CO-Horts Blog

Wednesday, September 24, 2014

I Think I Might have Crabgrass in my Lawn?

Alison O'Connor, CSU horticulture agent in Larimer County

Clumpy, fast-growing tall fescue is often incorrectly called
"crabgrass" by home gardeners

It seems that, whenever a grass growing in a lawn doesn’t look quite “right”, it is labeled as “crabgrass”. In the spring we receive calls and emails about ugly, quick-growing clumps of grass that home gardeners suspect to be crabgrass. It never is. Crabgrass doesn’t even think of germinating until late April or May, since it is a warm-season annual. But people will insist that crabgrass is making a mess of things in their lawns in March and April. They are usually seeing tall fescue, bromegrass and quackgrass – perennial, wide-bladed, cool-season weedy invaders of bluegrass lawns that green up early in the spring. Ironically, people rarely recognize true crabgrass in their lawns when it really does begin growing in July and August. We wrote about crabgrass - the real stuff, and its look-alikes - back in July.

Close-up of tall fescue - often mistaken for crabgrass
In the past few weeks, the cool-season grasses (including the clumpy, ugly ones) are perking up after the heat of August and early September - and home owners are once again seeing “crabgrass” in their lawns. The problem with that is the REAL crabgrass has recently been stunted or killed by cool nights and frost – and is no longer growing in lawns. However, tall fescue, bromegrass and quackgrass are growing happily in response to cool fall temperatures and rain - and people, as in the spring, believe they are seeing "crabgrass" in their lawns.

Crabgrass killed by frost
Even if you have some green, living crabgrass (the real stuff) in your lawn that wasn't hit by that frost on the morning of September 13, it will eventually be killed by frost very soon - so it would be irresponsible (and ineffective) to apply a crabgrass herbicide at this point in the year. But, it’s another story for those nasty patches of quackgrass, brome, or tall fescue. Now is an ideal time to apply glyphosate (aka Roundup…and other brand names of glyphosate products) to control these cool-season perennial grasses. Just remember that glyphosate is non-selective, so anything that it contacts – including bluegrass – will be harmed or killed. Apply glyphosate carefully to patches or clumps of these perennial grasses; apply in the morning when it is calm, and avoid walking where you have sprayed (glyphosate can be tracked onto desirable grass). Wait about 7 days and reapply to control any grasses that were missed or not killed with the first application. When the area is dead, rough up the dead grass with a rake – or use a “foot aerator” to punch holes (the more, the better) in the dead turf. Seed with the appropriate grass (bluegrass or ryegrass into a bluegrass lawn; tall fescue into a tall fescue lawn) and rake lightly to work the seed into the holes. There is no need to topdress with soil or compost. Simply water the spots to keep them moist and you will have new grass growing in the killed area in a few weeks.

So, you might have crabgrass in your lawn - but if it isn't already dead, the next hard frost will kill it. Any ugly, green, happily growing grass you see this in a lawn this time of the year is most likely tall fescue, bromegrass or quackgrass - and it's a good time to control it if you don't want it there next year.


  1. I would love to have printable versions of many of these for the board in the lunch room:)

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