Mulching Mower vs. Autumn Leaves

September 27, 2008 9:40 AM

Will mulching our leaves with our mower help or harm our lawn?

We have a huge amount of leaves of that fall or blow into our yard and we own a good mulching mower. Our soil also has a very high clay content, which seems to be why we have a lot of bare spots in our lawn that that look and feel like concrete after even short dry spells.

I was wondering if I can get rid of some or all of our leaves by mulching them with our mower, and in the process also improve our soil by adding organic material to it. Keep in mind that we have a LOT of leaves so I'm wondering if I can overdo it.

I spend at least 20 hours every year raking/blowing leaves. The chore has become worse since our town told us we can't compost them anymore because the pile was too close to our property line. That means I have to bag them and put them out with the trash, which bothers me for ecological reasons.

If it helps, we're in western NY and we don't exactly have a thick lush lawn. A couple of big Labrador retrievers beat it up and we have a lot of moss this year because of unusually wet weather.

posted by 14580 to home & garden (12 comments total)

Raising the carbon content of the soil will be good for increasing drainage and root penetration (especially in clay soil), and the organic nutrients will help the grass come back stronger next year - anything beats shipping your leaves out to the dump to be composted.
posted by acro at 10:26 AM on September 27, 2008

It seems to be recommended (more), though if you had inches of leaves it may be going too far...

Much like aerating your lawn, it will, of course, look like crap for a while, with chopped leaves all over your lawn. But given time, they'll work their way down and leave things better for next year.
posted by fogster at 10:40 AM on September 27, 2008

The nitrogen from the grass clippings will probably help the leaves rot down. If there isn't much grass, the leaves will take an age to rot, and look a mess for a long time. You need to get the mix of nitrogen/carbon right, or you'll end up with a mess. This site will help you work out the balance correctly.

If it's just a matter of disposing of them, but you're OK with raking them, just stick them in black plastic bags, out of the way, for a year. Some holes in the bags will help the air penetrate. Next year, you'll have honest-to-goodness leafmould, which you can dig in, or spread thinly over the grass to let it sink in. Once it's decomposed, it'll vanish much more quickly than chopped leaves will.

I have a feeling you'll have better results by doing the bagging method than expecting them to rot in the lawn. They'll get chopped up more quickly, but they're not going to rot quickly over the winter anyway. By mulching the lawn, you're going to take nutrients away from the grass that it needs to grow, because they're going to be rotting the leaves instead of making the grass grow.
posted by Solomon at 10:54 AM on September 27, 2008

If you want pretty, flat, smooth, putting-green-type turf, mulching in my opinion is the wrong way to go about it. Bag the clippings (and leaves) and discard.

If you want lumpy, uneven grass sitting on a thick spongy layer of decaying organic matter, by all means mulch. You can certainly smother your lawn if you cover it completely with leaf matter, so keep that in mind.
posted by ikkyu2 at 11:27 AM on September 27, 2008

Your ecological concerns are kind of a wash. Yes it would be nice to just mulch the leaves directly in your lawn, but the effect is really trivial. Numbers vary but one common rule of thumb is that it takes 100 years to create one inch of top soil. So whatever you chose to do, it probably won't have much effect on your clay soil.

The other consideration is that most of the leaf mold eventually is digested by microorganisms and released into the atmosphere as carbon dioxide. Very little of the carbon stays in the soil for any length of time (which is why soil takes so long to build up). Over a year or two it is almost the same as if you burned the leaves. On the other hand, if you send the leaves to the landfill, the carbon is permanently buried and there is a net reduction in atmospheric CO2. In fact some scientists suggest cutting and burying forests as a method of carbon sequestration. This, however, doesn't address the issue of limited landfill space or poor landfill management practices.

Solomon's idea of bagging the leaves and composting them on-site might get around your town rules.

The problem of composting in your lawn is that if the leaves are too thick they will smother the lawn. Remember that gardeners smother weeds by piling on large amounts of mulch. What I do is sort of a compromise. During the fall I make a quick pass with the mower each week to chop up the leaves as they start to fall. But toward the end, especially after a big rainstorm, the pile of leaves just becomes too thick. Then I rake them up and send them to a commercial composting center.
posted by JackFlash at 11:34 AM on September 27, 2008

I hae done this for years and unless the leaves are very thick, when done, it isn't really noticable. I'm pretty sure it's good for the lawn. Best way to do it is to mow frequently even if the grass dosen't really need it.
posted by Rad_Boy at 12:01 PM on September 27, 2008

The other consideration is that most of the leaf mold eventually is digested by microorganisms and released into the atmosphere as carbon dioxide.

Well, you don't have decomposition, and thereby compost, without releasing carbon. Carbon is essential for healthy soil systems. Without it, you have no decomposers or mycorhizae, and the assosciated plant life due to lack of nutrients and little or no soil structure or porosity.

Over a year or two it is almost the same as if you burned the leaves.

