Does your plant need a grow light this winter? Shorten the day and see

Around now, plants that were fine all summer start to sulk. Leaves drop, growth stops, stems stretch toward the glass. It is tempting to blame the radiator or the watering can, but the biggest change is usually simpler: the day got shorter, and the light that does arrive is weaker.

Two things shrink in winter, not one

The first is day length. This is pure geometry, and it depends on how far north you live. Around the winter solstice (21 December), the daylight hours are roughly 9 at the latitude of Madrid, 8 in Paris, 7½ in Amsterdam or Berlin, and about 6 in Stockholm or Helsinki. Midsummer in the same cities runs from around 15 to 19 hours.

The second is intensity. In winter the sun stays low, so its light crosses more atmosphere and hits the window at a shallow angle, and a lot of it bounces off the glass. Cloud, a net curtain or a building opposite take more. So even the hours you have are dimmer than summer hours. Day length is the part you can calculate, so that is what the planner below shows. Treat it as the best case.

What the planner is telling you

Drag the date from late October towards 21 December. At 52°N you lose roughly two hours of daylight in two months, and the loss is fastest right now, which is why the change feels sudden. After the solstice it turns round: the days grow again, slowly at first.

The gap card is a rule of thumb, not a law. Many growers aim for somewhere around 12 to 14 hours of light a day for leafy houseplants, and plants also need a dark stretch to rest. Tough, low-light plants such as snake plants or ZZ plants cope with far less and may simply need less water in the dark months. Plants that came from bright places, such as succulents, citrus or herbs on the sill, are the ones that stretch and fade.

Do you need a lamp? Check before you buy

Many homes do not. Start with what costs nothing:

• Move the plant closer to the window. Light falls off quickly with distance from the glass, so a plant on the sill gets far more than one a metre back. Take it off the floor and onto a sill, shelf or stool.
• Wipe the glass. A dusty window can block a surprising share of the light.
• Rotate the pot a quarter turn each time you water so it grows evenly.
• Water less. A slow plant uses less water, and wet soil in the dark is how roots rot.
• Hold off feeding. Many plants rest in winter and do not need fertiliser.

If a plant still stretches, drops leaves or goes pale after those fixes, a plant lamp can bridge the gap. A lamp only helps if it is close and the right kind, so it is worth reading the specifications on the box: look for a stated light output for plants rather than just "LED", and follow the distance the maker gives. Many are meant to hang within a few tens of centimetres of the leaves.

Put the lamp on a plug-in timer so it runs the same hours each day. Run it in the daytime or early evening, not through the night, so the plant keeps its dark period. Plant lamps can be bright for people too, so point them away from eyes and keep them away from fabric, paper and anything that can heat up. Use a plug-in timer rather than rewiring anything.

A calmer way to see winter

A plant that rests for a few months is not failing. Long nights are part of its year, and most hardy houseplants simply wait for the days to lengthen. You do not need to turn a home into a greenhouse. A bright sill, a cleaner window and a lighter hand with the watering can carry most plants through, and the ones that need more can have a small, timed lamp.

When you do add light, measure first: note the window's width and the shelf's depth and height before choosing a lamp, so it fits where the plant lives.