Why Late Frost Is the Sneakiest Threat of Spring

There is nothing quite so dispiriting as walking out on a bright May morning to find the courgettes you lovingly raised on the windowsill collapsed and blackened. Late frost does its damage quietly, usually between two and five in the morning, when the sky clears, the wind drops and the heat your soil has been hoarding all day simply radiates away into space.

It catches people out because the daytime weather can be glorious. A sunny 16°C afternoon tells you nothing about what happens at 3am. In most of the UK, the risk of a ground frost runs until roughly mid-May in southern and coastal areas, late May across the Midlands and the north, and into early June in upland Scotland and exposed rural spots. Gardens in valleys and low-lying ground can be two or three degrees colder than a nearby town, because cold air drains downhill and pools like water. If your plot sits at the bottom of a slope, treat every forecast with suspicion.

Fleece: Simple, Cheap and Remarkably Effective

Horticultural fleece is the workhorse of frost protection. A lightweight 17g fleece will lift the temperature around your plants by two or three degrees, which is often all you need. Heavier 30g grades offer more insulation for a hard snap, and doubling up a lightweight fleece works almost as well.

The trick is to keep it off the leaves. Drape it over hoops, cane arches or even a few bent sticks so the fabric sits like a tent rather than a blanket. Where fleece touches foliage, that contact point freezes first. Weight the edges down properly with stones, pegs or a spadeful of soil, because a gusty night will whip a loose sheet off in minutes and leave your seedlings completely exposed.

Timing matters too. Put fleece on in the late afternoon, while the soil is still warm, so it traps that heat rather than simply blocking the cold. Take it off mid-morning once the temperature has risen, and don't leave it on through a warm, sunny day — plants under fleece can overheat surprisingly quickly.

Cloches, Cold Frames and Improvised Covers

For individual plants, nothing beats a cloche. Bell cloches are excellent for a single tender specimen, while tunnel cloches suit a short row of lettuce or beetroot. You can improvise perfectly well with what you already have:

• Cut the base off a large plastic drinks bottle and pop it over a young plant, lid removed for ventilation on mild days.
• Turn an old cold frame into a nursery for pots, propping the lid open slightly during the day.
• Use a traditional Victorian-style glass cloche if you have one — heavy, elegant and superbly insulating.
• Lay a double thickness of newspaper or an old curtain over a row as a stopgap on a single cold night, removing it first thing.

Whatever you use, ventilation is essential. A sealed cloche on a sunny morning can reach 30°C inside and cook your plants far faster than frost ever would.

Hardening Off: The Step Most of Us Skip

Frost is only half the problem. A seedling raised indoors has soft, thin leaves and no tolerance for wind, strong sun or fluctuating temperatures. Even if the mercury stays above zero, plunging it straight into the garden is a shock it may not recover from.

Harden plants off gradually over one to two weeks. Start by putting them outside for an hour in a sheltered, shady spot on a mild day, then bring them back in. Add an hour or so each day, gradually moving them into more sun and more breeze. After about a week, leave them out for a full day and bring them in at night. By the end of the fortnight they can stay out overnight — but only once the forecast is reliably frost-free. If a cold night is predicted, bring them in or cover them. A fortnight of patience here prevents a whole season of sulking.

Small Habits That Make a Big Difference

A few simple routines will see you through the tricky weeks:

• Water in the morning. Damp compost and wet foliage freeze faster than dry, so avoid evening watering before a cold night.
• Skip the nitrogen feed. Rich, sappy growth is far more frost-tender than slower, tougher growth. Hold off until the risk has passed.
• Mulch around plants. A layer of compost or bark locks warmth into the soil and keeps roots insulated.
• Use a windbreak. A row of fleece on canes, or a low hedge of hardy plants, stops cold air sweeping across your beds.
• Group pots together against a south-facing wall, where bricks and paving release stored heat overnight.
• Check the forecast daily and pay attention to the minimum overnight temperature, not the daytime high.

If Frost Catches You Out Anyway

Don't panic and don't reach for the secateurs. Frosted foliage often looks far worse than it is, and plants frequently regrow from basal buds you can't yet see. Water the affected plants gently, keep them protected from the next cold night, and give them a fortnight to show signs of life before you decide their fate. Tender seedlings that have been completely blackened are usually a lost cause, but it's worth sowing a few spare seeds as insurance rather than mourning the originals.

Late frost is part of gardening in Britain, and every grower loses something to it eventually. Cover what you can, harden off what you've raised, watch the overnight forecast like a hawk, and you'll come through the spring with far more plants than you lose.