Houseplants in the heating season: light, water and humidity

When the heating comes on, houseplants enter the hardest half of their year. In Estonia three things hit them at once: very little light (in Tallinn the sun is up for only about six hours in late December), air dried out by radiators, and the summer habit of watering. Almost all of it can be fixed by moving the pot, changing the watering rhythm and choosing the right plants for the sill above the radiator.

What happens to the air and light in winter

Frosty outdoor air holds very little water. Warm it to 20–22 °C indoors and the relative humidity drops: usually 30–40% in a thaw, 20% or lower in a cold spell, and in flats with old windows single-digit readings have been measured at –20 °C. Tropical plants are comfortable at 40–60%, the same range recommended for people.

Dry air shows on the leaves: brown tips and edges, curled young leaves, buds that shrivel. Spider mites multiply fast in warm, dry air, so check the undersides of leaves once a week. Pale pinprick dots and fine webbing mean they have already moved in. Watering does not help here: it is the leaf that dries out, not the compost.

In Estonia light is just as big a problem. From November to January the sun is low and often behind cloud. Light falls off sharply with every step away from the glass: three metres from an ordinary window there is not enough even for shade-tolerant plants. In winter, plants move closer to the window, not deeper into the room.

What to do in the first week of heating

1. Get plants out of the hot air stream. Keep thin-leaved tropical plants 30–50 cm from the radiator. If the radiator sits under the sill and there is no brighter spot, stand the pot on a wooden board or cork mat so the root ball does not overheat. A shelf or cover above the radiator will deflect the warm air into the room.
2. Keep leaves off the glass. In a frost the pane is ice-cold and a leaf touching it gets chilled: first a water-soaked patch, then a dark one.
3. Do not shut plants behind curtains. Curtains drawn at night cut the sill off from the warm room.
4. Air the room away from the plants. A short burst of fresh air is fine if the pot is away from the open window; an icy draught can ruin tender leaves in minutes.

Humidity: what works and what does not

• A humidifier is the only thing that noticeably raises humidity across a whole room. Aim for 40–50%. A hygrometer tells you the real figure rather than a guess.
• Grouping plants – water evaporating from leaves and compost raises humidity slightly around them. The effect is noticeable only in a small room.
• A tray of damp expanded clay or gravel does no harm, but measurements show a gain of only 2–3% a few centimetres above the tray. The base of the pot must not sit in water.
• Misting – in a dry flat the droplets evaporate within 10–15 minutes. The leaf is refreshed, the humidity is not.

How to water in winter

With short days a plant grows slowly and drinks little, while the compost inside the pot stays wet much longer than the dry crust on top suggests. Overwatering is the main cause of winter houseplant deaths, so water by the compost, not by the calendar.

• Tropical foliage plants – push a finger into the compost to the second knuckle, about 3–4 cm. Dry – water. Damp – wait.
• Succulents and snake plants – only when the compost is dry right to the bottom of the pot, usually every 4–8 weeks.
• Water – at room temperature. Tip out whatever is left in the saucer after 15–20 minutes.

No feeding is needed from November to February: the plant cannot use it, and salts build up in the compost and scorch the roots. Start again in March with a half-strength first dose. Plants that keep growing under a grow light can have half strength once a month through winter. Leave repotting until spring; in winter repot only if the roots are rotting.

Plants that cope with a radiator

• Snake plant, Sansevieria (Silver Flame, Tiara Grand Gazelle) – its stiff leaves store water, and dry air does not bother it. It tolerates part shade, but in winter it is better moved closer to the light. Water every one to two months and keep water out of the leaf rosette. Not below 10 °C.
• Haworthia – the most shade-tolerant succulent, happy on an east or west window. In winter it needs a rest: cooler, but not below 10 °C, with occasional light watering, just enough to stop the leaves shrivelling.
• Crown of thorns, Euphorbia milii – handles dry air superbly but needs the brightest sill you have. It drops some leaves before its winter rest; that is normal. The milky sap irritates skin and eyes, so wear gloves.
• Spider plant, Chlorophytum – survives almost anywhere. Its brown tips come more often from chlorine and fluoride in tap water and from excess fertiliser salts than from dry air. Water with standing or filtered water and snip the dry tips off with scissors.

Plants to move away from the radiator

Alocasias, anthuriums, calatheas and other plants with large, thin leaves need 50–60% humidity and warmth. Their place is by a bright window with no radiator below it, in a bathroom that has a window, or next to a humidifier. A windowless bathroom will not do: there is no light.

Alocasia often drops its leaves in winter and goes dormant. The tuber is alive: keep it at 15 °C or above, water sparingly once the top third of the compost is dry, and wait for a new leaf in spring.

Do you need a grow light?

In Estonia from November to January, yes, for light-loving plants (crown of thorns, succulents other than haworthia, flowering plants) and for anything on a north-facing window or far from the glass. Total day length including daylight should be 12–14 hours; a timer makes it easy. The lamp usually hangs 15–30 cm above the tops of the plants, but follow the maker’s instructions.

What not to do

• Do not water on a summer “once a week” schedule – only when the compost is dry
• Do not put tropical plants directly above a radiator or in the draught from an open window
• Do not feed from November to February unless the plant is under a lamp
• Do not leave water in the saucer, and do not mist succulents or hairy leaves
• Do not throw away an alocasia that has lost its leaves: the tuber regrows in spring

Frequently asked questions

Does misting help against dry air?

Hardly: in a heated flat the droplets evaporate in 10–15 minutes and humidity goes straight back to where it was. Only a humidifier really works; grouping plants helps a little.

Why do leaves turn yellow and drop in winter?

Most often it is overwatering, a cold draught when the room is aired, or lack of light. Check the compost with your finger, move the plant away from the open window and closer to the light.

Do houseplants need a grow light in winter?

Light-loving plants and anything on a north-facing window do: 12–14 hours of light a day including daylight. Snake plant, spider plant and haworthia will get through winter without one.

When should I start feeding again?

In March, when the days are noticeably longer and new growth appears. Give the first feed at half strength.

From our catalogue

More plants in the houseplants section; spring fertilisers in for houseplants.