A frosted citrus gives itself away: the leaves fall over and dry out yet cling to the stems, the fruit turns soft, and the bark splits. Hardiness runs from key lime, damaged at -3°C, to deciduous Poncirus trifoliata at -20°C, but only a healthy, well-settled tree reaches the figure given for its species.
This depends mainly on the variety itself, but how they’re being grown also has an impact on their frost resistance.
The most important factor is the variety that’s being grown. Secondary factors are also relevant: how old the tree is, how old its organs (branches) are, when it freezes and how long the frost spell lasts.
A short bout of frost, shortly before dawn, with a nice warm sunny day and above-freezing temperatures shouldn’t lead to any problems at all. This is true as long as the temperature didn’t drop below the threshold known for each variety, listed lower down in this article.
However, if the freezing lasts for several days or even weeks, the tree may truly be in danger: citrus trees can only stay dormant for short while, after that they start dying.
A healthy citrus that’s well-settled in will resist frost much better than a young one, or one that’s weak because of a disease or pest.
This is the reason many treatments are sprayed preemptively just before winter, against scale and aphids for instance.
A frozen citrus tree has telling symptoms: its leaves fall over and dry out, but they still cling to the tree.
Frozen fruits are easily identifiable: they’re soft instead of firm, and may show signs of necrosis or even burst open. Bark on branches cracks open.
If your citrus has already frozen over, and if some portions only of your citrus show the symptoms above, then you must go for a drastic pruning of all dead branches and any branch which has burst bark.
Also remove all fruits, because they would infect the tree with diseases if they’re allowed to rot on the tree.
Only perform this pruning at the end of winter (when the vegetation phase begins again).
If even just a small portion of the tree didn’t die, then there’s a chance that the citrus may survive.
Even so, and even if the citrus tree seems to come back to life, it will stay very fragile and will have trouble producing fruit.
The figures below are for the tree itself, not the fruit, and assume a vigorous, well-settled specimen of five to six years with no disease or pest weakening it. They run from the most frost-sensitive to the toughest.
| Citrus | Hardy to (settled tree) | Notes |
|---|---|---|
| Mexican lime / key lime (Citrus x aurantifolia) | 26°F / -3°C | The most frost-sensitive of the commonly grown citrus |
| Persian lime / Tahiti lime (Citrus x latifolia) | 25°F / -4°C | Drops leaves in strong winds and low temperatures; used for punch and perfume |
| Citron, hand of Buddha | 25-26°F / -3 to -4°C | Hand of Buddha is used in fruit jams and jellies |
| Limequat | 25-26°F / -3 to -4°C | — |
| Finger lime | 25+°F / -4°C at least | Slightly colder if winterized. Grow finger lime here |
| Bergamot tree | 21-23°F / -5 to -6°C | Bergamot is used in making perfume, bergamot tea, and as a flavor for candy |
| Lemon tree | 21-23°F / -5 to -6°C | Including the 4 seasons lemon tree |
| Calamondin (indoor orange tree) | 21-23°F / -5 to -6°C | Variegated calamondino isn’t as hardy, only 23-25°F or -4 to -5°C |
| Combava or makrut lime (kaffir lime) | 23-25°F / -4 to -5°C | Fruit and leaves are a spice in cooking |
| Grapefruit tree (Citrus maxima) | 19-21°F / -6 to -7°C | The original tropical species |
| Clementine tree | 17-19°F / -7 to -8°C | All you need to know about the clementine tree |
| Common mandarin orange tree | 17-19°F / -7 to -8°C | growing this one? here! |
| Orange tree | 17-19°F / -7 to -8°C | Same bracket as clementine and common mandarin |
| Pomelo (Citrus paradisi) | 17-19°F / -7 to -8°C | The ‘Star ruby’ pomelo variety produces pink-flesh fruit |
| Bitter orange | 16°F / -9°C | Used in perfume, and a key ingredient of Cointreau and Grand Marnier spirits; also delicious in orange marmalade and beautiful as an ornamental tree |
| Satsuma mandarin orange | 10-14°F / -10 to -12°C | Tree is hardy but fruit already freezes at 25°F (-3 to -4°C) |
| Kumquat | 10-14°F / -10 to -12°C | Fruit freezes at around 25°F (-3 to -4°C) |
| Yuzu | 5°F / about -15°C | Leaves have superior taste, fruit less impressive. More about growing yuzu |
| Poncirus trifoliata | -4°F / -20°C | The hardiest, and the only citrus with deciduous foliage |
Young stems, flower buds and fruits give way well before these thresholds, while larger, older branches come through.
Growing citrus in the ground, outdoors, is only possible in areas where temperatures never drop below freezing. This isn’t the case anywhere in the British Isles, nor in Northern States in the USA.
Take note, though, that all these areas aren’t necessary frost-free all year round: you’re still running a risk of losing the trees if not protected.
Indeed, for example, along the Mediterranean ocean, cold winters in 1985 and 1986 devastated a great number of citrus trees that were growing outdoors. Many magnificent lemon trees and orange trees never recovered and were lost.
For potted citrus, it’s best to bring them indoors in a place that has none or little heating. Such places used to be called “orangeries”, they’re the ancestors of today’s greenhouses.
Temperatures of around 40°C (4 to 6 degrees Celsius) are perfect to protect citrus during winter.
During the rest phase, you must keep a watchful eye on the health of your citrus plants. In particular, scale insects tend to particularly love wind-free, sheltered places.
You can treat citrus trees even in winter if they’re in a sheltered spot: what matters is that the temperature doesn’t freeze.
If you can’t move your pot around, you’ll have to make do with protecting it with some sort of thermal insulator. Bubble wrap works fine if that’s all you’ve got.
For the branches, it’s best to use winterizing fleece. This is a product that makes your plant feel that it’s 3 to 4 degrees warmer than the air outside it.
Read also: how to winterize your plants
>> Learn about the different citrus varieties <<
Some parts of the tree are more vulnerable to freezing, like young stems, flower buds and fruits, but larger, older branches will survive freezing.
