How to Get More from Your Compost Heap in Warm Weather

By July, the compost heap has usually stopped apologising for itself and started doing something quite alarming. What was a tidy mound in April is now a steaming, collapsing, occasionally pungent presence in the corner of the garden, doing its own thing with minimal supervision. That is, of course, exactly what you want. Except that in warm weather, a compost heap's independence can tip from productive autonomy into something closer to neglect, and there are a handful of summer-specific habits that make the difference between a heap that finishes well and one that simply sits there, caked and motionless, until October.

The heat that speeds decomposition in early summer is a genuine asset, but it comes with conditions. Warmth accelerates microbial activity, which is the engine of the whole process, but those microbes need moisture and air in roughly the same way a fire needs oxygen and fuel. Dry spells, which in a Surrey garden can arrive with very little warning even in an unremarkable British summer, have a particular effect on the outer layers of the heap. They dry out, shrink slightly, and then form a crust that acts as a barrier against both water penetrating inward and air circulating through. The interior may still be working beautifully while the outside has essentially given up. You will not know this is happening unless you look.

Watering the Heap: More Important in July Than Most Months

The test for moisture is simple and a little unglamorous. Take a handful of material from several inches into the heap and squeeze it. It should feel like a wrung-out sponge, damp but not dripping. If it crumbles, it is too dry. If water runs freely, it is too wet, though in a British summer the latter is the rarer problem.

In a dry spell, water the heap directly with a can or a slow trickle from a hose, aiming for the centre rather than the surface. A fast jet of water will run off the outer crust without penetrating. Give it time, turn the material gently to draw the drier outer layers inward, and water again. Some gardeners drape an old piece of carpet or a square of thick cardboard over the heap during dry weeks to slow evaporation; it is not elegant, but it works, and the cardboard will eventually join the heap itself.

Do not water a heap that smells of ammonia. That sharp, eye-watering smell indicates too much nitrogen and too much moisture already, and adding water will compound both problems. More on that shortly.

The Green and Brown Balance Shifts in Summer

The green-to-brown ratio that every composting guide mentions is not a fixed constant. In practice, the balance you need in July is rather different from the one that served you well in October, and getting this wrong is one of the more common reasons a summer heap stalls.

In autumn and winter, gardeners tend to have a ready supply of brown material: fallen leaves, spent stalks, woody prunings that have been through the shredder. By summer, the garden is producing almost entirely green material. Grass clippings arrive in quantity every week. Spent vegetable haulms from the kitchen garden, soft annual weeds, the debris from deadheading the borders. Courgette plants, which appear to be in a competition to overwhelm everything else by August, add great volumes of soft wet material. All of this is nitrogen-rich, and without sufficient carbon to balance it, the heap becomes dense, airless, and starts that ammonia note.

The summer rule of thumb is to match every bucket of grass clippings or soft green material with a roughly equal volume of brown. Torn cardboard, paper bags, the inner tubes from kitchen roll, wood chip from a summer pruning job, dried straw if you have access to it. If you find yourself reaching the bottom of the brown pile in June, it is worth building a small reserve now. Scrunch cardboard into loose balls rather than laying it flat, which helps air move through it rather than forming an impenetrable layer.

Turning Frequency and What It Actually Achieves

Turning a compost heap in summer is not a ritual you need to perform with obsessive regularity, but it does more work in warm weather than in cool. The core of an active heap in July can reach temperatures between 55 and 65 degrees Celsius, hot enough to break down material quickly and, usefully, to kill many weed seeds. The problem is that decomposition is uneven. The hot core processes fast while the cooler edges move slowly. Turning brings the outer material to the centre, exposes everything to air, and redistributes moisture.

In practice, turning every two to three weeks through summer is enough for most heaps. If you have time only for one significant turn, do it at the end of June or in early July when the heap is likely at its most active. Use a fork rather than a spade, and move material from the outside edges to the new centre pile if you can manage it. Even a partial turn, dragging the outer crust inward and loosening the top layer, is considerably better than nothing.

Restarting a Heap That Has Stalled

Every gardener who composts seriously has stood before a heap that simply refuses to cooperate, a dry, greyish mound that looks more like rubble than anything in the process of becoming. This is usually a combination of the problems already described: too dry, too much green material compacted without air, or both. But there is a fourth cause that is less often mentioned, which is that the microbial population has dropped to a level where decomposition can no longer sustain itself.

The simplest remedy is also the one most frequently overlooked. Adding a spadeful of finished compost or good garden soil to a new layer of material introduces a fresh population of microbes and fungi directly into the heap. You do not need much, a single generous spadeful worked into the material as you build a new layer is sufficient. Comfrey leaves, a classic activator in the British kitchen garden tradition, do the same job differently: their high nitrogen content gives the microbial population a boost and speeds up decomposition of the surrounding material. If you are growing comfrey in the garden, a handful of leaves added every few weeks through summer will keep things moving. If you are not growing comfrey, it is worth considering. It takes up little space, asks very little of the gardener, and earns its place many times over.

What to Add and What to Keep Out in Summer

The volume and variety of material available for composting changes considerably through summer, and a few additions that seem obviously useful can cause problems in a warm heap.

Grass clippings are the defining summer compost material, and in large quantities they form a dense mat that excludes air and turns slimy. Add them in thin layers, mixed with brown material, rather than in a single dump. If you have more clippings than the heap can absorb without compacting, spread them thinly between vegetable rows as a moisture-retaining mulch instead, which serves another useful purpose while the surplus is dealt with.

Brassica material should go in with caution if clubroot has been a problem in your kitchen garden. Hot composting, where the internal temperature is consistently high, will kill clubroot spores, but many garden heaps do not reach or sustain those temperatures reliably enough to make this safe. When in doubt, bag and bin brassica roots rather than add them.

Cooked food, meat, and dairy have no place in an open compost heap at any time of year, but in summer the risk is higher: flies are active, the smell carries further, and the heap becomes a genuine problem for neighbouring gardens. A sealed hot composter or a bokashi system handles kitchen waste better than an open heap in warm weather.

Seaweed, if you happen to return from a seaside visit with a car boot full of it (and stranger things have happened in a dedicated gardener's household), is an excellent summer addition. It is rich in trace minerals, breaks down quickly, and provides useful potassium. Rinse it if you can to reduce the salt content, then add it in modest amounts layered with your browns.

The Payoff: What Good Summer Management Produces

A heap that has been properly managed through a British summer, turned regularly, kept moist, balanced between green and brown, will be producing usable compost by late September or October. That timing matters. It aligns almost perfectly with the moment when the garden is beginning to wind down and the ground is crying out for a generous mulch before the first frosts. Well-made compost spread over beds in early autumn feeds the soil, retains warmth, suppresses late weeds, and gives overwintering crops the best possible start.

If you are planning to mulch raised beds or dig compost into borders before winter, now is the moment to start managing the heap with that goal in mind. Decide which heap is intended for autumn use and treat it accordingly: no new large additions after August, consistent turning through July, and enough moisture to keep it active through any dry spells. A second, receiving heap can take everything you produce in September and October, to be ready for the following spring.

The heap you tend carefully this month will earn its keep twice over: once in the ground this autumn, and again in the confidence of knowing you are not starting from scratch.