Should I Cut the Grass Before Applying Weed Killer?
Many homeowners have asked the question, should I cut the grass BEFORE applying weed killer. I’ll answer that question for you today and give you what I believe is the best way to handle cutting the grass when you have to apply weed spray.
Remember that there are two different types of herbicides that we need to talk about – POST emergent (the stuff you spray on weeds after they visibly appear in your yard) and PRE emergent (the stuff you spray on your lawn to prevent weeds from growing.) Both of these types of herbicides are discussed below and I’ll tell you when you can mow your lawn when you need to apply either of these sprays.
Table of Contents
How Do Weed Killers and Herbicides Work?
So let’s start out with the basics on how weed killers actually work and then we’ll answer the main question, should I cut the grass before applying weed killer. The idea behind spraying weeds is you want to coat the weed itself with a herbicide that will stop the weed growth by preventing photosynthesis. So in order to stop that growth, you want to make sure the spray comes in contact with the leaves of the weed. That way it’s absorbed by the plant by ultimately getting into the roots, inhibiting its growth and killing it.
Different herbicides are made for different weeds and some are made to tackle any and all weeds. But the idea remains the same. If you want to kill the weed, you have to get the herbicide in contact with (or absorbed into) as much of the weed as possible and that includes the leaves, stem and roots.
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Why You Should Apply Weed Killers BEFORE Cutting the Grass
Spraying weeds before mowing your lawn is the ideal situation in my opinion. By applying the herbicide during this time before mowing, you ensure that the weed is at a stage where its leaves and stem are large enough to be coated with the spray. This is important because you want to make sure that there is enough surface area on the weed to absorb the herbicide so it can do its job……which is ensuring its DEMISE! 🙂
The only issue with using his method is that you’ll have to live with some unsightly weeds in your lawn for a little while. A few weeks back my front yard started to have dandelions popping up here and there so I held back mowing to ensure I could get the herbicide on as much of the plant as possible. Those little dandelions can go from just a few to hundreds if you don’t kill them before they go to seed so it was imperative I sprayed them at the ideal time. I used the very effective Roundup For Lawns and spot treated those little dandelions and in just about 7-10 days later you would have never known they were there.
The reason the Roundup was so effective was because I allowed those plants to continue growing so they were just large enough to get a good dousing of the herbicide. So the rule of thumb in this case is to NOT mow weeds down because you want to ensure there’s enough of the weed present to spray. Had they been mowed down you may have a small step sticking out and your herbicide quite possibly wouldn’t be as effective in killing the plant because the spray only has a small surface area to attack.
Is There a Time When You Should Apply Weed Killer AFTER Mowing?
Spraying weed killer after mowing your lawn really comes down to personal preference AND what type of weeds are present. If you have a lawn full of taller weeds that grow erect (like dandelions, crabgrass, nutsedge, lambsquarter milkweed, Canada thistle, etc.) and your lawn is just unsightly, I can see you making a case to mow the lawn to make it look a little better. You can still spray your lawn for weeds in this case, but you will more than likely have to broadcast the spray across the entire lawn instead of on individual weeds because they will blend in with the grass more easily. This will be a more expensive way to handle the weed issue for sure but your lawn may temporarily look better when everything is mowed.
If your lawn has more of those ground-cover types of weeds like clover, purslane, chickweed or my arch nemesis Creeping Charlie, sometimes mowing the lawn first won’t affect the weed very much since they’re so low lying. I normally mow my lawn at the second highest setting on my mower so many times the blades won’t even touch those ground covering weeds. If anything, the blades may clip off the top of the weed or the flowering part. In this case, applying a weed killer after mowing will not make much of a difference so nothing to worry about.
WEED WACKIN’ ADVICE!
One thing you’ll want to do if you’re mowing the lawn that has weeds is to use the mower’s bagger to capture the seeds of the weeds. Mowing a weed-filled lawn that has gone to seed and spreading those seeds on your lawn with the mulcher or side discharge chute is not a good idea. The following season you’ll have those same weeds pop up again because those seeds were spread around your lawn by the mower. Make sure you bag and dispose of the clippings either in your trash or better yet in a composting bin that will get hot enough to break down the seeds.
Mowing Your Lawn When Applying Post Emergent Herbicides vs Pre Emergent Herbicides
One other thing I consider when I’m trying to figure out should I cut the grass before applying weed killer is which type of herbicide I’ll be using. There are two types of herbicides you may need to apply and those are PRE-emergent herbicides and POST-emergent herbicides.
If you are applying a PRE-emergent herbicide (which is something you apply to your lawn early in the season to PREVENT weeds from growing) you’ll want to make sure you do two things:
- Water in the herbicide as the directions advise or wait to apply the herbicide on a day before it rains. This ensures the herbicide will be absorbed into the soil and that it will lay down the barrier you need to prevent weeds from growing.
- Do NOT mow the lawn until after the herbicide has been watered into the soil. If you mow the lawn without watering in the herbicide, you’re taking a risk that your mower will bag up or destroy a good portion of the herbicide you just put down (if it’s the pellet variety.) With the cost of a bag of weed and feed averaging $30-60, you do not want to waste that money so it’s best to water it in and wait until the lawn is dry before mowing.
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If you are applying a POST-emergent herbicide (which is something you apply to your lawn after weeds have started popping up), my recommendation sticks with what I mentioned before:
- Do not mow your lawn right away. Mowing over the weeds that are already present may end up cutting leaves and stems from the weed that you really want to coat with the post-emergent herbicide (aka weed killer.) Spray the weeds as you see them and then wait for at least a couple of hours before mowing your lawn to ensure the chemical has soaked into the leaves and roots of the plant.
- If you cannot wait to mow your lawn, go ahead and spray the post-emergent herbicide prior to getting out your lawn mower. But understand it may not be as effective as waiting to mow as mentioned in #1 above. At the very least, give at least a few hours for the chemicals to soak into the weed before mowing.
So there you have some answers to should I cut the grass before applying weed killer. In most cases I would say YES you should wait to mow just to ensure that weed gets a full blast of weed killer on all parts of the plant. But I totally understand that in certain situations you’ll want to mow first. We’ve all had outdoor parties or get-togethers and can’t stand the sight of an unkept unmowed lawn so the desire to mow first could be important in that case. But in my view, waiting a day or two and blasting those weeds with the best weed killer is probably your best bet.
Good luck and go blast some weeds!
