When It Is Hot, Hurt My Lawn?


Treating crabgrass is my question, if you wouldn't mind. 

I'm in Missouri and it's been nearly 100 degrees during the day. (August, 2010). I have quite a bit of crabgrass that has sprouted over the last month that I just haven't had time to deal with. I'd like to apply a liquid weed killer to it.

Would I be safe to put this down with a pump sprayer in the evening once the temp goes down a bit? I have an in-ground sprinkler that would run about 5:00 a.m. and give the rest of the yard a nice drink before the sun gets hot and has it's way with the yard.

Do you think this would be ok? I hate to let the crabgrass just keep dominating. Thanks for your help.

Editor's Note: This may sound like a rather simple question that could get a yes/no answer. Please read on to discover multiple factors regarding temperature and vulnerability issues that cause a wide variation in how your lawn might react as you apply a crabgrass treatment.

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Garden Counselor ANSWER:

Treating crabgrass is a procedure that encounters all manner of circumstances, yours being a good example.

As mentioned in some of the reference articles on the website, crabgrass has an extremely broad growing season, so all climate conditions come into play.

Allow me to make several comments/observations, and you apply whatever is most appropriate to your situation.

or E.T. phone home...

1. Generally, it is NOT wise to apply any garden chemical to your lawn when temperatures are above 90 degrees. Even 85 degrees is too hot and pushing it for some chemicals on certain plants. 

The effects of  the chemical is amplified and your lawn grass may show more negative impact in the heat, while it might be fine in cooler temps. 

If your grass is well established, and the crab grass infestation is quite severe, you may be inclined to try it and hope any damage is minimal. But consider this first before you decide.

It seems reasonable, treating crabgrass in the evening, and allowing the cooler air through the night to work in your favor. Maybe it would, but the effects may last longer than overnight.

It also depends on the type of grass, the overall health of the lawn, and the type of weed killer. The residual effect during the next day can still be quite potent in the heat. Look what happens.

The heat during the day kicks in a process known as Evapotranspiration, or ET. A simple way to understand that? 

Water passes from the soil through the plant and is drawn out of openings in the surface of the leaves (blades of grass). 
Water is also drawn out of the soil by the dry air being heated in the sun. The higher the temperature, the greater the ET. 

What is the impact then, by treating crabgrass when this occurs?

High ET rates can stress any plant, just under normal conditions. By adding chemicals, you affect the grass plant both internally and externally. Many sprays are oil based, and this magnifies the intensity of the hot sun on the outside surface for a time. 
Also, the chemicals absorbed into the plant can diminish the ability of the grass to translocate the water from the soil up into the tissue. Wilting and burning can result, in a very short period of time.


2. Yes, you have the right idea, to soak the lawn well before treating crabgrass with a spray or granular weed killer. This would be essential.

But it sounds like you were also planning to let the sprinklers run the morning after you spray? That would compromise the effectiveness of the spray.

The label instructions typically specify no irrigation or rain for a set number of hours, usually 24/36/48 hours, after treating crabgrass with a liquid spray.

So in order to cause the greatest damage to the weed, you have to submit the lawn to a significant trial. 
(It’s not unlike a cancer patient undergoing chemotherapy — no insult intended to any oncology patients by minimizing their plight to this level — but chemicals are harsh stuff for a soft tissue plant.)

before applying oil dressing.

 So toss all this around — as a test:

  • Is your lawn mature (well-established), healthy and deep rooted?
  • Is it a sensitive variety of grass?
  • or a tough, durable, heavy bladed species?
  • Have you recently applied some other treatment,insecticide, fertilizer or another weed killer?

As you make your decision, considering all the possible stress factors, realize that the only way to know for sure is to try a small area first and test it.

(That may seem like a pain to do, if it works great. If it results in trouble, you'll be so glad you started small!)

If you do a test spray, treating crabgrass in a small plot first, and things seem to be reacting well, you should be good to go. Keep in mind that the concentration you use can make a big difference in any harmful effect.


If your lawn grass is young, planted in the last 3-4 months, do NOT treat with any weed killer when the weather is quite hot. 

The grass is not well enough established to endure the stress.

Don't Read, to Reseed Weeds

What weed killer do you plan to use? Is it selective, non-selective, combination, concentrated or RTU? See our Weed Killer Guide before buying.

Read the label on any herbicide at least twice before you plan to apply it. 

Read the label at least twice. Yes, I am repeating myself. 

Seriously, three times is better, when treating crabgrass, because it is usually a very potent chemical. 
Look for every note that pertains to your situation, type of grass, and circumstances.

Does the label indicate it WILL DO specifically what you NEED it to do? …
Without causing any harm or negative results beyond what we have discussed here?

Again, pardon my persistence. I know from experience that many people cause more trouble for themselves because they skip this part.
They trust that the picture and info on the front of the bottle sounds like it might work.

IMPORTANT TIP: Many people do not realize that the back panel on typical bottle labels will open up with multiple pages of detailed info inside, so I always make this clear.

Don't mix anything stronger than recommended. And in some cases, (when it is edging toward really hot), it might make sense to use a lower dose of chemical. 

The weed kill rate might be lower and require additional applications, but it might be safer for your grass and you will probably be needing to do repeat sprayings at regular intervals anyways.

Treating tenacious crabgrass requires fortitude on your part!

and all the weeds did drink..

5. One other point I’d like to clarify, (for those treating crabgrass, or just having trouble making their lawn thrive). This reader's question implies the ability to water his lawn everyday if he desires. Please don't do that.

Watering the lawn with your sprinklers everyday aggravates your problem.

It is better to water less frequently, but for a longer interval, to encourage deep root growth.
Watering everyday keeps roots near the surface and makes the grass more vulnerable to heat stress.
The top layer of soil constantly loses moisture, so you want a deep rooted lawn that has a longer lasting reserve of water.

Avoid daily watering of lawns that keeps the soil constantly moist on top.  Tip #7 in Getting Rid of Crabgrass states that: Abundant moisture makes a prime germination setting for weeds.

This is especially true for crabgrass seeds in lawns that have bare spots or thin areas. Letting the soil surface dry out in the sun may interfere with the weeds germination process. 

CAUTION: If this principle needs to be applied to your lawn, don't change anything immediately during the hot season. When the lawn is accustomed to one pattern, you want to make changes gradually, or during the cooler season, and eventually train it to get by with longer intervals between watering.

do it yourself pest control

Go to Top of Page

Don't delay if it is time to apply that crabgrass treatment. Just be smart to be effective. Any crabgrass that you can kill before it goes to seed will help you out in the future.

Further assistance is available in many articles by the Garden Counselor.

Go to Crabgrass: General Info page.

Go to Getting Rid Of Crab Grass.

Go to Lawn Weeds General Info page.

Go to Home Page of Lawn Care