Gadenin日本語Get the appMove it before the first frost, not after it
A lemon tolerates a light touch of frost and dislikes everything about it. Bring the pot in when night temperatures start settling below about 41F (5C), which is usually a few weeks before your first real frost. Waiting for the forecast to threaten means you move a tree that has already been chilled, and chilled roots are what causes the leaf drop three weeks later.
Do not move it in one step from the garden to a heated room. Give it a week in a porch, unheated conservatory or garage with light, then move it to its winter position. The tree is adjusting to a drop in light of ninety per cent or more, and a week of half measures cuts the shock.
- Inspect before it crosses the threshold Go over the undersides of leaves and the joints of branches for scale. A few insects outdoors become an infestation in a warm, still room within two months. Deal with them outside, where you can spray without ruining the furniture.
- Hose the canopy Give the whole tree a firm spray of water, top and underside of the leaves. It removes dust, spider mite and a proportion of the scale crawlers before they get indoors.
- Check the drainage hole Turn the pot on its side and look. Roots blocking the hole means the tree will sit wet all winter. Trim them back flush and clear the mesh.
- Cut back the feed now Stop the high nitrogen feed as soon as the tree comes in. A tree in low light cannot use it, and unused nitrogen builds up as salts in the compost.
- Photograph the tree on the day it moves One picture from the same angle each autumn is the only honest record of whether it is gaining or losing leaf year on year. Note the date and the room it went into.
Cool and bright beats warm and dim
The mistake almost everyone makes is putting the tree somewhere comfortable for people. A lemon in a warm living room at 68F (20C) with weak winter light through one window keeps trying to grow, cannot photosynthesise enough to pay for it, and drops leaves to balance the books. It also flowers at the wrong moment and drops those too.
The winter position you want is bright and cool: 41F to 55F (5-13C), by the largest south facing window you have, or in an unheated but frost free room. At that temperature the tree slows down, its water demand falls, and it holds its leaves until spring. If the only spot available is warm, you must add light. A grow light run for ten to twelve hours a day, positioned about a foot (30 cm) above the canopy, compensates for a warm room.
| Winter location | Temperature | What happens |
|---|---|---|
| Unheated frost free room, big window | 41-50F (5-10C) | Best case. Holds leaves, rests, wakes in spring |
| Cool porch or conservatory | 45-55F (7-13C) | Very good. Watch for freezing on the coldest nights |
| Heated room by a bright window | 60-68F (15-20C) | Some leaf drop. Needs supplementary light |
| Heated room away from a window | 68F+ (20C+) | Heavy leaf drop, spider mite, often fatal |
| Unheated garage, no window | Any | Loses all leaves. Only for a fully dormant tree, not a lemon |
Cold roots and yellow leaves
Leaves that turn a flat, uniform pale yellow over several weeks indoors are usually telling you the roots are cold and wet, not that the tree is hungry. Roots below about 55F (13C) stop taking up nitrogen efficiently even when there is plenty of it in the compost, so the tree strips nitrogen out of its oldest leaves and they go yellow from the bottom of the tree upwards.
A pot standing on a cold tiled or concrete floor beside a drafty door is the classic setup for this. Get the pot off the floor onto a wooden block or a thick mat, move it away from the door, and cut watering back so the compost dries between waterings. The yellow leaves already there will not turn green again, but new growth in spring comes back the right color.
Yellowing with a different pattern means something else. If the leaf is yellow between the veins while the veins themselves stay distinctly green, and it shows on older leaves near the fruit, that is magnesium, which a heavy fruit crop pulls out of the leaves. Yellow between the veins on the newest leaves at the tips is iron, and it comes from compost that has gone too alkaline, usually from hard tap water.
| Pattern | Where on the tree | Cause | Fix |
|---|---|---|---|
| Whole leaf pale yellow | Oldest leaves first | Cold roots or nitrogen shortage | Warm the pot, dry it out, feed once growth restarts |
| Yellow between green veins | Older leaves, near fruit | Magnesium | Epsom salts as a foliar spray in spring, 1 tbsp per gallon (15 g per 4.5 L) |
| Yellow between fine veins | Newest leaves at the tips | Iron, locked up by alkaline compost | Chelated iron, and collect rainwater instead of using hard tap water |
| Yellow with brown crisp edges | Scattered anywhere | Salt build-up from feed | Flush the pot with three pot volumes of water |
| Sudden yellow then drop, whole tree | All over within days | Shock from the move or a draft | Stabilize position and temperature, do not repot, wait |
Do not respond to yellow leaves indoors by feeding. A tree that cannot take up what is already in the pot will not take up more, and the extra salts make the problem worse. Feeding is a spring answer.
Water less than feels right
Winter losses are overwhelmingly from overwatering rather than from cold. In a cool room a lemon may want water once every two to three weeks, against every two or three days in summer. Watering on a schedule is what kills them.
Test by weight. Lift the pot, or tip it, after a full watering and again a week later; the difference is obvious once you have felt it twice. Alternatively push a finger two inches (5 cm) into the compost, and only water when it is dry at that depth. When you do water, water properly until it runs from the base, then empty the saucer within half an hour.
Use water at room temperature. A can filled at an outside tap in winter can be near 40F (4C), and pouring that into a pot chills the rootball for a full day and gives you exactly the cold-root yellowing described above.
Going back out in spring
Reverse the autumn process and take longer over it. Once nights are reliably above 45F (7C), put the tree out for a few hours a day in shade, building up over two weeks. Leaves grown in a dim room have thin cuticles and will scorch white in a single afternoon of direct spring sun.
Resume feeding when you see the first new shoots extending, not before. That first flush, encouraged by proper light and a high nitrogen citrus feed, is what replaces any leaves lost over winter.