To Mulch or Not To Mulch: Can Mulching Leaves Kill Grass?

It’s that time of year again! With the end of the summer season, many homeowners are looking forward to a respite from mowing their grass weekly, or, here in the southern USA, sometimes twice a week. But don’t be so quick to winterize and stow away your lawn mower just yet. There’s another use for that essential piece of machinery – mulching the fallen leaves on your lawn.

You may be asking, is mulching leaves really a good idea? There is some controversy out there regarding how mulched leaves will affect your lawn. Some sources even claim mulching will kill your grass…

But we’ve done the research for you, and rest assured, that’s not the case if it’s done properly. Mulched leaves can actually nourish and protect your lawn. Read on to find out why, and how best to incorporate this practice to benefit your lawn.

How Mulching Leaves Can Benefit Your Lawn

A thin layer of mulched leaves covering your grass during the winter season is beneficial in several ways.

1. Natural Fertilizer

As the smaller bits of leaves break down, they provide much-needed nutrients for your lawn, adding a boost of nitrogen, phosphorus, and potassium to your soil.

2. Weed Prevention

Leaf mulch can act as an excellent barrier against weed growth. This is because the minute leaf pieces will fall into tiny bare spots between turf grass, where weeds are otherwise likely to spring up, and smother them before they get started. This study by Michigan State University reported a huge decrease in dandelions and other weeds with the regular practice of leaf mulching lawns.

3. Moisture Balance

 A layer of mulch can both retain moisture during dry weather and regulate moisture content in damp weather, as the mulch absorbs excess moisture from the grass.

4. Insulation

 A layer of leaf mulch left on your lawn over the winter can help protect the turf grass roots from freezing temperatures, minimizing damage during extreme cold snaps.

Potential Risks of Mulching Leaves on Grass

There is a grain of truth in those rumors about the dangers of mulching leaves on grass. If done improperly, you can do more harm than good, and have an unwelcome surprise once the warm weather returns. Keep these warnings in mind to avoid harming your lawn.

1. Smothering

If your mulched leaf layer is too thick it can be detrimental, smothering your lawn. Aim for no more than 1” of finely mulched leaves on your lawn.

2. Moisture excess

If you have a rainy fall season, it’s best to rake those leaves away from your grass. Once mulched leaves become saturated and don’t have a chance to dry out, they can mat together and prevent air flow. This can lead to problems such as mold, fungus, or rotting of the turf layer.

3. Blocking sunlight

If your leaf mulch layer is too thick or too wet, no sunlight will penetrate. Your grass will suffer as it needs sunlight to photosynthesize. This is also the reason you should rake away leftover leaf mulch in the spring.

4. Changing pH levels

There is some concern that oak leaves will raise the acidity of your soil, but recent studies have shed light on the matter. If the leaves are finely shredded, they will deteriorate quickly. By the time they’re absorbed into the soil, they will have become largely pH neutral. However, if you have naturally acidic soil, you may want to go easy on oak leaf mulch.

Read our related post “The Leafy Dilemma: How Long Can Leaves Stay on Grass?” here.

Does Your Grass Type Matter?

No, all types of grass can benefit from a thin layer of leaf mulch. However, some grasses you may want to be careful with how much leaf mulch you choose. 

Cool-season grasses (e.g. Kentucky Bluegrass, Fescue, Ryegrass) are actively growing in the fall when leaves are dropping most. Mulching can be beneficial for these grasses.

However, for warm-season grasses (e.g. Bermuda, Zoysia, St. Augustine, Centipede), these go dormant in the fall and a thicker layer of mulch can potentially cause a delay in their greening up in the spring.

Warm-season grasses may not benefit as much as cool-season grasses from decomposed leaves. 

Does the Type of Leaf Matter?

Be aware of what tree varieties you have on your property. Some trees have leaves which can be toxic to other plants. Both black walnut trees and buckeye trees should not be used for leaf mulch as they contain natural chemicals that may inhibit or even kill other plant life, including grass. Remove these leaves from your lawn instead of mulching them!

Here is a quick list of leaves you may not want to use as much on your lawn: 

  • Black Walnut
  • Walnut 
  • Oak Leaves
  • Pine Needles
  • Leaves from Diseased Trees
  • Thick Leaves
  • Leaves from Invasive Species

So, Should I Mulch Leaves into My Lawn This Fall?

Absolutely! Besides the obvious advantages such as:

  1. Less work than raking
  2. Provides free nutrients for your lawn.
  3. Insulates and protects grass in freezing temperatures.
  4. Balances moisture
  5. Cuts back on weeds

Mulching with leaves is also good for the planet! Every year, a harmful chain reaction occurs when people dispose of leaves using other methods. Burning leaves releases carbon into the atmosphere. Bagged leaves often end up in a landfill taking up space. Blown leaves can wind up drifting and clogging up sewer grates.

Mulching your leaves is a natural and responsible solution.

Consider Alternative Uses for Mulched Leaves!

If you have more than enough leaf mulch on your lawn, why not use it in other areas? You can add leaf mulch to garden beds, around the base of the trees of your property, and spread it under the canopy of shrubberies. Just leave a little breathing space between the trunks/stems and the mulch, and don’t overdo it. If you still have leftover leaf mulch, add it to your compost pile.

I hope you found this article informative, and it answered any questions you may have had about mulching your leaves this year. If we’ve missed anything, feel free to drop a comment in the space below.

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