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Everything you have always wanted to know about citrus trees and overwintering them

Autumn has barely started and the first colds are already doing the rounds. "I'll make you up a hot lemon drink" can often be heard, which gives rise to the following question:

Can I sow pips to grow a lemon tree?

Well, as with all seedlings, the offspring will be different from the mother plant because they also contain genetic material from their "father". Therefore - like for all fruits - grafting is the only way to maintain lemon varieties. To achieve this, branches from the desired type are grafted on to "root donors" which act as grafting bases.

Although you can often successfully plant citrus pips, the resulting trees often only produce fruit many years later, and the quantity and quality of the fruit is uncertain because nobody has tested and reported on the exact gene combination of the seedlings beforehand. However, interestingly enough in some cases citrus seedlings can produce true-to-variety fruit when pips are sown. Trying it out for yourself pays off. Germination is triggered when the seeds get cold (place them in the vegetable compartment of a fridge at 1-5° C for 14 days).

You are sure to enjoy seeing the lemon's germination leaves push up through the sowing soil to see the light of day. And talking of light: Citrus pips should be sown at the end of February or start of March when light levels and daytime temperatures are rising again. The weather should be bright and warm with constant germination temperatures of around 21-25°C. The seedlings need a very bright location.


You can find out about the best way to care for your citrus tree in another GARDENA Newsletter Special next spring, at the start of the new open-air citrus season.

But since citrus trees will soon be requiring winter care, we wish to provide specific information on this matter in this GARDENA Newsletter Special:

Citrus trees are a little tricky when it comes to overwintering, as you may already know. People often ask what you can do to prevent leaves from being shed, which often occurs in late winter - usually due to incorrect care.

Understanding the tree

The species thrives in tropical and subtropical climates but can also cope with the Mediterranean climate which - in winter - can reach zero degrees or just below. This means the following when it comes to their care: Citrus trees need a lot of light in both summer and winter, just as they would get in their natural habitats.

Light and temperature are mutually dependent since they act both together and alone to activate the tree's metabolism - that's what's tricky about caring for citrus trees in the winter. To put it plainly: The more light the tree receives (which triggers photosynthesis), the warmer it needs to be to achieve a sufficient metabolic rate. The more heat the tree receives (which speeds up its metabolic rate), the more light it requires in order to achieve sufficient photosynthesis.

If the tree's metabolism is too fast during winter because the temperature is too high (e.g. if the plant is in a living room, in particular behind a curtain) and if it does not have sufficient light, it metabolises more substances than it can generate through photosynthesis - it consumes itself.

However, it makes sense to hibernate the plant by placing it in a very light location at a lower temperature (this also includes frost-free ventilation of the tree's winter location). This is because at our latitude the brightest location possible is like a dark room compared with tropical light conditions, especially in winter. For those among you who love numbers: I measured a brightness of 190,000 to 210,000 lux around lunchtime in Zanzibar in February this year. At the same time, it was maybe 10-15,000 lux outside in Germany. A very bright winter home (such as a small greenhouse or orangery) can often only provide 10,000 lux or less. A normal winter home (people often say "I put it in a cool corridor near a north-facing window where the light is at its brightest") only reaches around 1,000 lux; the brightness of living rooms is normally 500 to 800 lux. You see: In such conditions it's more or less a case of crossing your fingers and hoping for the best in countries such as ours when it comes to overwintering citrus trees, due to the difference between the light they need and the amount of light they actually get.

Now you've had the information, let's get back to practical measures.

Three overwintering strategies have proven to be successful.

A.: Overwinter the tree in a nursery's greenhouse or at home in a small greenhouse, conservatory or orangery (www.florino.de).

B.: Overwinter the tree in a bright location at a temperature of between 5 and 10°C; ventilate the room frequently and provide shade if warm sunlight penetrates the room (from the end of January, especially for south-facing windows).

C.: Overwinter the tree in a very bright location at a temperature of around 16/18 to 20°C.

Variant C is the most difficult, as such conditions are almost impossible to create. In these parts, we have poorer light conditions as stated above and we normally heat living rooms up to at least 21°C on cold winter days. Rooms at just 16 to 18 degrees, which would be preferable, are at best only rooms which are rarely used or cool corridors. In such rooms, too, trees should be placed as near to a window as possible.

On the topic of temperature:

Leaves are often also shed because the trees are too wet and because there is a major temperature difference between the roots and leaves: "Hot head and cold feet" as gardeners call it - or indeed "hot feet and cold head". The reason for this is rarely noticed: As the room heats up, most of the tree warms up to the room temperature of, say, 21°C. But it is standing on a floor with a temperature of only 16 to 18 degrees. The colder roots work more slowly than the warm leaves; the tree becomes stressed and sheds its leaves. The remedy: Separate the base of the tree from the cold floor by placing it into a mobile stand or a pot with castors and sufficient ventilation underneath or place a thick polystyrene sheet between the floor and the pot to insulate the tree against the cold of the floor.

The opposite situation: The tree might be standing on underfloor heating so the roots heat up to 20 to 28 degrees and the crown is only at 18 to 22 degrees. A similar thing happens here: The tree becomes stressed because now the roots are more active than the leaves. Result: The leaves are shed.

Well, that's provided a lot of reading material for you! However: The better you understand citrus trees and how you can influence them during the overwintering phase, the better you can juggle the situation and develop the best overwintering strategy for your situation at home. And you can respond more quickly with alternative actions if problems do occur.

These light/temperature relationships are comparatively simple when compared with fertilising and watering strategies in the colder six months of the year:

Do not fertilise from September to February; if the plant is overwintered in the cold (Variant B above), do not fertilise from September to March. Treat a lack of iron (green leaf veins, yellow leaves) by adding ferrous feed.

Water using soft water at room temperature. Do not use hard water as this could result in a lack of iron. For warmer overwintering, water very carefully in accordance with the situation; in particular, prevent over-watering and water-logging. More trees drown than dry out! For cool overwintering, keep the tree drier (slightly earth-moist soil) - the soil block should not be straw dry and the tree should not dry out.

Leave the tree outside for as long as possible and put it back outside as early as possible in spring.

In particular in late winter and early spring, you can place the tree in a protected location outside. The location should not heat up to the extent that the temperatures at night and during the day differ greatly, e.g. night-time 5°C, daytime 20-25°C.

Before moving the tree inside, clean it thoroughly and do everything you can to ensure that no snails or lice overwinter with it. If necessary, you can prune it at this time. Thin out very tight crowns so that more light can penetrate. At the end of January, check the pest threat (on the rise at this time of year). Scale insects and the first mealy bugs are already on the move now. The last to show up are the aphids, but even they appear from March onwards.

We wish you every success in overwintering your citrus trees!

One more tip:

People wanting a small, pretty citrus tree which can also deal with warmer, slightly less bright locations in winter should switch to fortunella - the kumquat!