What Happens to Your Compost Heap in Summer and How to Help It Along

The compost heap in July is not the same creature it was in April. You may have left it in good shape at the start of summer, a reasonable balance of greens and browns, a little moisture in the middle, the whole thing ticking along quietly at the back of the garden. Then the heat arrived, or the dry spell did, and something shifted. Either the heap went strangely still and began to smell faintly of nothing, which is not as reassuring as it sounds, or it became a hot, collapsing, slightly alarming mass of half-rotted grass clippings and wilted courgette leaves. Both of these outcomes are common in British summers. Neither is a disaster, but both are asking you for something.

Understanding what summer does to a compost heap is not especially complicated once you have the basic picture. Composting is a biological process driven by microorganisms, fungi, and the occasional cooperative worm, and like all living systems it has preferences. It likes warmth, but not desiccation. It likes a mix of materials. It likes a little air. When July tips into a dry fortnight, the conditions inside the heap can shift quite sharply, and the microorganisms either race ahead without the materials to sustain them, or they slow down and wait for water.

When the Heap Dries Out and Stalls

A compost heap that has dried out in summer is one of the quieter failures in the garden. There is no smell, no activity, no visible change from one week to the next. You might push a fork in and find the interior dusty, the materials barely broken down, sitting in much the same state they were in three weeks ago. This is what happens when the microbial community runs out of moisture: the organisms responsible for decomposition cannot function without it, and the whole process effectively pauses.

The fix is straightforward but needs doing properly. Water the heap slowly and thoroughly rather than drenching the top and assuming the moisture will travel. Use a watering can rather than a hose on full pressure, and work around the edges as well as the centre, which is often drier than it looks. If the heap is in a bin with a lid, remove it on overcast days or during light rain to let natural moisture in. A properly moist compost heap should feel roughly like a wrung-out sponge if you pick up a handful: damp but not dripping. If it crumbles, it needs water. If liquid runs from it, that is a different problem.

Covering the heap with an old piece of carpet, cardboard, or a dedicated compost blanket helps to retain whatever moisture you have managed to restore, particularly during hot spells when surface evaporation is rapid. This is not a permanent measure, just a practical one for the height of summer.

The Opposite Problem: Overheating and Burning Through Greens

Some heaps in July do the opposite of stalling. They become intensely hot at the centre, rising to temperatures that can exceed sixty degrees Celsius in an active heap, and they burn through green materials very quickly. This sounds positive, and in some respects it is: high heat kills weed seeds and pathogens, which is one of the reasons a well-managed heap is worth more than cold composting in a forgotten corner. But a heap that is running very hot on a diet of summer greens without enough carbon-rich material will produce a slimy, nitrogen-heavy mass that smells unpleasant and breaks down unevenly.

In high summer, the garden generates a great deal of green material. Lawn clippings come in volume. The vegetable patch offers bolted lettuces, spent pea plants, bean tops, soft herb cuttings, and the endless surplus of courgette foliage. All of this is nitrogen-rich and adds valuable material to the heap, but it needs balancing. The common guidance about a roughly equal ratio of greens to browns applies all year, but in summer it requires more conscious effort because the browns, the dried stems, cardboard, scrunched newspaper, straw, and wood chip, are less automatically available.

Make a point of stockpiling brown materials during summer precisely because the green supply is at its peak. Cardboard boxes from deliveries, torn into pieces and dampened slightly before adding, are one of the most accessible sources. Dried grass collected after a period of hot weather, brown paper bags, spent compost from pots that have finished their season, and the woody tops of herbaceous plants once they have dried on the plant are all useful. Adding a layer of browns between each significant addition of greens is a habit worth forming in July; it prevents the clumping and matting that turns a heap anaerobic and gives it that sour, silage-like smell.

What to Add from the Summer Garden

The summer garden produces some of the best compost materials of the year if you use them wisely. Soft-leaved spent crops from the vegetable patch break down readily. The tops of broad beans after you have harvested them, the straggly ends of cut-and-come-again lettuce, and the faded foliage of garlic once the bulbs are lifted are all excellent. Do not add anything that shows signs of disease, particularly clubroot-affected brassicas, rust-covered leaves, or the blackened foliage of plants that have suffered from botrytis. These can persist in the heap and reintroduce problems into the garden when the finished compost is used.

Spent flowers from the cutting garden are welcome, as are the first faded blooms of border plants if you are deadheading through summer. Keep an eye on any material from plants treated with persistent herbicides, and if you are adding grass clippings from a lawn that has been treated with a selective weedkiller, it is sensible to leave a gap of several cuts before the clippings go in, as some products remain active through the composting process and can affect seedlings in finished compost.

How Often to Turn in Summer

Turning the compost heap is the part many gardeners do least often and think about least clearly. In summer, the question of frequency depends on what the heap is doing. A very active, hot heap benefits from turning every two to three weeks: this reintroduces oxygen, redistributes moisture, and moves the outer materials that have had less microbial attention into the warmer centre. A stalled, dry heap needs water first and then turning once the moisture has been restored, otherwise you are simply moving dry material around.

From my own experience, the most useful summer turning is often in early July, before the real heat of the month settles in. At this point the heap has had the benefit of June growth, the materials are varied, and a thorough turn with a fork, moving material from the outside edges to the centre and vice versa, sets the heap up well for the weeks ahead. A second turn in late August, once the garden is beginning to slow slightly and there is more time for these tasks, rounds out the season nicely.

If you have a two-bay system, summer is the ideal time to transfer material from an active bay into a maturing bay and start the working bay fresh. The matured compost from the previous year, by now dark and crumbling and smelling of earth rather than vegetation, can be used immediately as a mulch around established plants, pressed into service around courgettes and squashes that appreciate the moisture retention, or stored under cover for autumn use.

How to Tell Whether the Heap Is Doing Its Job

A functioning summer heap should be noticeably warmer than the surrounding air at its core, particularly in the days following a turn. It should smell earthy rather than sour, and the materials should be visibly reducing in volume over a period of weeks. If you added a specific identifiable item, say a spent leek or a sheet of torn cardboard, and you can still find it completely unchanged six weeks later, the heap is not working as well as it should and the checks above are worth working through.

The finished compost, when it arrives, should be dark, crumbly, and pleasant to handle. It should not be possible to identify the original materials. If the heap is producing something patchy, with fully composted material at the centre and recognisable additions around the edges, that simply means the outer layers need more time or another turn to move them into the active zone.

Worm activity is one of the simplest indicators of heap health. Brandling worms, the reddish, vigorous species that prefer composting environments rather than mineral soil, will colonise a heap that is moist, aerated, and well-balanced. Their presence in reasonable numbers is a reliable sign that the process is working. Their absence in a heap that has been standing for months is a sign worth investigating.

The practical upshot for July is this: check the moisture level now if you have not done so since the warmer weather began, add browns if you have been putting in a great deal of lawn clippings, and give the heap a turn if it has been sitting undisturbed since spring. None of these tasks takes long, and they make a considerable difference to what you will have available to work with later in the year.