No, not really. Carbon is used by decomposers for energy, and for creating nutrients that plants need. Burning the leaves releases carbon dioxide, but also carbon monoxide, hydrocarbons, particulates and nitrogen oxide (which easily forms nitric acid in the atmosphere); while heterotrophic decomposers release carbon dioxide and water as their by product (just like we do). Carbon dioxide is used by plants for photosynthesis, unlike the carbon monoxide produced by burning. By burying leaves in a landfill you also tie up nitrogen that is needed by plants, necessitating the purchase of commercially produced nitrogen- organic or not, that means more carbon released into the air in the form of mining and shipping raw material, processing, packaging, shipping again, and obtaining. Add to that the fact that most fertilizers are made up of ammonium nitrate which is easily lost to the atmosphere or leached from the soil (due most often to improper application of fertilizers), and the artificial system that most people engage in is far more leaky and sends much more energy out of the cycle than proper composting does. It is always more sensible to strive for the nearlyclosed loop of a natural system in which elements are circulated with minimal loss, and ecosystems are maintained.

If you are mulching some green grass with your leaves and not doing too many leaves at a time, you should be OK. Strive for about half grass and half leaves, and don't allow a buildup of more than half an inch or so in the winter, 3/4 of an inch when the soil begins to warm up in spring. Be sure to de-thatch and aerate your lawn on a regular basis; and keep in mind that if you use herbicides or pesticides on your lawn the mulch will take a much longer time to break down because your soil will be devoid of a lot of important living things. I've brought many lawns back to life with compost, aeration, and de-thatching. A good layer of decomposing organic material makes lawns more resilient to use than a lawn on hard-packed soil. It won't be lumpy if you rake your lawn (an extremely underrated practice that seems to be going the way of the dinosaur).

Numbers vary but one common rule of thumb is that it takes 100 years to create one inch of top soil. So whatever you chose to do, it probably won't have much effect on your clay soil.


It's completely untrue that you can't have a noticeable effect on clay soil. I've worked with gardens in clay soil for years as a professional Bay Area gardener. Topsoil is the "A" layer of soil, where organic matter is recycled. You don't have that layer at all if you don't allow for natural decomposition. If you have a healthy collection of decomposers, you have topsoil. However, you have to give them something to eat if you want them to exist.
posted by oneirodynia at 1:00 PM on September 27, 2008 [2 favorites]

It matters what kind of leaves we're talking about. Oak leaves in particular are quite acidic and though they will eventually return nutrients to the soil as they rot, they're likely to kill most things in the soil before that happens. The resulting soil will also be acidic, so that will limit what you can grow in it. Western NY tends to have a lot of oak trees, so it's something to look into.
posted by valkyryn at 1:25 PM on September 27, 2008 [1 favorite]

Over a year or two it is almost the same as if you burned the leaves.

No, not really.

If we are talking about atmospheric carbon, then it is true. Almost all of the carbon in mulch eventually ends up in the atmosphere as CO2. Soils really don't retain that much carbon in the long run. It is carbon neutral. In terms of atmospheric CO2, the end result is not much different than burning. And I'm not saying that burning is a good thing. It is not because of the pollution effects. But as far as carbon is concerned the net effect is the same.

On the other hand, burying does remove carbon from the atmosphere. Burying does not tie up nitrogen because there is almost no nitrogen in falling leaves. Most nutrients have already been removed by the tree which is part of the reason for changing colors. All that is left is cellulose. In fact leaves alone are very slow to compost because of the low nitrogen content.

It's completely untrue that you can't have a noticeable effect on clay soil.

I didn't say you can't improve clay soils. I did say that naturally falling leaves will have little effect on clay soils in short time scales. You can improve clay soils by bringing in lots and lots of compost from other sources. But spread over an entire yard, the leaves have little measurable effect.

The point is, that the OP can mulch the leaves on the lawn if he wants to save some time and work, but it won't really do all that much to improve the soil. And he can also send the leaves to the landfill, if that is more convenient, without concern that he is harming the environment (assuming it is a well-managed landfill). All you are doing is burying carbon. Sending green grass cuttings to the landfill, on the other hand, can be harmful because it is nitrogen rich and can decompose into methane which is a powerful greenhouse gas.

This has nothing to do with the merits of organic gardening or composting. We are discussing here the practicalities of mulching leaves on a lawn, a more limited question.
posted by JackFlash at 1:53 PM on September 27, 2008

Years ago, I had a house with many. many, many trees. I didn't want to rake, so I mulched. The following spring... dead lawn. There were just too many leaves.

If, after you mow/mulch, you can see chunks/clumps of mulched leaves, you will want to think seriously about going back over it with a bag.
posted by jknecht at 5:09 PM on September 27, 2008

You can definitely overdo it with leaves. You could end up with not enough nitrogen to get the leaves to break down. This could actually lead to the leaves sucking nitrogen out of your soil and away from your grass. When mulching your lawn, do not let the nitrogen/carbon ratio get too lean. As ikkyu2 said, you could also smother the lawn if the leaves are too deep.

If you want organic matter top dress the lawn every spring and fall for several years with a mixture of rich loam and sand. If you mulch some of your leaves onto the lawn, put the fall top dressing on after. The soil on top of the leaves will help break it down faster. Of course with a real clay issue you may need to till the soil, work in a six inch layer of quality loam and reseed.
posted by caddis at 6:23 PM on September 27, 2008

According to this article from Grounds Maintenance, which sites 3 studies from Michigan State University's Hancock Turfgrass Research Center, mulching deciduous leaves into your lawn is not only okay, but good for it. The potential Carbon / Nitrogen balance issues mentioned above were not observed in all three studies.

One study went so far as to test up to 450 lbs per 1000 sq ft, or an 18 inch layer of leaves, mulched into a mixed-turf area.
posted by mdebruic at 12:07 PM on October 16, 2008 [1 favorite]