CITRUS IN POTS: GETTING THROUGH WINTER WITHOUT LOSING THE TREE

What kills a potted lemon tree is not the cold. It is the living room.

The usual sequence goes like this: October turns chilly, the tree comes indoors next to the most convenient window, it gets watered as if it were still summer because the leaves are right there, and by February it is bare. The winter did not do that. The position did.

First of all: each citrus takes a different amount of cold

Bringing them all in "just in case" is a mistake, because several are far tougher than people assume.

Kumquat. The hardiest of the lot, down to about -8 °C. In most of Spain it stays out without trouble.

Mandarin and sour orange. Around -6 °C. Outside in a sheltered spot, perfectly happy.

Sweet orange and grapefruit. Between -4 and -5 °C. Outside unless you get hard frosts.

Lemon. Between -2 and -3 °C, and it shows the cold sooner than the others.

Lime. The most tender of them; below zero it already suffers. This one comes in.

In a pot, though, take a couple of degrees off all of those: a root ball freezes far sooner than garden soil, and frozen roots are what actually kills the tree.

If it stays outside

Protect the pot, not the canopy. The weak point is the roots. Wrap the pot in bubble wrap, hessian or an old blanket, or stand it inside a larger pot with dry leaves packed between the two.

Lift it off the ground. Two battens or a few blocks. Standing on cold ground the pot freezes from below, and lifting it also stops water sitting underneath.

Against a south-facing wall. A wall that has taken sun all day gives that heat back at night. It is worth two or three degrees, which is often the whole difference.

Fleece only on the bad nights. On at dusk, off in the morning. Leaving it on all week stops the tree breathing.

If it comes indoors, the spot matters more than the temperature

This is where it really goes wrong. A citrus indoors does not want to be comfortable, it wants to be cool and well lit.

Ideal: 8 to 12 degrees. A conservatory, an enclosed porch, an unheated room, a garage with a window. At that temperature the tree spends the winter half shut down, which is what it should be doing.

Worst: the living room at 22 degrees. The warmth keeps it active and growing, but indoor winter light cannot possibly sustain that. The tree draws on its reserves, and when they run out it drops its leaves. It is the single commonest reason for bare lemon trees in February.

Away from radiators, without exception. Dry radiator air fills the foliage with red spider mite. If there is nowhere else, stand it on a tray of damp pebbles and mist the leaves now and then.

The brightest window you have. South or west facing. And even in a cool room, air it occasionally: still air is what brings fungal problems.

Watering: the mistake that comes round every year

Water far less. In winter a citrus barely drinks. Keeping up the summer rhythm rots the roots, and the cruel part is that the symptom, yellow leaves falling, looks so much like thirst that the instinct is to water more. That is how the tree is lost.

Check before watering. Finger a few centimetres into the compost. If it comes out damp, leave it. It may be once every ten to fifteen days, depending on the spot.

Tepid water. A root ball at 10 degrees watered with very cold tap water is a shock to the roots.

Never leave it standing in a saucer of water. Water, let it drain, empty the saucer.

And no feeding from October to February. Feeding a dormant tree burns the roots.

Yellow leaves: how to read them

Yellow between the veins, veins still green. Chlorosis, iron shortage. Common with hard water. Chelated iron, but in spring.

Whole leaf yellow and dropping by itself. Overwatering. Let it dry out.

Wilting leaves although the compost is damp. The roots are already damaged. Lift the root ball, smell it for sourness and cut away the rot.

Dry brown leaf tips. Air too dry, or salts built up. Water heavily once to flush the pot through.

Fine webbing and tiny dots underneath the leaves. Red spider mite, a central-heating problem. A jet of water on the undersides, repeatedly.

Going back out in spring

In March, once the frosts have finished, the tree goes out. But not straight into full sun: after a winter in low light the foliage scorches exactly like skin on the first day at the beach.

Ten days in the shade, then half shade, then its proper place. That is when feeding starts again, and the moment to pot it on if it has outgrown its container.

In one line: cool, bright, and much less water. Do that and a potted citrus gets through winter without losing a leaf.

For the nights coming up, here is what to do the evening before: how to protect the garden from the first frosts.