The May garden check that protects tender plants overnight

The risk in May is sneaky: days feel warm, but nights can still drop low enough to nip dahlias, pelargoniums, tomatoes and other tender plants. If you’ve carried trays of young plants in and out, or filled patio pots already, this is when a single chilly night can undo weeks of care.

If your bedding plants look fine in the evening but sulky and limp next morning, it’s often light cold damage, not lack of water or feed.

The one evening check that makes the difference

Your May “safety check” is simple: look at tonight’s forecast low and match it to where your plants are sitting.

Do this an hour or two before dusk:

• Check the temperature: if the forecast is below 7°C, treat tender plants as at risk.
• Check how exposed they are: anything on an open patio, balcony rail, or by a garden gate is more vulnerable than plants tucked against a house wall or in a greenhouse.
• Check how soft the growth is: fresh, sappy leaves on dahlias, cosmos, runner beans and tomatoes mark them out as needing protection.

If the night looks cold and your plants are soft and exposed, protect them before you go to bed, not when you see damage in the morning.

How to protect tender plants quickly

You don’t need anything fancy. The aim is to hold a little warmth around the plants and keep off cold air and wind.

Choose one of these, depending on what you have:

• Moveable pots and trays: slide them into a greenhouse, porch, conservatory or against a sheltered house wall. A bright kitchen or hallway floor is fine for one night. If you lift the pot and it still feels heavy, wait before watering; cold and wet together stress roots.
• Plants already in beds or big containers: cover with horticultural fleece, an old cotton sheet or newspaper held down with stones or pegs. Make a loose “tent” so the cover isn’t pressing on the leaves.
• Wall-hugging climbers and tomatoes: a simple sheet of clear plastic or a bit of fleece pinned to canes makes a quick screen. Leave a gap at the bottom for some airflow.
• Greenhouse or cold frame plants: close vents and doors in late afternoon to trap warmth, then crack them open again in the morning so the sun doesn’t scorch the plants.

Before you water in the evening, push a finger 3–4 cm into the compost. If it’s still cool and damp lower down, don’t drench it just before a cold night; slightly drier compost holds warmth better than a pot that is cold and sodden.

When you can stop worrying about cold nights

In much of the UK, the last frost is often around mid– to late May, but it varies. Higher or more exposed gardens can see a cold snap into early June.

Use these as your guides rather than the calendar:

• Night temperatures are consistently above 8–9°C.
• You’re no longer seeing your breath when you go out late.
• The greenhouse stays mild overnight without extra covers.

At that point, you can ease off the nightly check and start hardening plants fully, but still keep fleece handy for the odd cool spell. If this is happening on your plants – new leaves blackening or turning glassy after a cold night – trim off the worst, keep them just moist, and give them a week before deciding they’re lost.

A quick look at the forecast, a hand on the compost, and a decision about shelter is usually all it takes. Make that two‑minute May check part of your evening routine, and your tender plants are far more likely to sail through to summer intact.