Spring is the best time to get serious about compost. The heap wakes up as the weather warms, the garden produces a flood of green material from spring pruning and the first mowings, and the finished compost from last year is needed now for mulching and planting. A well-run compost heap turns garden waste into the best soil improver there is — free, rich in nutrients and biology, and the foundation of healthy soil. A badly run heap, by contrast, is a slimy, smelly pile that never breaks down. This guide explains how to make compost work in spring.
How composting works
Composting is the work of microbes, fungi and worms, which break down organic material into a dark, crumbly, earth-smelling substance. They need four things: food, air, moisture and warmth. Food means a balance of green material, which is rich in nitrogen — grass clippings, soft prunings, vegetable peelings, manure — and brown material, which is rich in carbon — dry leaves, straw, shredded cardboard, woody prunings. Air means the heap must not be compacted, because the microbes need oxygen. Moisture means the heap should be damp like a wrung-out sponge, not wet and not dry. Warmth comes from the microbes themselves: a working heap heats up in the middle, and that heat is the sign that it is working.
The balance of green and brown is the key. Too much green and the heap becomes wet, slimy and smelly, because the nitrogen-rich material collapses and excludes air. Too much brown and the heap is dry and slow, because the microbes do not have enough nitrogen to work. A rough rule is equal parts green and brown by volume, layered alternately, with a thin layer of soil or finished compost between layers to introduce the microbes. The compost ratio calculator helps you work out the balance for your materials.
Starting and turning a heap in spring
Spring is the time to turn the heap that has been sitting over winter. Turning mixes the material, lets air in, and moves the unrotted material from the outside into the hot centre, where it breaks down fastest. Turn the heap with a fork, moving the whole pile into the next-door bay or onto a sheet, then back again, and water it lightly if it is dry. A heap that is turned every few weeks in spring and summer breaks down in a few months; a heap that is never turned can take a year or more.
Spring is also the time to start a new heap with the season's first materials: spring prunings, the first grass cuttings, and the last of the autumn leaves. Shred or chop woody material before adding it, because whole branches take years to rot. Add materials in layers, keep the heap damp, and cover it with a lid or old carpet to hold in heat and moisture. A heap needs to be at least a cubic yard in volume to heat up properly — a small heap never gets warm enough to break down quickly. Check your local extension service if you want advice on composting systems for your garden.
What to compost, and what not to
Good green materials in spring include grass clippings, soft prunings, annual weeds before they set seed, vegetable and fruit peelings, coffee grounds and manure from herbivores. Good brown materials include dry autumn leaves, straw, shredded newspaper and cardboard, sawdust from untreated wood, and chopped woody prunings. Eggshells, crushed, add calcium. Do not compost meat, fish, dairy or cooked food, because they attract rats and smell. Do not compost diseased plants, because home compost does not get hot enough to kill the disease — see our spring pest control guide for how to dispose of diseased material. Do not compost perennial weeds with roots, or weeds that have set seed, because they survive and spread when the compost is used. Do not compost treated wood, glossy paper, or cat and dog waste.
Grass clippings deserve a warning: a thick layer of fresh clippings mats down, excludes air and turns slimy. Mix clippings with brown material, or spread them thinly and let them dry for a day before adding them. The same applies to any soft, wet material — always mix it with something dry and bulky. Our vermicompost guide explains worm composting, which suits small gardens and kitchen waste.
Using finished compost in spring
Finished compost is dark, crumbly and smells of earth, with no recognisable pieces of the original material in it. Sieve it if you want a fine product for seed sowing, or use it as it is for mulching and soil improvement. Spring is the main season for using compost: spread it two to three inches deep as a mulch around shrubs, flowers and vegetables — see our spring mulching guide — dig it into beds before planting, and mix it into potting mix for containers. Compost feeds the soil slowly all season, improves its structure, and helps it hold moisture, which is why it is the single most valuable thing you can add to a garden.
Compost is not a fertiliser in the ordinary sense: it releases nutrients slowly, and a plant that needs a quick feed still needs one — see our spring fertilizing guide. But for the soil itself, nothing beats compost. A garden that receives compost every spring builds better soil year after year, and the plants show it. If your heap has not finished by spring, use what is ready and leave the rest to finish over the summer; partially rotted compost can be used as a mulch around established plants, but not dug into beds, because it robs the soil of nitrogen as it finishes breaking down.
Fixing a heap that is not working
A heap that smells bad is too wet and too green: add brown material, turn it to let air in, and cover it to keep rain off. A heap that is dry and inactive needs water and green material: water it, add grass clippings or manure, and turn it. A heap that is not heating up is usually too small, too dry, or too brown: add green material, water it, and make the heap bigger. A heap full of flies needs covering with a layer of brown material or soil, and a heap that attracts rats needs meat and cooked food removed and the heap kept hot and turned. If a heap still will not work, check your local extension service for advice — they can usually diagnose a compost problem from a description.
Frequently asked questions
How long does compost take in spring?
A heap turned regularly in warm weather can finish in two to three months. A heap that is never turned can take a year or more. Turning and the right green-brown balance are what make it fast.
What is the green to brown ratio for compost?
Roughly equal parts green and brown by volume, layered alternately. Too much green makes a slimy, smelly heap; too much brown makes a dry, slow heap.
Can I put grass clippings in compost?
Yes, but mix them with brown material or spread them thinly. A thick layer of fresh clippings mats down, excludes air and turns slimy.
What should I not compost?
Meat, fish, dairy, cooked food, diseased plants, perennial weeds, weeds that have set seed, treated wood and pet waste. These attract pests, spread disease or survive the heap.
When is compost ready to use?
When it is dark, crumbly and earth-smelling, with no recognisable pieces left. Use it as a mulch or dig it into beds in spring. If you are unsure, check your local extension service for advice.