Welcome to Top of the Crops. Today, you will learn to grow citrus trees and fruits in a polytunnel for UK gardens! Prepare for a zesty harvest, and for further gardening advice, remember to check out our blog Polytunnel Gardening.
Citrus trees such as orange, lemon, lime, grapefruit etc. thrive in a warm, humid climate and will not survive outside year round in the UK. The good news is that it is still possible to grow citrus fruits in the UK – you just need to make sure that they are protected during the colder months.
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Trees and shrubs in the Citrus genus give us many important fruits. They are native to subtropical and tropical regions of Asia, islands of Southeast Asia, near Oceania and parts of northeastern and central Australia.
Of course, in their native range, they grow in climate conditions very different to our own in the UK. But it is possible for us to provide conditions that allow us to grow these subtropical or tropical species where we live as long as we have somewhere to keep citrus trees warm enough, generally above around 10ºC., over the winter months.
Most citrus form small trees or shrubs that are ideal for growing in containers. They will tend to be less than 1.5m in height. It is best not to grow them as houseplants as they won't enjoy the low humidity in most centrally-heated homes. They will do best in a frost-free greenhouse or polytunnel, a glazed porch or conservatory.
Varieties of citrus fruit that you might consider include oranges, lemons, limes, grapefruit, mandarins, calamondin oranges, kumquats, limequats, tangelos, citrons, kaffir limes etc... some for eating fresh from the tree, some used for juice or in cooking.
Some are available from garden centres or plant nurseries but you will get more choice from specialist online suppliers. Just remember that some will be much more challenging to grow than others.
When choosing which citrus fruit to grow, consider first where you will place your new tree and whether you can provide it with the conditions it needs.
Citrus trees need:
A bright, sunny location, which might be outdoors in summer, but which will almost always have to be undercover during the winter months.
A frost free location, usually needs to be above 10ºC. though some citrus trees are a little hardier than others.
Reasonably high humidity.
A suitable container (if your tree did not come in one you like or needs potting up).
A nutrient-rich soil-based compost/ potting mix with 20% by volume of grit added to improve drainage.
Ideally rainwater for harvesting.
Fertilisers suited to citrus for summer and winter.
You may also need a trowel/spade for potting up, pruning tools, and gardening gloves if you do not have these already.
It may also be helpful to have a trolley of some kind, to make it easier for you to move around the citrus tree – taking it indoors in autumn and perhaps outdoors again in spring once the weather is warming well. A citrus tree in a pot, especially a larger one, can be quite heavy to move around.
Citrus trees will usually come in pots of sufficient size to keep them happy without repotting for several years.
However, if you wish you can choose a more attractive pot of a similar size. Carefully transfer the plant to its new pot, placing it in a suitable growing medium as described above. Take care to keep root disturbance to a minimum.
A citrus tree will only need repotting when the roots fill the current container completely and start appearing through the holes at the base of the pot. Repot in spring, ideally, and choose only a very slightly larger container.
Citrus trees are, it must be said, much more challenging to care for than trees better suited to our climate. Though they can be rewarding, visually and in terms of the yields they can provide, growing these trees in the UK will take quite a lot of time and effort.
One thing that you will have to take care of all year round when growing citrus fruits in the UK is meeting their water needs. Citrus trees will need to be grown in containers in the UK, and like other plants in containers, will need to be watered consistently throughout the year, more frequently than plants growing in the ground.
It is best to water citrus trees with rainwater whenever possible. Remember that water needs will differ with the seasons. In summer, containers need to be checked daily and watered often, especially in hot weather.
In winter, you need to be careful not to overwater. Water thoroughly whenever you water, but let the potting mix partially dry out before watering with tepid rainwater once more. Watering too little in winter is less of an issue than watering too much, so err towards keeping things on the drier side through the coldest part of the year.
Humidity is also important. You can ensure humidity remains high enough by standing pots on trays of pebbles, misting, and/or grouping plants together.
Citrus are pretty 'hungry' plants. They need to be fed year round, with fertiliser specially formulated to meet their needs.
