If a houseplant droops or develops dark leaves beside a cold window, move it away from cold glass and draughts, then check the compost before watering. Chilling, overwatering and thirst can look similar. The RHS troubleshooting guide lists blackened leaves, drooping and collapse among cold-damage symptoms, but also warns that overwatering can cause wilting.[2]
What makes a windowsill risky in cold weather?
The brightest daytime position is not automatically the safest overnight. Check whether leaves touch the pane, a window catches poorly, or a nearby external door opens repeatedly. Also look at the curtains: a plant left between a closed curtain and the glass can occupy a very different environment from the room.
The University of Connecticut advises keeping foliage off cold glass and plants away from draughty doors and windows.[3] Illinois Extension also highlights cold windows, heat vents and large temperature swings as winter stresses.[4]
Photograph the plant and its surroundings before moving it. Record the side facing the pane and the date symptoms appeared. This gives you something more useful than trying to remember whether a patch has grown.
How can you separate a draught from a watering problem?
Start with three separate checks rather than choosing a cause from one leaf.
Was there a plausible cold exposure?
Think about an open window, a cold night behind curtains, a move into an unheated porch or a recent journey home. Check the plant's stated minimum temperature rather than applying one threshold to every houseplant. RHS guidance makes that species-specific check part of diagnosing temperature damage.[2]
Put a minimum–maximum thermometer at plant height and inspect the overnight reading. The University of Arizona Extension recommends this type of thermometer to identify differences within a building.[6] A comfortable reading by the sofa does not tell you what happened beside the glass.
What is happening in the root zone?
Lift the nursery pot out of its cover pot and check for collected water. Feel accessible compost below the surface without digging through roots. Compare its weight with the plant's usual watered and drained weight.
Wet compost with drooping foliage is not a reason to add more water. If the pot feels light and the root ball is dry, water need may be part of the problem, even if the window is also unsuitable. Illinois Extension notes that wilting can occur with either underwatering or overwatering and that species requirements differ.[19] Our overwatered versus underwatered comparison covers that distinction.
Is the damage local or widespread?
Mark which leaves are affected: those touching the glass, those facing a draught, or leaves throughout the plant. Local damage can help you decide where to investigate, but it does not prove the cause. Continue checking water, light and temperature together rather than treating a pattern as a laboratory test.
What should you do first?
Choose a bright, stable position away from the pane and the path of cold air. Do not transfer the plant straight onto a radiator or beside a hot air outlet. UConn recommends avoiding both hot and cold air sources.[3]
If the compost is appropriately moist, leave the watering can alone. If it is dry enough for that species to need water, water thoroughly using room-temperature water, let it drain and empty the saucer. Do not adopt a universal winter interval.[19]
Keep a short log of the new position, moisture checks and fresh symptoms. Make one reasoned adjustment at a time where possible. Our winter houseplant care guide gives the wider seasonal context; this check is specifically about the window and root zone.
Will cold-damaged leaves turn green again?
Completely blackened or dead leaf tissue will not recover. The RHS advises removing fully damaged leaves while retaining predominantly green ones until fresh growth appears.[2] Avoid stripping a partly green plant simply to make it look tidy.
Do not force growth with feed as an immediate response. UConn advises fertilising plants when they are actively growing.[3] First establish whether the plant has a suitable position and functioning roots.
When should you investigate beyond the window?
If the plant keeps declining after relocation, or the compost remains wet and roots are deteriorating, investigate the root system rather than repeatedly blaming a draught. UConn guidance on ailing houseplants explains how damaged roots and repeated watering can perpetuate decline.[20] See our root rot guide for the next checks.
Use Green Sage's identification aid if the plant's identity is uncertain. Treat the suggestion as help with choosing species guidance, not a certain diagnosis of cold injury.
Sources
[2] How to Help a Poorly Houseplant | RHS Advice
[4] How to overwinter house plants | The Garden Scoop | Illinois Extension | Illinois
[6] Indoor Plant Selection and Care | UA Cooperative Extension