Do I need to water anything in winter?

The four situations where plants genuinely dry out in a British winter despite the rain, how to tell which of yours are at risk, and how to water without doing harm.

Difficulty: beginner
Time: 10 min
Read: 4 min

Short answer

Four groups need checking: containers, anything under an overhang or against a wall in the rain shadow, evergreens in cold drying wind, and anything under cover in a greenhouse, cold frame or cloche. Everything else in open ground almost never needs water in a British winter. Check by weight and by feel rather than on a schedule, water sparingly in the morning of a mild day, and never onto frozen ground.

It sounds absurd in a British winter, and yet winter drought is a real and common cause of loss — particularly for pots and for evergreens. The rain is falling; it is just not reaching the plant, or the plant cannot take it up.

Step by step

Work out which of your plants are actually cut off from rain.Containers, anything under eaves or a dense canopy, anything under cover, and anything under a solid overhang. These are the four categories; everything else in open ground is generally fine.
Check by weight and touch, not by schedule.Lift a pot — dry compost is obviously lighter. Push a finger into the top of the compost, or use a trowel in the ground. Winter watering on a routine is the fastest way to drown something.
Water in the morning of a mild day.Water applied in the evening sits cold in the compost overnight and can freeze around the roots. Morning gives the day to drain and the plant to take some up.
Water sparingly and check it drains.Enough to moisten the root ball, not enough to leave the pot standing wet. Water until a little emerges from the drainage holes and stop. Never leave a saucer under a pot in winter.
Keep water off foliage and crowns.Water at the base. Wet foliage in cold still weather stays wet for days and encourages rot, and water sitting in a crown freezes there.
Never water frozen ground or a frozen pot.It cannot be taken up, it can freeze on the surface and make things worse, and the plant's problem in a freeze is not the water supply. Wait for a thaw.
Water before a cold spell, not during it.For evergreens especially, going into a cold dry snap with moist soil around the roots is what limits desiccation damage. Once the ground is frozen there is nothing you can do.

Tips

Evergreens are the winter-drought casualties people never expect, because they keep transpiring all winter while the soil is too cold for the roots to work properly. A newly planted evergreen in its first winter is the most exposed of all.
Grouping pots together against a wall reduces exposure and slows drying, as well as buffering temperature.
Rainwater from a butt is better than tap water for anything ericaceous, and in winter there is no shortage of it.
A pot that has dried out completely will shed water straight down the gap between the compost and the pot wall. Stand it in a bucket of water for an hour instead.

Common mistakes

Assuming winter rain waters everything — It waters open ground. Pots, rain shadows under eaves, dense evergreen canopies and anything under cover can be completely dry after months of rain.
Watering on a schedule through winter — Plants use a fraction of the water they do in summer, evaporation is minimal, and routine watering keeps compost saturated at exactly the temperature at which roots rot.
Watering in the evening in a frost period — Wet compost that freezes solid overnight around the roots is a worse outcome than the mild dryness you were correcting.

If it doesn't work

A pot plant looks dry and lifeless in midwinter even though it has rained

Cause: Rain does not penetrate a dense evergreen canopy, and a pot in the lee of a wall or under an overhang may get almost none of it — Fix: Lift the pot. A dry pot is startlingly light. Water sparingly on a mild morning until it just begins to run from the drainage holes, then leave it.

Evergreen leaves browning at the tips and edges through a cold dry spell

Cause: Winter desiccation — the leaves lose water to cold drying wind while frozen or cold soil stops the roots replacing it — Fix: You cannot water frozen ground. Reduce exposure with a temporary windbreak, and water thoroughly in a mild spell before the next cold snap so the plant goes into it with a full reservoir.

A border against a house wall is dust dry in February

Cause: The rain shadow under the eaves. That strip can receive almost no rain all year and it is easy not to notice in winter — Fix: Check it with a trowel rather than by eye. Water it in mild spells, and mulch it so what it does get is retained.

Plants in a greenhouse or cold frame are wilting in winter

Cause: Nothing under cover gets any rain at all, and they still transpire on bright days — Fix: Check weekly and water very sparingly in the morning, keeping water off the foliage. Overwatering under cover in winter causes more losses than underwatering.

Questions people ask

How often should I check pots in winter?

Every couple of weeks is plenty for most, weekly for small pots and anything in a sheltered dry spot. The check takes seconds; it is the routine watering rather than the routine checking that causes harm.

What about plants in an unheated greenhouse?

They need checking weekly and watering very sparingly, in the morning, at the base. Ventilate on mild days as well — still, damp air under glass in winter is where grey mould starts.

Should I water before a frost to protect plants?

Watering the soil, yes — moist soil holds and releases more heat overnight than dry soil, which helps slightly on a marginal night. Watering the plant itself, no.