The Frost Map: Finding Warm Corners and Cold Sinks in a Small Garden

November is the month when a garden stops looking generous and starts telling truths. The leaves are down, the soil is colder than it was a fortnight ago, and the first proper frost is somewhere in the forecast. That is exactly the moment to stop guessing where things should go and start measuring. A frost map is not a fancy landscape plan. It is a rough sketch of your plot, balcony or windowsill, marked with the places that run cold and the places that hold heat. It takes a week, costs very little, and it will save you from repeating the same winter mistakes.

Why a frost map beats guesswork

Most of us garden by feel. We know the sunny end of the garden and the shady bit by the shed, but cold is sneakier. It moves. It drains down slopes, pools behind solid barriers and gathers in low corners. Two beds a few metres apart can have different first-frost dates, and a cold frame that works beautifully in one spot can be a soggy failure in another. If you are planning to overwinter crops, protect a few salads or get an early start in spring, those differences matter.

The kit: two thermometers and a pencil

You do not need a weather station. A couple of cheap min-max thermometers are enough. Digital ones that record the highest and lowest temperature since last reset are ideal, but old-fashioned spirit min-max thermometers work fine. If you can stretch to three, put one at ground level, one at crop height and one somewhere you suspect is a cold pocket. Otherwise, move one thermometer around every few days.

Pick spots that represent real growing conditions: an open bed, the base of a wall, the middle of a patio, the bottom of a slope, a corner behind a fence, a balcony floor, a windowsill. Put the thermometer where the leaves will be, not high up on a post. Reset it in the afternoon, then read it at the same time each morning. Note the minimum overnight, the maximum during the day and anything obvious like wind, rain or a clear sky. A phone note is fine. A scrappy notebook is better because you will keep it.

Walk the plot: where cold air goes

Cold air behaves a bit like water. It is heavy, it sinks and it collects in the lowest available space. That is why the bottom of a sloping garden, the dip in front of a retaining wall and the low corner by the back fence are usually the first places to freeze. Solid fences and hedges can make things worse by blocking the flow, so cold air sits behind them instead of draining away. A frost pocket is not always the coldest place on the map by day; it is often the place that stays cold longest in the morning.

Warm spots are easier to spot once you start looking. A south-facing wall is obvious, but any wall with a bit of sun and thermal mass helps. Brick, stone and concrete soak up heat during the day and release it slowly at night. Paving does the same. A raised bed tucked against a house wall will usually run a degree or two warmer than an exposed bed in the middle of the lawn. Sheltered courtyards and balcony corners can be surprisingly mild, as long as they are not also dark and damp.

Balconies and windowsills count too

Small spaces have microclimates as well, just at a smaller scale. On a balcony, the floor, the back wall and the railings all change how cold air moves. Pots standing on cold concrete may be colder than pots raised on feet, while a pot jammed into a sheltered corner against a warm wall may sail through a frost that scorches one out by the railing. Wind is the other factor. A balcony can be sheltered at ankle height and brutal at waist height, which is why a thermometer at pot level tells you more than a forecast for the nearest town.

Windowsills are trickier still. Glass is a poor insulator, so the pane can be several degrees colder than the room. A sill above a radiator is warm and dry; a sill beside a draughty sash window is not. If you are overwintering herbs or trying an early sowing, put a thermometer on the sill for a few nights. You may find that the bright window is actually the coldest place in the house after dark.

Turning the map into planting decisions

Once you have a week of readings, draw a simple plan. Mark the cold sinks, the warm banks and the neutral middle ground. Then match crops to places rather than forcing plants to cope. The warmest sheltered spot is where a cold frame earns its keep, and where you might try overwintering winter lettuce, land cress or parsley. A sunny wall is a good home for early sowings under cover in late winter, provided the soil is not waterlogged. Open, neutral ground is fine for hardy crops such as broad beans, garlic and overwintering onions, as long as drainage is decent.

The cold pocket is not useless. It is just honest. Use it for things that do not need cosseting, or for infrastructure: a water butt, a leaf mould bin, a stack of pots. If it is wet as well as cold, do not fight it with tender crops. Grow something tough, or leave it alone. The point of the map is not to make every corner productive. It is to stop wasting money and hope on the wrong corner.

A week is enough

You do not need a full winter of data. A week of consistent readings in November, plus a walk round on the first frosty morning, will tell you most of what you need. Look at where the frost sits longest, where the grass stays green, where the wall is still warm at dusk. Then draw it, photograph it and stick it in the shed. Next spring, when you are tempted to sow early, you will have evidence rather than optimism.

Good gardening is often just paying attention in a slightly more organised way. A frost map is that idea in its simplest form: a few thermometers, a pencil and a proper look at the space you have got. Do it before the frost does the mapping for you.