What the …??  Where’s this coming from and what does that have to do with weeds?

A few years ago, I noticed that when I was wearing my cool polarized sunglasses it was like I had a superpower for spotting smooth crabgrass in turf.  I haven’t done a lot of research on this but it appears that the light reflected (or absorbed, defending on your figurative point of view) makes crabgrass pop out.  At least it does in tall fescue. I haven’t had a chance to do much other observational work with other turf species.  At any rate, with a little practice, I found that could pick out a single crabgrass seedling in an area of tall fescue from 10 feet away.  This is slightly similar to how automated weed spot sprayers work except their sensors only detect green (the weed) vs brown (bare ground). At least for now it looks like humans are smarter than machines!

The benefit of this is that if a turf manager wants to monitor for crabgrass early on, it may be possible to check during bright days using polarized sunglasses. Be sure they are truly polarized, not just dark or color-changing.

Because crabgrass is a summer annual that produces numerous seed quickly, the sooner seedlings or small patches are removed, the better the chances are for long-term control.  After the crabgrass is found, it can either be hand-pulled, spot-sprayed, or if a number of seedlings are found in an area, do a larger spot spray.  You can also consider using a preemergent too or something like dithiopyr, which has preemergent activity, but also will has some activity on very young crabgrass.  Remember, weeds are much easier to control when young than when mature and you definitely want to control them before they produce seeds.

For more information about crabgrass, see the Pest Note on crabgrass at http://ipm.ucdavis.edu/PMG/PESTNOTES/pn7456.html and the Weed Photo Gallery: http://ipm.ucdavis.edu/PMG/WEEDS/smooth_crabgrass.html

For more about weed control in commercial turf see the UC IPM Pest Management Guidelines for Turfgrass at

http://ipm.ucdavis.edu/PMG/selectnewpest.turfgrass.html

Try it – I’d be interested in knowing what your experiences are.

Smooth crabgrass seedling