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The main reason your lemon tree is losing its leaves is probably stress. Here are four reasons that your tree may be under stress and what to do about each one.
Temperature and/or humidity
Lemon trees need to be in a warm, humid environment, even when they’re in their dormant state in winter. The minimum temperature they can handle is 10°C but they don’t like really hot places either. And some gardening experts state that these trees need 50% humidity.
A centrally heated room in your home is not the best place to keep a lemon tree. Choose somewhere cooler and more humid. If this isn’t possible, build a more moist environment by placing gravel on a tray and stand the pot on that. Fill the tray to just below the top of the gravel with water, and keep it filled to produce humidity for the plant. And turn the heating down in that room.
You may be under-watering or over-watering your lemon tree and sometimes it’s difficult to tell which. In general, if the fallen leaves are a bit crispy, you’re under-watering; if they’re somewhat leathery, you’re over-watering. There are a couple of general rules about watering lemon trees: Water freely in summer and sparingly in winter; and ensure any excess water drains out of the pot after watering so the roots don’t become waterlogged. Experts disagree about how often to water lemon trees, so you have to work with your own plant to determine what’s best for its health.
Lack of sunlight
Lemon trees need to receive about six hours of sunlight per day. If you’re losing the bottom leaves on your tree, it’s probably been in too dark an environment.
Start to slowly increase the hours of off-peak (that is, morning and evening) natural light that your tree is exposed to. An abrupt change could intensify the problem of the tree losing leaves– see Environmental Shock below.
If you purchased your lemon tree in a greenhouse or garden centre it may succumb to environmental shock once you place it in your home. Lemon trees don’t like changes, to temperature, light conditions, humidity, nutrients, watering, and so on. They respond to any or all of these by dropping their (usually lower) leaves.
As a result, you need to acclimate your lemon tree very slowly to its new environment. Keep its initial environment in your home or garden as close as possible to the one it was originally in. In particular, gradually increase the amount of sunlight it receives in the morning and evening slowly over a few weeks. The tree will become more active so you need to keep an eye on its watering needs, increasing your watering schedule as needed.
Some garden experts recommend not to fertilise a new lemon tree for the first year in its new environment until it has settled down. That is unless you have other signs of malnutrition in your tree (yellow leaves, etc.).
If your lemon tree losing leaves is not its only problem, I have some other articles to help you. Please read How to look after a lemon tree, Why is my lemon tree sticky? and Why are my lemon tree leaves turning yellow?