An organic summer citrus feed high in nitrogen should be applied between late March and October. A more balanced organic winter citrus feed should be provided between November and mid-March.
In addition, you should, each May or June, remove and replace the top 5cm of potting mix.
You don't necessarily need to grow more than one citrus tree because these trees are self-fertile. You do not need to hand pollinate. However, pollination can be poor if humidity is too low, so ensure that this is reasonably high when the plants are in flower.
Citrus trees will sometimes produce more fruit than they can sustain. So it is often necessary to thin fruits to ensure that some reach maturity and ripen well.
Most citrus trees around 1m in height should not be allowed to carry more than around 20 fruits. Kumquats are the exception – they are bushy with smaller fruits and you don't need to thin their fruits at all.
Remember that citrus plants will not survive a winter outside in the UK. They must therefore, if placed outside in summer, be brought back under cover or indoors as soon as nighttime temperatures begin to dip in autumn and before the first frosts arrive.
Overwintering citrus trees means making sure their basic growing requirements are met.
Make sure you know what temperatures a particular citrus tree can cope with before you choose it, to make sure you can deliver the conditions that it needs. Some can cope with a frost-free polytunnel or greenhouse as long as temperatures remain above 5ºC.. But others need 10 or even 15ºC to survive.
Look out for cold draughts or sudden temperature fluctuations, which can cause citrus trees to shed their leaves, or to fail in producing flowers or fruits.
Commercially, citrus trees are usually propagated by budding or grafting, as this means the trees can fruit in 2-3 years rather than the usual 7-10. But if you wish, you could also consider propagating citrus through semi-ripe cuttings, for clones of the parent.
Citrus can also be grown from pips. But this will take a long time, and fruits on a tree grown from a pip will not necessarily be the same as those of the parent and may be lower quality.
If you feel like giving it a go, you should ideally sow in March, and provide temperatures of 16ºC. for germination to take place.
Wear gloves to protect against sharp thorns that are often present on citrus trees when pruning. Not much pruning is required but in February, it is a good idea to thin branches that seem overcrowded and cut back straggly or leggy ones by up to 2/3.
In summer, you can also if you wish pinch back the tips of the most vigorous branches in order to encourage branching.
If vigorous, vertical water shoots arise, prune these out lower down and shorten those near the tips of branches. Remove any shoots that emerge from below the graft on a grafted tree.
Citrus fruits ripen very slowly, often taking almost a whole year to develop their full size and mature colouration. Once ripe, you can either pick the fruits, or leave them on the tree until they are needed.
Of course, citrus fruits are good for your health, and you are sure to be able to find plenty of ways to enjoy eating them, or using them in other ways around your home.
In addition to eating them, you can also potentially use citrus fruits to make your own homemade health and beauty products, and cleaning products for your home.
Most of the problems that can crop up when growing citrus fruits revolve around issues with the environmental conditions or care. Make sure you choose the right location for the tree both in summer and in winter, and care for it correctly.
A range of pest problems can also arise when growing citrus – while the plants are indoors look out for sap-suckers like scale insects, mealybugs and red spider mites for example.
Yellow leaves on citrus trees can denote a pest problem, but can also show environmental problems such as temperature issues, underwatering, overwatering, or nutrient deficiencies. When troubleshooting any problems, taking things back to the basic growing requirements is usually a good place to begin.
An unheated polytunnel might not always be warm enough for citrus fruits in winter, but may be a good spot during the summer months if a sheltered and warm outdoors location is not available or when a sudden cold-snap threatens.
Some polytunnels however may keep off the cold enough for some citrus trees to survive the winter months. And of course you might heat a polytunnel to the right degree for the citrus tree you wish to grow.
Consider companion planting to keep your citrus tree healthy. Plant aromatic herbs in pots close by, for example, to attract beneficial insects and to confuse, distract or repel pest species.
Nekrich, A., (2022) The Powerful Health Benefits of Citrus Fruits. The Whole U. [online] Available at: https://thewholeu.uw.edu/2022/05/23/citrus/ [accessed 15/12/23]
