Citrus are not tropical plants and will grow naturally in fairly specific climatic conditions. The southern Mediterranean is in many ways an ideal climate as is parts of Asia around the Himalayas and south-east Asia where they have grown for thousands of years. They like to be warm during the day and cool at night, warm in summer and cold in winter – without any frost. A few modern citrus hybrids will withstand some light frost, but oranges in particular like warmer temperatures.
Citrus fruits came to Europe from Asia in numbers in the early middle ages, although there are reports of lemons being grown in Italy in the third century AD, and also in the middle east after around 700 AD.
Lemons arrived in the Caribbean with Christopher Columbus on his second voyage west, and spread slowly through the islands and on to the north American mainland until mass cultivation began in Florida and California in the early 1800s.
These days most citrus grows in Italy, Spain, Greece, Turkey, Cyprus, Lebanon, South Africa and Australia. Australia has native citrus varieties which have evolved in isolation away from the trading routes of Asia, Europe and most recently the Americas. Citrus grows rather well in hilly rather than mountainous terrain. High enough for it to get cold at night and in winter, but not high enough for ice and snow.
In northern Europe, growing oranges and lemons in glasshouses became very popular for the wealthy from the 1600s onwards. It was only the rich who could afford to build big enough glasshouses to house their trees and to keep the servants who would haul the trees, planted in huge wooden crates with handles, outside in late spring and back in October before the first frosts struck.
As the vast majority of citrus plants are non-hardy, it’s a mistake to think that any garden or patio in the UK is sheltered enough to grow them outside on a permanent basis. So grow them in pots, and keep them outdoors in summer, but bring them in in winter.
The citrus plants we have at the Secret Garden Club are:
- 2 different types of lemon, a Eureka, and a Meyer lemon. Meyer lemons can withstand the occasional low temperature. That still means you bring it in overwinter.
- A kaffir lime - the aromatic leaves are used in Thai cuisine, as is the zest of the knobbly fruits.
- A grapefruit tree – this particular tree is relatively new to me. I don’t even know if this is the sort of size of fruit I should expect.
- A calamondin – another example of a relatively hardy citrus. There are always plenty of these little fruits on the tree – they are startlingly tart to taste, but make a fantastic lime pickle (calamondin pickle), great in a gin and tonic, and I’m told, make very good marmalade.
- A kumquat tree.This has already produced a heavy crop of ripe fruit and a second crop is developing now.
You might be surprised to know that these are all hybridised types of citrus, except possibly the kumquat, which is odd as until a few years ago, the kumquat was thought not to be a citrus at all but a more distant relation.
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| Citrus fruits for tasting at the Secret Garden Club. Clockwise from top left: pomelo, pink grapefruit, limettas (sweet lemons), clementines, yellow limes, green Mexican limes, oranges, lemons, kumquats |
Origins and hybridisation
The ancestry of citrus plants would keep several series of Who Do You Think You Are going for months. There are over 300 different varieties, with many hybrids, either varieties occurring naturally, or cultivars which have been deliberately crossed by horticulturists. In the last few years botanists have been able to use DNA testing to ascertain precisely how some varieties have occurred and which plants are their ‘parents’, as it were. Before DNA testing, they had to look at external characteristics, habitat, etc.
Keeping track of citrus is complex, but if you’re interested, there is a fantastically comprehensive website called Citrus Pages, which attempts to catalogue all varieties of citrus, with photos and descriptions. Although it's quite scholarly in places, it's also immensely readable, and if you've ever wondered how a lime is related to a grapefruit, or just how many varieties of tangerine there might be, this is the place to visit.
I have also attempted to show the relationships between the common varieties you find in UK shops in this diagram here. Looks complicated? It’s actually over-simplified and any botanist would probably look at it and say, “We-e-e-ell …”
All the evidence suggests that there are three ‘original’ citrus types which have hybridised and crossed to create all the others: the citrons, the mandarins and the pomelos.
Citrons (Citrus medica)
Citron is the oldest known variety of citrus, and were originally used as perfume and a insecticide. Citron seeds have been found in archaeological excavations in Babylon (now Iraq), in ruins which date back to 4000 B.C. They’re known to have been cultivated in the middle east since around 700BC.
