The bags of leaves at the curb every fall are one of the garden's great wasted resources. Those leaves are free carbon, and with almost no effort they become two of the best soil amendments you can get: leaf mold and finished compost. Here is how to turn a yard full of fallen leaves into black gold, and how to keep a compost pile working through a Canadian winter.
Why leaves are worth keeping
Fallen leaves break down into material that loosens heavy clay, helps sandy soil hold water, and feeds the soil life that makes everything grow better. Trees spend all summer pulling nutrients and carbon out of the air and ground, and they hand it back to you every October. Sending that to the landfill and buying bagged soil in spring is paying twice for something you already have.
Leaf mold: the lazy method
Leaf mold is simply leaves that have broken down on their own. Unlike compost, which is driven by heat-loving bacteria, leaf mold is made by fungi that work slowly even in the cold, so it practically makes itself.
- Shred the leaves first by running a lawnmower over them. Shredded leaves break down in about a year; whole leaves can take two or three, and tend to mat into a slimy layer that sheds water.
- Contain them in a simple wire ring, a pile in an out-of-the-way corner, or a few garbage bags with holes punched in the sides.
- Keep them damp, about as wet as a wrung-out sponge. Dry leaves do nothing; moist leaves feed the fungi.
- Then wait. In a year you have a crumbly, sweet-smelling soil conditioner. Dig it into beds, use it as mulch, or add it to potting mixes.
Put some leaves to work right now
You do not have to store them all. This fall, shredded leaves can:
- Mulch your beds for winter, a couple of inches over bare soil to stop erosion and feed the ground.
- Insulate perennials and roses after the ground freezes (see our winterizing guide).
- Feed the lawn: a thin layer mowed back into the grass breaks down over winter and feeds the soil.
- Balance your compost as the "brown" ingredient all winter (more on that below).
Composting through a Canadian winter
A compost pile does not die in winter, it just slows down or pauses when frozen, then roars back in spring. You can keep feeding it right through the cold.
- Keep adding kitchen scraps and leaves all winter. Decomposition mostly stops once the pile freezes, so material simply stockpiles and breaks down when it thaws. That is fine.
- Layer greens and browns. Kitchen scraps are "greens" (nitrogen); your stockpiled leaves are "browns" (carbon). Cover each addition of scraps with a few handfuls of leaves. That keeps the balance right, controls smell, and hides food from pests.
- Insulate the pile. A bigger pile holds heat longer, and a thick jacket of leaves or straw around it keeps it active later into the fall. In the coldest zones it will still freeze solid eventually, and that is normal.
- Use a closed or rodent-resistant bin in cold climates, since a pile is a tempting winter shelter and food source. Bury fresh scraps in the center rather than leaving them on top.
- Turn it in spring, not in the frozen depths of winter. Once it thaws, give it air and it finishes fast.
A few things to leave out
- Black walnut leaves, which contain juglone, a compound that stunts many garden plants. Keep them out of compost and mulch.
- Diseased foliage, especially from tomatoes, squash, and anything with blight or mildew. A cold backyard pile never gets hot enough to kill the spores.
- Thick, wet, unshredded mats, which go slimy and airless. Shred first and they break down sweet instead of sour.
Rake once, and you have made next year's soil for free. The gardeners with the best beds are almost always the ones who stopped throwing their leaves away.