Why covered plants still lose water in winter

Plants under cover—whether in an unheated greenhouse, a cold frame, a porch, a conservatory or tucked against a house wall—are sheltered from rain, but they are not sheltered from water loss. Even in January, a mild, sunny, breezy day can pull moisture from leaves and compost. Hardy evergreens are especially vulnerable because they keep their foliage through winter. They continue to transpire, albeit slowly, and if their roots are frozen or their compost is dry, they cannot replace that lost water. The result is not lush growth but desiccation: curled, scorched or dropped leaves that may only become obvious in spring. Winter watering is therefore not about encouraging growth. It is about keeping plants alive until the growing season returns.

Hardy evergreens: tough, but not invincible

Hardy evergreens such as holly, ivy, euonymus, skimmia, choisya, bay, rosemary, hebe, camellia, photinia, Viburnum tinus and winter heathers are the backbone of many UK gardens. They shrug off frost and wind, but they still need moisture. Under cover, they miss out on natural rainfall, and their leaves keep losing water whenever the air is mild or windy. A sunny day in February can warm leaves well above air temperature, increasing demand for water. Then a sharp frost at night can freeze the compost, locking away the moisture that is already there. Evergreens in pots are particularly at risk because their roots have a limited volume of compost to draw from. Do not assume that "hardy" means "no care". A little attention now prevents brown, brittle foliage later.

Check pots weekly, not on a fixed schedule

Make a weekly check part of your winter routine, and increase it to every few days during mild, sunny or windy spells. The aim is to water only when the compost is actually drying out. A fixed schedule—every Sunday, say—is unreliable because temperatures, wind and light vary so much. Instead, use your senses. Push a finger 2–3 cm into the compost. If it feels dry at that depth, it is time to water. If it feels damp or cold and clammy, wait. For larger pots, check near the edge rather than in the middle, where compost can stay wetter. You can also lift the pot: a dry pot feels surprisingly light, while a wet one is heavy. A wooden stick or moisture meter can help if you are unsure, but your finger is often the best tool.

• Finger test: push 2–3 cm into the compost; water only if it feels dry.
• Weight test: lift the pot; light means dry, heavy means wet.
• Drainage check: clear blocked holes, empty saucers and raise pots on feet.
• Plant signals: drooping, curling or dull leaves mean check immediately, but do not wait for damage.

Water sparingly, in the morning

When you do water, do it in the morning. This gives foliage and surface compost time to dry before the cold night returns. Wet leaves on a freezing night invite rot, fungal problems and unsightly marks, especially on evergreens. Use tepid water rather than icy water straight from an outdoor tap. Water the compost, not the leaves, and aim for the rootball rather than the gaps around it. Water sparingly: a trickle, not a drench. The goal is to moisten dry compost, not to soak it. Let any excess drain away freely, and never leave pots standing in water. If the compost is frozen, do not water at all; wait for a thaw. If a hard frost is forecast, water early in the day so the compost has time to drain before temperatures fall. In an unheated greenhouse, water in the morning and open vents briefly on mild days to reduce humidity, but close them again before evening.

Ways to reduce the need for winter watering

You can make winter watering easier by slowing water loss in the first place. Group pots together so they shelter one another from wind. Mulch the surface of the compost with bark chips or gravel to reduce evaporation. Wrap terracotta and ceramic pots with bubble wrap or hessian to protect roots from freezing, but leave the foliage uncovered. Move smaller pots to a sheltered spot against a wall, ideally out of the strongest sun and wind. Check that pots are raised on feet so drainage holes do not sit in puddles. Use rainwater collected in a butt if you can, but let it come up to a reasonable temperature indoors before using it. Avoid repotting or feeding in winter; the aim is steady survival, not new growth.

Common mistakes to avoid

• Assuming rain reaches plants under cover—it usually does not.
• Watering on a fixed schedule instead of checking the compost first.
• Watering in the evening, leaving foliage wet overnight.
• Soaking the compost and leaving pots in saucers of water.
• Wetting evergreen leaves unnecessarily.
• Ignoring small pots, which dry out far faster than large ones.
• Watering frozen compost, which can damage roots and waste water.
• Letting a mild spell pass without checking, then finding scorched leaves after the next frost.

Winter watering is one of the gentlest jobs in the garden, but it makes a real difference to hardy evergreens under cover. Check pots weekly, water sparingly in the morning, and keep foliage dry before cold nights return. Your plants will reward you with healthy leaves and a strong start in spring.