Lemons, limes and bergamots all have a citron variety as one of their 'parents'.
Mandarins (Citrus reticulata)
Mandarins originate from China and Japan (hence two of the names to describe mandarin varieties – mandarin and Satsuma – and were brought over to England at the beginning of the 19th century. During the course of 1800s they were introduced into Italy and other Mediterranean countries and into the USA (a variation from the more conventional wetsward Mediterranean to GB route).
Satsumas, tangerines and clementines are all varieties of mandarin – clementines a hybrid between mandarins and sweet oranges. Other hybrids with mandarins as a parent include minneolas, tangelos, ortaniques and other less well-known varieties.
Pomelos (Citrus maxima)
You mainly see pomelos in Asian stores rather than supermarkets. It’s a deceptively big fruit, as the peel and pith are often quite thick; the taste is like a milder, less acidic - and much less juicy - version of the grapefruit.
Pomelos originate from south east Asia, mainly Malaysia. These days it is also grown in Fiji and other Pacific islands but although it was introduced into the Caribbean and the Americas as were other citrus it has never really caught on there.
Pomelo hybrids include grapefruit and sour and sweet oranges (pomelo x mandarin) - the sour varieties inheriting more pomelo features, the sweet versions with more mandarin characteristics.
How to grow a citrus plant
You can raise a plant from sowing citrus pips fairly easily. With oranges and lemons in particular, if you sow in good compost and keep the pot warm, you should get several seedlings from a single pip. If you’re just after an ornamental tree, then that’s a highly cost-effective way to do it.
But if you want fruit I wouldn’t recommend sowing a pip. If your seedling grows true to type (it may well do so, but there’s no guarantee), it will take about 10 years before it produces any fruit. If you want to guarantee fruit, buy a named cultivar from a reputable dealer. This will be a grafted plant, where the top of one plant is melded on to the rootstock of another – grafted citrus will fruit much earlier.
Garden centres will often get deliveries of citrus trees around now, just before Christmas – because they make good Christmas presents, and then again in spring at the start of the new season. Beware of buying a Christmassy citrus – I’ve seen some unsuitable ones. Oranges, for example, like it warmer than other citrus fruits and their fruit are rarely good to eat unless kept in a proper greenhouse – lemons are more suited to our climate. A table of mandarin plants up at a garden centre near here were being kept in conditions that were too warm and sure enough when I lifted up a pot to take a closer look, all the developing fruit fell off. £40 is a lot to ask for a plant that has just lost all its fruit.
You can buy from a specialist, or an outlet which regularly sells citrus - the Secret Garden Club likes M Lanza in Crews Hill - or you can go online. I've had good experiences with the following:
So, you’ve bought your plant, whether it’s a lemon, a Kaffir lime, or a calamondin, and let’s say you’ve bought it round about now. How to care for it? Citrus plants have very specific care requirements depending on the time of year and all relating to the ideal conditions they would like in the wild. If you’re able to meet these requirements, then you’ll find citrus an easy plant to look after. Otherwise, they can appear to be very fussy. It’s worth working out in advance whether you have a good environment for a citrus plant as they can be expensive to buy – but then, looked after well, they can last a lifetime.
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| Standing plants in a moist gravel tray keeps the air around the plant relatively humid |
- Find a cold room – they like a winter temperature of between 7-13 degrees C, no draughts and with relatively humid air. A light unheated bathroom is your best bet, an unheated spare bedroom the next best. Below 7 degrees, the plant will become dormant and won’t flower. Over 13 degrees, and developing fruit may drop (so may the leaves).
- Ensuring relatively humid air can be a challenge. One way to increase the ambient moisture around the plant is to create a gravel tray as we have here. Stand the pot on a tray of gravel and keep the gravel moist – don’t let the water come up to the level of the bottom of the pot, though, you don’t want to waterlog it.
- Another way is to get one of those spray misters and keep it full near the plants. Whenever you go past, give the plants a squirt from the mister.
