Will Grass Clippings Kill Plants And Trees?

One of the byproducts of mowing your lawn and keeping your grass in shape is the yard waste you leave behind in the form of all those grass clippings. Of course, there are a lot of uses for grass clippings, so long as you gather them up. But do they cause damage? Are grass clippings harmful?

Will Grass Clippings Kill Plants And Trees?

Grass clippings, if left in large piles over pretty plants, may do damage, but it’s highly unlikely. They won’t damage trees either unless the grass is coated in herbicide, which can easily carry over to a tree and cause a lot of damage. 

The trees that you have to worry about the most are young. It’s doubtful that grass clippings covered in herbicide will kill a large oak or pine. They won’t do much as far as killing weeds either since weeds are always so resilient. 

Potential Dangers of Grass Clippings

It’s not necessarily the grass clippings that are the danger. However, the herbicide that coats them could potentially cause harm, especially if you are into the practice of piling up large amounts of grass clippings around the trunk of a tree. 

If the grass clippings are thick enough, they might kill small plants by depriving them of the necessary sunlight. If you’ve ever left a large object lying out in the yard and picked it up a few days later, you’ve probably seen the damage it can do. 

Heavy grass clippings will exacerbate the damage if your lawn is already damaged and infested with yellow spots or disease. They can also be a pain if they end up in areas like gutters or if you live near a body of water. Herbicides on the grass clippings will effectively pollute it. 

Benefits of Grass Clippings

Grass clippings don’t have to be a bad thing, especially if you know what to do with them when you are finished mowing the lawn. One good way to properly collect them is to use a bag catcher on your lawn mower; then, you can distribute the grass clippings as needed. 

  • Grass clippings are often used as an effective mulch material
  • Thick layers of grass clippings will slow the growth of weeds
  • Grass clippings can serve as a barrier to stopping the spread of weeds
  • Grass clippings can serve as a decent fertilizer
  • Decomposing grass clippings can feed your garden (as long as they aren’t covered in herbicide)
  • Grass clippings hold moisture underneath

Yes, grass clippings are potential hazards to trees and healthy plants if they are coated in herbicide. 

However, they have significant benefits if you use them correctly. Without herbicide, grass clippings alone provide potassiumphosphorous, and nitrogen to your garden.

While they aren’t the most fast-acting fertilizer in the world, you might consider them a “slow-release” type of fertilizer for your garden. If you have enough grass clippings, you can even use them as a shield to protect your garden from the spread of weeds outside it. 

It’s like building a wall to defend a city, with the garden serving as the inner town. Grass clippings also retain moisture reasonably well. So if you use them as mulch around your plants, they will help the plants retain that moisture for longer. 

What if Your Lawn Has Herbicides in It?

Using grass clippings for much of anything becomes more complicated if you want to use them for mulching, as a weed shield, or pack them around trees. However, if you use herbicides on your lawn, there are a few steps that you can take to ensure that you adequately protect your trees. 

One of the most obvious things you need to do if your lawn is covered in herbicide is not to let any clippings gather near the base of your trees or around roots exposed to the air (surface roots).

In the future, use only approved herbicides for lawn use in residential areas. These herbicides are not as potent as the synthetic variety and are far less dangerous to your garden or your trees. 

Make sure you have a bag on your lawnmower or some method for catching the clippings as you mow your lawn. So long as you have any kind of synthetic herbicide on your grass, you should use only mulch that you have purchased or make separately from the grass clippings. 

How to Use Grass Clippings as a Mulch

If it hasn’t been clear enough to this point, avoid using grass clippings as mulch if there is any synthetic herbicide on them. If your neighbor uses the stuff, you will have to be careful that you only gather up grass clippings that are well away from the neighbor’s yard, depending on how they dispersed their herbicide. 

If you have used herbicide (especially the harsher, synthetic varieties), you must wait at least two months after the last application before applying your grass clippings as mulch. This is because the grass needs time to rid itself of the herbicide. 

If any of the grass you cut contains weeds or vines, never use it as mulch in your garden, as it will only facilitate the spread of weeds in the garden, which is precisely what you are trying to avoid. 

Never use freshly mown grass clippings as mulch. Instead, give the grass clippings a total of 24 hours—48 hours is better—before gathering them up. Let them sit on the lawn while you wait. 

If you collected the grass clippings in a bag, you should spread them out somewhere and give them time to dry out before you apply them as mulch. 

This is because grass clippings create a ton of heat as they die and dry out. Of course, it’s probably not enough heat that you would recognize it or even feel it. However, when it comes to the delicate ecosystems in roots and garden plants, that heat is enough to kill roots and, possibly, the plant.

Applying the Grass Clippings

After the grass clippings have had a chance to dry out, gather them up and start going through them to ensure you don’t have any stray material that could potentially be harmful. Apply the grass clippings in 3″ to 4″ layers around your plants. 

If you palace your grass clippings in heavy weed areas, lay it on thick. The idea is to smother the weeds completely. If some of the weeds are a bit long or are capable of growing to a decent length, you may want to pull them or cut them down to size. 

Make sure that any grass clippings used as mulch around your good plants don’t lay up against the plants themselves. Grass clippings can have the same effect on your good plants as they do on the weeds, so be careful about how you apply them as a mulch. 

Final Word

While grass clippings covered in potent, synthetic herbicides can be extremely dangerous to your trees and garden, clean grass clippings are highly beneficial. 

They can ward off weeds, provide nutrition for your garden plants, help your garden to retain moisture, and more. 

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