled experiment in the greenhouse. We found that the golf course population was not controlled by a mitotic-inhibiting herbicide, but the research farm population was 100% controlled (figure below). Both populations were 100% controlled by an herbicide with a different mode of action.
In the Southeast, herbicide resistance to mitotic-inhibiting pre-emergence herbicides (dithiopyr, prodiamine, and pendimethalin) and oxadiazon has been implicated as the cause of goosegrass issues on many golf courses and was first reported over 30 years ago in 1984 (by Mudge et al.). However, I’m not aware of resistance to mitotic-inhibiting herbicides in the Northeast despite heavy reliance on these herbicides for annual grassy weed control.
Goosegrass resistance to pre-emergence herbicides typically begins with one plant surviving the application due to a genetic mutation. The goosegrass plants with the mutation probably goes unnoticed and sets seed, which leads to a small patch of plants the next year when the same pre-emergence herbicide is used again and does not control plants with the mutation. The patch may become larger each year as that population is spread throughout the property via seed eventually becoming problematic.
At the recent NJ Green Expo in Atlantic City in December, I briefly discussed the topic of goosegrass resistance to pre-emergence herbicides during a panel discussion. Responses suggested this is an issue that deserves more investigation.
If you suspect that resistance to pre-emergence herbicides is an issue on your property, please contact me via email (firstname.lastname@example.org) or Twitter (@RUturfweeds). Your will be extremely helpful to help determine if this is a widespread issue and potentially collect plants/seed samples for further testing.. All correspondence will be kept confidential.