- In winter, let the soil in the pot dry out between watering. When you do water, give it a good soak and make sure that any excess water can drain away. Standing plants in the (empty) bath is a good way to do this.
- Ideally, water with rainwater. (I have to confess I do this in summer, but not winter.)
- Buy a pot of citrus winter feed – this contains a balance of nutrients that will supplement the nutrients in your soil. Use the feed according to the instructions when you water, or about once every fortnight.
- Check your plant regularly for signs of pests or disease. We’ll talk about these in more detail, but you can keep plants clean and healthy by getting rid of any problems as soon as they arise and before they take hold.
- Remove any leaf fall, bud fall straightaway to keep the areas below and around the plants clean and debris free.
- In springtime, you may well see fruit starting to form – although both lemons and oranges will flower in autumn, winter or spring, so you can get fruit and flowers on the tree at the same time.
- Once all danger of frost is past, and the night temperature no longer goes below 7 degrees, the plant should go outside. It’s not really a kindness to keep them in. Oranges, however, which like warmer temperatures, may well benefit from being kept in an unheated, well-ventilated greenhouse in summer. Generally, citrus will benefit from the fresh air outside and also the difference in temperature between night and day – the bigger the difference, the juicier the fruit, or so people say. This temperature difference is also what deepens the red colour in a blood orange.
- Put the plant somewhere light and bright and preferably south facing to make the most of summer warmth. BUT, not in full sun – under another bigger plant would be ideal. Even more important is that it should be sheltered – a light breeze may be welcome, but it won’t like being exposed to wind.
- Once you see fruit forming, keeping the plant sheltered is even more important, so that the fruit doesn’t drop. If a plant has a lot of immature fruit forming, some fruit drop will be inevitable and not unhealthy.
- Now switch to a citrus summer feed – this will be one high in nitrogen and feed the plant around once a week.
- In summer the soil can be kept lightly moist, but never let it become waterlogged – it must always have good drainage. Ideally water with rainwater.
- If you want to transfer you citrus to a bigger pot, first, desist. Citrus do not need yearly repotting, but will be happy in the same pot for 2-3 years.
- When you do repot, you can use a proprietary citrus compost.
Lemons like slightly cooler conditions than oranges or grapefruit - its green fruit will ripen to yellow during the colder winter nights, but it will still be killed off by any frost. Oranges, however, are more of a sub-tropical plant, more suited to the southern Mediterranean than the cool UK, even in summer. Even so, they, like lemons, need a wide difference in temperature between day and night to help ripen the fruit and turn it from green to orange.
This requirement for warm days and cooler nights also partly explains the colouration of blood oranges. Blood oranges are a mutated variety of sweet oranges, and the greater the difference in temperature, the deeper the red colour of the flesh.
Limes are related to lemons and to papedas, a particular type of ancient wild citrus originating from Asia. The kaffir lime, whose leaves and zest are used in Thai curries, is perhaps the best-known example in the UK of the papeda subgenus.
The most common pests most likely to attack your citrus plants are scale insects, mealybugs and citrus leafminers. Scale insects look like little brown and white discs (or indeed scales) adhering to the trunks and branches. Mealybugs look like wisps of cotton wool stuck in the crooks between stem and leaf, or on buds. Inside that cotton wool there is a little bug, which will look superficially like a tiny woodlouse has been dusted with talc.
Both of these can be most easily removed with a damp toothbrush. Pinch out any badly affected leaves.
Leafminer larvae will actually burrow inside the leaves, leaving discoloured trails on the surface. Any damaged leaves should simply be removed from the tree.
Remove scale, mealybug, or leafminers as soon as you see them – operate a zero tolerance policy. If any of these get a hold on your plant they will debilitate it severely.
The other main problem often suffered by citrus trees is the propensity of the leaves to fall off. The most common reason for this is overwatering in winter, when the plants really can be left to dry out a bit. However, draughts, being moved around too much, or being kept too cold, or, more likely in winter, being kept too warm, will also lead to leaf fall.
You can worry less about the fruit that fall off when they are still very tiny. Some fruit drop is inevitable - like airlines overbooking their flights, many types of fruit tree will set far more fruit than the tree can handle.
