Cold damage on houseplants

Before doing anything, work out where the cold came from. If the source is still there, the next night does the same thing again.

This is the common one indoors. An entrance door that opens through the day, a window left tilted open on a cold night, or a gap under a balcony door sends a cold stream across a plant that is otherwise standing in a warm room.

Air moving over the leaves does more harm than still cold air at the same reading. That is why one plant on a windowsill can be marked while another two feet away on the same sill is untouched.

Hold the back of your hand where the plant stands, at leaf height, on a cold evening. Moving cold air is easy to feel and hard to spot any other way.

Move the plant, or close the gap. A curtain drawn between the plant and the glass at night takes most of it out.

Glass gets much colder than the room behind it. A leaf resting against the pane loses heat directly into it all night, so the mark follows the shape of the contact rather than the shape of the leaf.

Look for a patch that matches where the leaf was pressed, often a band across the blade or a dark edge along one side. Leaves half an inch off the glass usually come through fine.

Pull the pot back a hand’s width from the window, or turn it so the foliage no longer leans on the pane. Large leaved plants near a window in winter need this checked every few weeks, because they keep growing toward the light.

A plant bought on a cold day can be damaged before it reaches the room, in the walk to the car or in an unheated trunk. The signs appear two or three days later, once the collapsed cells have darkened, and by then the purchase looks like the problem rather than the journey.

Deliveries left standing outside or in a cold hallway do the same thing.

Wrap a plant in paper for the walk out of the shop in cold weather, and take it out of the sleeve as soon as you are home.

A plant in this state needs its usual spot and no extra intervention. Repotting and feeding a cold shocked plant adds a second problem on top of the first.

An unheated bedroom, a hallway, a conservatory or an office over a weekend can sit far below what a tropical plant wants without ever getting near freezing. The damage then arrives slowly instead of overnight, as stalled growth, yellowing lower leaves and soil that stays wet for weeks.

The plant is using very little water at those temperatures. Watering on the summer rhythm in a cold room is how cold conditions turn into root rot, and the symptoms that follow look nothing like the original cause.

Move the plant to a warmer room, or accept the slow spell and water far less until growth restarts.

Species vary widely in what they tolerate, and the plant’s own care guide carries its range. A snake plant and a calathea in the same cold hallway will not report the same result.

Move the plant out of the cold and leave it alone. A warm, bright spot with no draft is the whole treatment, and it needs a few days to show what survived.

Do not cut anything yet. Tissue that looks finished on day one can firm up again, and stems that still have live tissue inside will push new growth from a node lower down. Take off only leaves that have gone soft and slimy, because those rot against the stem.

Water less than usual. A plant that has lost leaves takes up less water, and wet soil around cold damaged roots is the second injury.

Hold off on fertilizer until new leaves are opening. Feeding damaged roots adds salt to a plant that cannot use it.

Marked leaves stay marked. That tissue is dead and will not green up again, so judge recovery by the new growth rather than by the old leaves.

Stems are the part worth waiting on. Scratch a stem lightly with a fingernail. Green under the surface means it is alive and worth keeping, brown and dry all the way through means you can cut back to the next live point.

Give it several weeks in the growing season, longer in winter. A plant that has lost all its leaves but keeps firm green stems often comes back from the base.

If nothing has moved after a couple of months and the stems have gone soft, the cold reached the roots and the plant is not coming back.