Leaf Mulch and Grass Clippings

Like most suburban gardeners, I have two resources in abundance:  Grass clippings  and Fallen leaves.   Every time I mow my lawn during the summer, I replenish my supply.  And every fall, the leaves from the maples cover the ground inches thick.  Here, we need to discuss how to best use these resources to nourish the garden and avoid buying materials from the store.

Grass Clippings.  

The first and obvious application is that I use grass clippings to mulch the soil underneath my vegetable beds.  The classic case is to cover the ground under the tomatoes with fresh grass clippings. 

This does two things:  

1.  The mulch provides a protective layer that keep the soil beneath it moist.  It protects from the baking sun in the heat of summer; it allows rain water or hose water to percolate into the soil slowly and not simply run off of the hard surface; and it keeps the surface of the soil cooler, preventing soil moisture from evaporating rapidly.

2.  It does an amazing job of suppressing weeds.  I am speaking directly from personal experience. Bare soil becomes a carpet of volunteer weed seeds that need to be addressed weekly.  But covered with a mulch layer of grass clippings, emergent weeds are reduced to a tiny handful poking through the grass barrier.  These are easily pulled and simply added to the mulch layer.

In addition to its two primary benefits, grass clippings slowly break down over time, adding nutrients to the soil.  At the end of the season, the mulch layer can be turned into the surface of the soil and will break down entirely over the winter.

The other thing I use grass clippings for is to add it to my compost tumbler over the summer as a green contribution.

Fallen Leaves.

The difference with fallen leaves is that they are only available seasonally, in the Fall.  And they don't decompose as readily so they have to be treated slightly differently.

Leaves decompose through an anaerobic process facilitated by fungi.  They need to be wet to produce leaf mold, unlike the aerobic process of composting, where air is incorporated into the mix.

Leaves by themselves are also useful as a top dressing mulch offering the same benefits as grass clippings for protection of the soil.  However, they have a much sturdier structural component and will break down much more slowly.  This makes them more long lasting, but a less available form of nutrients.

However, leaves will definitely improve soil fertility, introducing N, P, and K nutrients.  Leaves are the result of the trees extracting minerals from the soil depth and bringing it to the plant.  Some are deposited in the fallen leaves where they are available to enrich the surface soil.

Because of their particular structure, leaves improve soil moisture retention and also water drainage.  They add an organic element similar to peat in the soil profile.

Leaves and leaf mold are a much better habitat for worms and will attract worms to your garden bed.  Then, the worms themselves will aid in breaking down the leaves and incorporating them into the soil.

Create Leaf Mold

To create leaf mold, 

• In the Fall, gather the fallen leaves by using the mower to chop up the leaves and collecting them in the grass catcher bag.  If you gather some mowed grass at the same time, so much the better.
• Consider the species of leaves that you are collecting.  Generally you want leaves that are NOT waxy or tough. This would be trees like holly leaves, magnolia.  The needles from coniferous trees, Pine and fir species also don't break down rapidly. In addition, some trees have chemicals that are not good for your garden soil, such as black walnut and locust.  Better to leave them out, entirely.
  
• Depending on the structure of the leaves, you want to pour the leaves out onto the ground and mow them into the grass bag again.  This breaks them into smaller pieces with greater surface area.  Some gardeners recommend as many as 4 passes through the shredder.  This makes the leaf mold ready more quickly.
  
• Bundle the leaves into contractor bags.  Don't use the common flimsy 30-gal black trash bags.  They aren't sturdy enough to last the entire year, and will likely break down in the weather when left outside.  Instead, you want the thick black 40 gal to 50 gal bags that are 5 mil plastic or thicker.
• An optional step is to rake up a small handful of leaves from back in the woods and add them to the top of your bags as a fungal inoculant.
  
• Water the bags thoroughly and roll them around to distribute the moisture.  Close the tops securely.  You can tie them or use a drawstring that will allow a little air exchange, but for the most part you want them to retain the moisture you added in the first place.  You don't want them to dry out over the course of the year, because then the process will come to a stop. You will be left with dry leaves at the end.  A moist environment makes the process work
  
• Then leave them in a shaded place, undisturbed to allow the fungi to do their work.  What can happen is that the thinner bags break down in the sun and allows vines and roots to invade the bag, stripping the leaf mold of their nutrients before you ever get to add them to the garden. The holes allow too much air exchange, and areas of the leaves dry out, stopping decomposition.
• Instead: use thick contractor bags, water the contents thoroughly, close the top securely, stack them out of the sun, leave them undisturbed for an entire year.
  

This process typically takes a full year to render the leaves into something approximating a soil amendment.  Even if they haven't completely broken down into soil, the resulting leaf fragments will quickly be incorporated into the raised beds and add structure and organic material as they do so.

I typically gather 4-6 bags in the Fall and put them in storage.  I also gather another 1-2 bags to sit next to my composting station to add to the bins when I need brown material.  This is just a small fraction of the amount of leaves that fall each year, so I don't feel that I have to capture the entire volume of fallen leaves.  The rest of the leaves are mulched in place or go into the woods at the back of my house.

The stored leaf bags sit behind the shed until the next Fall, but I typically won't use them until the following Spring.  Then, I open them and use them to amend my raised beds before planting the spring garden.  This gives the fungi 16-18 months to decompose the leaves.

Note that in that second Fall season, I'm gathering another 4-6 bags of freshly fallen leaves to keep the cycle going.  Over the winter I have 1 batch of maturing leaf mold, and one batch of freshly gathered leaves.  This gives me enough finished leaf mold to add one contractor bag's worth to each of my raised beds every year.