Leaves are the cheapest and most effective mulch for weed control, when enough of them are used. This natural gardener has been using them professionally on clients’ yards for 13 years. All other mulches are seed beds by comparison. Leaves feed the soil and thus the plants; soften it for easy weeding; and can stop sprouting of most small seeds, just by keeping the sun off them. As long as leaves stick around, they stop germination of seeds that land on top of them, because the top few layers dry out quickly and don’t allow seeds on top to sprout.
One can smother nearly any annual weed, and many small perennials, with enough leaves. But to be efficient with a client’s money, a professional gardener must figure out the least that it takes to do the job effectively when one has to bring them in from other places.
There is no need to shred leaves to use them for mulch in most situations. Whole leaves are better for blocking weeds, but leaves can be mowed into a lawn, and shredded leaves will stay in place better where the wind would otherwise blow them around.
Only very large trees drop enough leaves beneath them to stop larger annual grass seeds like cheat and foxtails, but taking them from pavements, roofs, lawns, paths, and neighbors with too many and spreading them more thickly in beds and borders can smother grass seeds and young weeds while encouraging larger established perennials.
There are three basic classes of leaves as mulch: soft leaves; tough broad leaves; and tough conifers. Black walnut is a soft leaf in a class of its own, containing a pre-emergent herbicide, juglone, that stops smaller seeds all summer, though their leaves are eaten before winter is over.
Red maple leaves, blown onto a weed field from fine bark. Not thick, but still smothering groundsel seeds.
Although soft leaves may be eaten by soil life before summer is over, and sometimes before fall or winter is over, they can have great smothering ability until they are eaten. Many tend to lie flat over seedy soil and stop spring weeds like groundsel and bitter cress, even without extra leaves piled on. Piling them 6 inches or more deep will preserve the top layers of leaves over the entire season, as they quickly dry out on top while the worms happily eat the damp bottom leaves. But that’s a lot of leaves unless one wants to grow vegetables. In that case, a foot is even better.
Oak leaf mulch under a large oak
Hard leaves, like oak and sweet gum, stick around the entire season, but tend to be stiff and fluffy, and thus allow seeds to grow through them to the light. Again, it usually takes more than naturally falls under the tree to stop many weeds, particularly annual grasses with larger seeds. Merely covering the ground is insufficient, especially with weeds already growing.
Pine needles naturally fallen beneath the tree.
Pine and true cedar needles are good for covering other leaves to hold them in place and give a consistent, quiet look to one’s mulch. They are also good for covering compost spread on top of soil or leaves to aid seed germination, when spread just thick enough to hide the compost seed bed. Piled over an inch deep, they stop germination as well as other leaves, as they pack down fairly tight. They can be used for path cover, but pine are rather slick and not suitable for slopes. Flat or non-needle conifer leaves, like fir, incense cedar, redwood, and sequoia, are soft and decompose as fast as other soft leaves.
Pine needles spread as mulch. They are much used in the South, going by HGTV.
Six inches to a foot of leaves can grow big food plants. But piling one kind of leaf too deep can occasionally result in slow decomposition and slow growth of plants. A mixture of several kinds of leaves has more variety for the soil life to eat, more nutrition, and allows water to move down through the stack more efficiently. It is important to spread them in layers and keep the surface relatively flat, so the water doesn’t just run off the top or around bundles of leaves.
One can grow garden seeds in a thick bed of leaves if one spreads an inch of compost on top of the leaves and plants into and on it. This even works on top of black walnut leaves and soil, keeping the seeds separated from the juglone long enough to sprout. Compost also helps decompose leaves more quickly for early spring growth. But it is not necessary for planting large seeds, like beans, corn, and squash, into the leaves. Poke them in to where you can feel moisture for them to sprout in.
But for non-food-growing areas, a mulch of leaves with several thin layers, each just thick enough to hide what is beneath, can provide maximum weed control with minimal leaves. Use soft leaves on the bottom to smother and physically block seeds and small plants from growing. Hard broad leaves can be spread on top of those to keep the sun off the soil all summer. Pine needles go on top to keep the leaves in place and give a consistent, calming look to the landscape.
Leaves work well for mulching beds and borders, but weeds in paths have to be controlled, too. Just about any material one puts on a path will sooner or later become a seedbed for whatever lands on it, except for pavement, and even there, they will grow in the cracks and holes. Organic path mulches must be renewed every year or so. Gravel is a headache once it fills with seeds and dirt.
4 x 8 sand, surrounding beds.
But a half-inch of 4 x 8 sand, which is river sand screened ¼-1/8 inch, can let you defeat the weeds by bringing them up right away, so you can slide a scuffle hoe, AKA hula hoe, underneath their crowns and pull or cut them off their roots. Rake out the weeds, and your path is cleared without bending.
The key to how 4 x 8 sand works is that, when particles of different sizes are disturbed, the smaller sift to the bottom and the larger end up on top. Dirt and small seeds on the path are sent by hoeing and raking to the bottom, and rocks and sticks larger than the sand float to the top where they can be raked out or picked up and thrown in the bed. Quarter-inch rocks dominate the top, and don’t stick to shoes. When seeds sprout, their crowns form at the top of the sand, so they are easily cut off their roots. Very young plants whose crowns are cut off their roots are killed, as they have no food in their roots.
Gardening is easy, if you do it naturally.