Getting Indoor Plants Through Winter: Light, Water, Heat and the Cold Window

Indoor plants do not have a winter in the sense that garden plants do; the room stays warm and they keep their leaves. But four things change between October and March, and each one quietly kills plants if you keep treating them the way you did in July. Here is what shifts and what we change in response.

1. There is less light, and it comes from lower down

In most of the US, a window in December delivers a third to a half of the light it delivered in June: shorter days, a lower sun, more cloud. Plants respond by slowing down or stopping growth. That is normal and not a problem in itself. The problem is when you carry on watering and feeding a plant that is barely photosynthesizing.

What to do: Move plants closer to the glass, or up into it. A plant that was fine three feet from a south window in summer may need to be on the sill or hanging in the pane now. This is the season when a set of hangers in the upper window earns its place: that is where the light is. Wash the windows; a winter's grime costs more light than you would think. Rotate plants a quarter turn every two weeks, because the low winter sun makes them lean harder. Stop fertilizing from November to February; the plant cannot use it and salts build up.

2. Plants need much less water

This is the big one. Slower growth and lower light mean a plant that drank a cup every five days in summer may need one every two weeks in winter. Meanwhile the soil surface, in dry heated air, looks parched after two days. People see the dry surface and water; the root zone stays wet; the roots rot. More houseplants die of winter overwatering than of anything else.

What to do: Judge by depth, not by surface. Push a finger in to the second knuckle before watering. For reservoir pots, let the tank run dry and stay dry for several days before refilling; in winter we refill our 7-inch pots every three to four weeks rather than every ten days. Water with room-temperature water, never cold from the tap. Water in the morning so the surface dries during the day.

3. The air is dry

Central heating drops indoor humidity to 20 or 30 percent, roughly desert levels. Tropical plants that want 50 percent or more respond with brown leaf tips, crispy edges, and spider mites, which thrive in dry warmth. Ferns, calathea, maranta and palms suffer most.

What to do: Group plants together; each one transpires and the cluster makes its own humid pocket. Keep sensitive plants out of the direct blast of heating vents and radiators. A tray of pebbles and water under the pots helps a little; a small humidifier in the plant corner helps a lot. Misting feels helpful but the effect lasts minutes. Reservoir pots help indirectly: steady soil moisture means the plant is not also fighting drought at the roots. Check the undersides of leaves for fine webbing every couple of weeks; a shower in the bath knocks mites back.

4. The window is cold

The one place a plant can actually get too cold indoors is touching the glass. A single-glazed pane on a January night can be near freezing on the inside surface, and leaves pressed against it get frost damage: translucent, then black patches. Cold drafts from a leaky frame do the same more slowly.

What to do: Keep leaves a hand's width off the glass. Hanging plants are easy: hang them 6 to 8 inches out from the window. Sill plants may need to come forward or onto a stand just in front of the sill. If a window is very drafty, close the curtain or blind between the plants and the glass at night and open it in the morning. Do not put plants on top of radiators or directly above heating vents; the root ball cooks from below.

Plant by plant

Pothos, philodendron, spider plant. Easy. Less water, closer to the glass, done.

Ferns. The humidity plant. Bathroom with a window if you have one; otherwise cluster them and add a humidifier. Keep the reservoir just moist.

Calathea, maranta. Humidity plus filtered water plus no drafts. Brown edges are almost always dry air, not underwatering.

Peace lily. Fine, but it will droop dramatically if the room drops below about 55°F at night. Move it off a cold sill.

Succulents, snake plant, ZZ. Nearly dormant. Water once a month at most. A cool bright window is ideal.

Herbs. Basil will not make it through winter on a north or east sill; harvest it and start again in spring. Mint, parsley and chives cope on a south sill.

The winter checklist

• Moved plants closer to or into the window, a hand's width off the glass
• Washed the windows
• Watering by depth, at roughly half the summer frequency; reservoirs allowed to run dry
• No fertilizer until late February
• Sensitive plants grouped, away from vents, with a humidifier if possible
• Leaf undersides checked for mites every two weeks

Do those six things in early November and most plants come through to March looking much as they did in October. Growth resumes as the days lengthen, usually noticeably by the equinox, which is the moment to repot anything that has outgrown its pot and start feeding again. Until then, the winter rule is simple: more light, less of everything else.