Hot Composting vs Cold Piles

Hot composting can make usable compost in 12 weeks in an insulated bin and kills weed seeds, but it needs at least a cubic metre built in one go, and wet Scottish summers make it harder to keep hot. A cool heap takes four months to three years and cannot be relied on to kill weed roots or seeds, but you can add to it whenever you like. Choose hot if you have seeded weeds to deal with and a free weekend. Choose cool for most everyday home composting.

How the two differ

Both rely on organisms breaking plant material down. The difference is heat. A cool heap is the default: you add material as it comes, with little attention to the mix. It rarely achieves a heat above 60°C and can take up to three years to decompose fully (Garden Organic). Garden compost can take six months to two years to mature (RHS), or four months to over a year (Soil Association), depending on how the heap is managed.

A hot heap is a deliberate build. Chop the materials small and mix them before they go in the container, and within a few days the heap is likely to get hot to the touch, which destroys weed seeds (Garden Organic). It takes more labour, including regular turning. A heap built up over weeks is unlikely to have enough fresh material heating at once, which is why the build is done in one session. Use roughly 25 to 50 per cent soft green material, such as kitchen waste and grass clippings, with the rest brown material such as cardboard, straw and woody prunings (RHS), and do not let grass clippings dominate.

Why a hot heap is harder here

Scotland tends to stay under the influence of Atlantic depressions for much of the summer, unlike other areas of the UK (Met Office). A hot heap needs air pockets, and a heap rained on repeatedly loses them to waterlogging, which cools it. Soil Association advice for a waterlogged heap is to use aerators to create air channels, and the RHS says to cover compost to protect it against rain. A heap that has lost its air goes anaerobic, which usually shows as a slimy, sour-smelling heap, and that can attract flies and vermin: see keeping rats out of your compost heap. Cooler air is likely to be a second drag, since an exposed heap probably loses heat to it faster, most at the edges.

Read the table by region. The East Lothian and Fife coasts have about 27 wet days in summer, so a hot heap there has the best chance of staying dry. On the western coasts it is about 35 and over the high ground over 40, and July maxima on the highest western ground are under 15°C, so a heap there has the least margin.

Building a hot heap that works here

Size and speed. Containers smaller than 1 cubic metre are generally less efficient, though they still make compost, more slowly (RHS). A bigger heap has less surface for its volume, so it probably loses heat more slowly to cool, damp air. Fill the heap in one session from chopped, mixed material, not a little at a time.

Site. Choose a sheltered spot in partial or full shade. An earth base gives soil organisms access, and a hard surface is fine with a few spadefuls of soil added to get it started (RHS).

Cover. Cover or insulate the heap with a tarp, cardboard or an old carpet. This heats the compost and stops it getting too wet (Soil Association). If the heap cools faster than expected it is probably either drying out or waterlogged, and turning it shows which. Water it in summer if it looks dry (RHS). A working hot heap is hot to the touch in the middle within a few days of building or turning.

An insulated hot bin. Commercial insulated hot bins can produce usable compost in 12 weeks and take almost any waste, including cooked food and dairy. They cost more and need careful attention to the balance of browns and greens (Garden Organic). An ordinary bin can still get hot if it is built as a proper hot heap; an insulated bin makes it easier to hold the heat through a cold, wet spell.

Running a cool heap properly

Add material as it becomes available, aim for an even mix of green and brown, and turn it once every few months (Soil Association). Expect four months to over a year if you manage it, and up to three years if you leave it alone. It is slower, but it works with a small heap, irregular feeding and no free weekend. Do not put grains, meat or cooked food in a garden heap, since they attract vermin, and keep perennial weeds such as dandelion, bindweed and couch grass out because they can survive and multiply when the compost goes back on the soil. A cool heap is also slower in cold, wet conditions, so expect the slow end of its range here, but without the sharp failure of a hot heap. Seeded weeds and diseased plants are best kept out of a cool heap too, because it may not kill them. If you have a pile of them, a one-off hot build or the council green waste collection is the better route than the everyday heap.

Which to choose

Two questions decide it. Is there a reason the compost must be seed-free, such as a barrow of seeded weeds? And is there a free weekend to chop and fill a cubic-metre bin and keep it covered? If yes to both, build hot. If not, a cool heap is the method suited to how most people garden, and it still makes good compost on a slower schedule. You can run both: a cool heap for everyday waste and a separate hot batch when something needs dealing with properly.

Finished compost helps heavy, slow-draining soil, which is common across Scotland: see improving clay soil.

Common questions

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