Identifying Unwanted Lawn Grass and How to Eliminate - Ask Extension
Hi, thanks for reading my question.
I have a large lawn that has developed a (grass?) in patches that I want to get rid of. I am guessing it's crab ...
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Identifying Unwanted Lawn Grass and How to Eliminate #811264
Asked September 18, 2022, 3:36 PM EDT
Hi, thanks for reading my question.
I have a large lawn that has developed a (grass?) in patches that I want to get rid of. I am guessing it's crab grass in layman's terms, but I am not sure; see attached photos.
I have sprayed several weed killers on this grass, first Weed B Gone, then Roundup for Lawns (3.85% MCPA, dimethylamine salt, 1.80% Quinclorac, 0.43% Dicamba, dimethylamine salt, 0.22% Sulfentra) which indicates it is more specific for crabgrass, and last week I tried a product with 18% Quinclorac. So far the offending grass seems to only be slightly less green.
My questions; 1) am I correctly identifying this grass; 2) am I using the right products to eliminate this? 3) am I perhaps being impatient and need to wait for the products to work?
Thanks very much;
Floyd Frick .... OSU Engrg Class of '77
Clackamas County Oregon
Expert Response
Hello,
It looks like the grass is tall fescue, a cool season grass. None of the products you are spraying are for cool season grasses. The products you sprayed are designed to be safe on cool season grasses (assuming you spray at the correct rates) but will kill broadleaf weeds. These formulations allow you to spray over your whole lawn and it kills only the broadleaf weeds but does not kill your lawn. Quinclorac does have activity on crabgrass, which is a warm season grass, but that is not what you have growing in your lawn.
If you want to kill tall fescue, you need to spot spray spray glyphosate (e.g., Roundup) at a 3% solution using a 4 lbs. per gallon glyphosate concentrated formulation. If you buy a ready to spray product, follow the instructions, which simply means it is pre-mixed and you spray it straight out of the bottle. You might have to make two applications as tall fescue can be slow to die.
You need to understand that Glyphosate is a non-selective herbicide meaning it will kill all plants. If you use it on your tall fescue, you will kill some grass around it as it is impossible to spray glyphosate and only get it on the tall fescue. As a result, you will need to re-seed after the tall fescue dies.
In general, you don't get good weed kill with herbicides if the plants are under drought stress. However, in this situation, it might be worth trying to spray now because you might get selective control meaning you might not kill part of your lawn that is next to the tall fescue because it is dormant now. If that doesn't work, I would fertilize your lawn and wait for it to green up with the fall rains. When it gets green and healthy, I would spray the tall fescue again. It will certainly die then. Just keep in mind that tall fescue is slow to die. It takes at least 14 days to show significant injury and longer to completely die. It may take even longer if you spray it when it is under drought stress.
I hope this helps. Good luck.
It looks like the grass is tall fescue, a cool season grass. None of the products you are spraying are for cool season grasses. The products you sprayed are designed to be safe on cool season grasses (assuming you spray at the correct rates) but will kill broadleaf weeds. These formulations allow you to spray over your whole lawn and it kills only the broadleaf weeds but does not kill your lawn. Quinclorac does have activity on crabgrass, which is a warm season grass, but that is not what you have growing in your lawn.
If you want to kill tall fescue, you need to spot spray spray glyphosate (e.g., Roundup) at a 3% solution using a 4 lbs. per gallon glyphosate concentrated formulation. If you buy a ready to spray product, follow the instructions, which simply means it is pre-mixed and you spray it straight out of the bottle. You might have to make two applications as tall fescue can be slow to die.
You need to understand that Glyphosate is a non-selective herbicide meaning it will kill all plants. If you use it on your tall fescue, you will kill some grass around it as it is impossible to spray glyphosate and only get it on the tall fescue. As a result, you will need to re-seed after the tall fescue dies.
In general, you don't get good weed kill with herbicides if the plants are under drought stress. However, in this situation, it might be worth trying to spray now because you might get selective control meaning you might not kill part of your lawn that is next to the tall fescue because it is dormant now. If that doesn't work, I would fertilize your lawn and wait for it to green up with the fall rains. When it gets green and healthy, I would spray the tall fescue again. It will certainly die then. Just keep in mind that tall fescue is slow to die. It takes at least 14 days to show significant injury and longer to completely die. It may take even longer if you spray it when it is under drought stress.
I hope this helps. Good luck.
Thanks Brian.
It’s really good to know exactly what is in the lawn!!
… and thanks so much for the advice!
One thing, I’d rather avoid though is glyphosate. Could I use herbicides with other active ingredients?
I’ve been using Spectracide Weed and grass killer, not sure what the active ingredient is in that though.
Thanks.
Floyd,
Yes, Spectracide Weed and Grass Killer concentrate has a grass herbicide in it (fluazifop which is sold at Fusilade II) which will kill tall fescue but you will need to make 2 applications about 3 weeks apart. It is not as effective as glyphosate but you should be able to kill most of it.
Keep in mind that Spectracide also has dicamba in it which can damage trees if you spray it over tree roots at too high a rate. It also has diquat, which doesn't do anythingother than burn the leaves a little bit so homeowners think the product is working faster than it is. No matter what herbicide you are using, it takes anywhere from 2 - 8 weeks to kill weeds completely. In general, grasses die much quicker than broadleaf weeds.
Good luck.
Yes, Spectracide Weed and Grass Killer concentrate has a grass herbicide in it (fluazifop which is sold at Fusilade II) which will kill tall fescue but you will need to make 2 applications about 3 weeks apart. It is not as effective as glyphosate but you should be able to kill most of it.
Keep in mind that Spectracide also has dicamba in it which can damage trees if you spray it over tree roots at too high a rate. It also has diquat, which doesn't do anythingother than burn the leaves a little bit so homeowners think the product is working faster than it is. No matter what herbicide you are using, it takes anywhere from 2 - 8 weeks to kill weeds completely. In general, grasses die much quicker than broadleaf weeds.
Good luck.
So much very useful information!!
Thanks very much
Floyd Frick
