Harriet Coles
Why Heat Matters in a Small Orchard
A heap that reaches 55–65°C is doing something a cold bin never will. At those temperatures, weed seeds are destroyed, most fungal and bacterial pathogens are knocked back, and the whole mass breaks down in eight to twelve weeks rather than eighteen months. For anyone growing fruit on a domestic scale, that speed matters. You want finished compost ready to mulch around apple, pear and plum trees in spring, not a half-rotted pile you're still turning in July.
The good news is that heat isn't mysterious. It comes from the respiration of microbes working through a well-balanced mix of nitrogen-rich and carbon-rich material. Get the ratio, the air and the moisture roughly right, and the heap does the rest.
Getting the Green and Brown Balance Right
The old rule of thumb is roughly one part green to two parts brown by volume, but treat that as a starting point rather than gospel. If the heap smells sour or slumps into a soggy mat, you have too much green. If it sits inert and dusty, you have too much brown.
Greens supply nitrogen and moisture:
- Grass mowings, added in thin layers no deeper than 5cm, never in wet clumps
- Kitchen peelings, tea leaves and coffee grounds
- Comfrey leaves and nettles, both excellent activators
- Spent annuals and soft prunings, chopped small
- Windfalls and bruised fruit, buried in the middle of the heap
Browns supply carbon and structure:
- Torn cardboard and scrunched newspaper
- Straw, hay and dried bracken
- Dead leaves, kept in a separate bin and added gradually
- Woodchip and shredded prunings, used sparingly — they're slow to break down
Chop everything. A heap of whole cabbage stems and intact twigs will stall no matter how good your ratio is. Aim for pieces no bigger than your thumb.
Building a Heap That Can Hold Its Heat
Size is the single most common reason a heap fails to heat. Below about one cubic metre, there simply isn't enough mass to insulate the core. A bay measuring 1.2m wide, 1.2m deep and 1.2m high is the sweet spot for a garden — big enough to cook, small enough to turn with a fork.
Start with a 15cm layer of coarse twigs or broken sticks to keep air moving underneath. Then build in alternating layers of green and brown, roughly 10cm at a time, watering lightly as you go. Don't compress the layers. A heap that's been stamped down or overloaded with wet grass becomes anaerobic, and anaerobic heaps smell like a drain and never heat properly.
Pallets lashed into a three-bay system work well: one bay filling, one cooking, one curing. If you have room near the orchard, put the bays there — fewer wheelbarrow trips means you'll actually keep it up.
Turning, Moisture and the Temperature Curve
A well-built heap should be noticeably warm within 48 hours. Stick a metal bar or a compost thermometer into the centre and check daily for the first week. The temperature will climb, hold for several days, then fall. When it drops below about 45°C, turn the heap, moving the outside to the middle and the middle to the outside. This reintroduces oxygen and re-ignites the microbes.
For fast results, turn every second or third day for the first fortnight, then weekly. Most gardeners settle on turning once a week — still enough to keep things hot without turning compost into a part-time job.
Moisture should feel like a wrung-out sponge. Squeeze a handful: one or two drops should appear, no more. In a dry summer, water each layer as you turn. In a wet autumn, cover the heap with a lid, old carpet or a straw bale — but never seal it with plastic sheeting, which starves it of air.
Where the Compost Should Go — and Where It Shouldn't
Finished compost is best used as a mulch rather than dug in. Spread a 5–8cm layer around the drip line of fruit trees in early spring, keeping it clear of the trunk by a hand's width to avoid collar rot. It feeds the soil slowly, improves structure on heavy clay, and holds moisture through a dry July. A thin scatter over raspberry rows and strawberry beds does the same job.
One important exception: keep it well away from a wildflower meadow. Meadows thrive on poor, low-fertility soil. Enriching them with compost or manure encourages coarse grasses, docks and nettles at the expense of ox-eye daisies, knapweed and birdsfoot trefoil. Mow the meadow in late summer and add the cuttings to the compost heap instead — you get the fertility, and the flowers keep theirs.
Keeping It Going Through the Year
Winter slows everything. A heap built in October may sit cool until March, so try to build one final batch in early autumn while there's still warmth in the air, and insulate the sides with straw bales. Keep adding to a second bay over winter rather than disturbing the cooking one.
If the heap turns slimy and sour, it's too wet and too green — fork in cardboard, turn it, and leave the lid off for a day. If nothing happens at all, it's too dry or too brown: add grass clippings or comfrey, water thoroughly, and turn. A hot heap is rarely perfectly behaved, but with a bit of attention it will quietly turn your orchard prunings, kitchen scraps and mowings into the best soil amendment your fruit trees will ever get.
