Citrus fruits and cold: 10 golden rules protect them from frost

Generous branches of lemon are increasingly peeking out from the balconies even in the cities of Northern Italy, where it is now not uncommon to be surprised in front of a orange or a mandarin grew up in the buildings. And just in these days, FAI is celebrating the charm and virtues of these symbols of the sun with the tenth edition of Agrumi, an exhibition-market held in Milan. What happens? The increasingly mild winters and the heat islands of urban centres, combined with the protected microclimate of the terraces are transforming the metropolitan botanical landscape, but beware of simplifications. Global warming, in fact, also means climate extremes and therefore abrupt drops in temperature with perhaps short but intense frosts that can catch plants unprepared. To enjoy citrus fruits even in cold areas on the safe side, however, we can choose some of the incredible varieties capable of withstanding temperatures well below zero, or adopt easy precautions able to protect oranges and lemons from the risks of frost.

How is resistance to cold measured?

Until minimum temperature Will my plant be able to resist? The answer depends on many factors. First, let’s check the tolerance to low temperatures reported in the literature for our variety (in botanical jargon this resistance is called “hardiness”). The advice is to consult reliable sources, for example the Internet sites of nurseries specialized in citrus fruits only. Below we report the minimum temperatures tolerated by most of them. The mandarin, for example, can survive down to -11°C. This value, however, always refers to the robustness of the adult plants, at least five or six years of age, with well lignified branches. Furthermore, these minimums are admitted only during the vegetative rest, when the plants do not have tender branches, and only by the healthy specimens planted in the ground, because the cultivation in pot also exposes the roots to the cold. Finally, we talk about minimum temperatures touched by occasional frosts, not of frosts that last many days. Furthermore, the survival capacity of the plant is taken into consideration, not the resistance of the fruits, which instead freeze and fall on sub-zero nights.

Citrus fruits champions of frost resistance

The extreme frost proof citrus they are also a resource for mountain areas. The most classic is the poncirusor trifoliate orange (Poncirus trifoliata), which resists up to – 20°C. This bush has leaves made up of three leaflets which turn gold in autumn and then fall off, leaving the scene for fruits similar to ping pong balls, golden yellow, with a velvety and very fragrant, but very bitter, skin. The branches are very thorny, perfect for creating fallow and roe deer-proof hedges, as was customary in Tuscan villas. The ‘Flying Dragon’ variety, with twisted stems, is particularly ornamental. The other resistance champions are hybrids. Among them, i citrange, crosses between poncirus and orange, resistant up to -12°C, but also citrumelos, with poncirus and grapefruit genes, rustic up to -15°C. We are talking about small deciduous trees of 3-4 meters in height, with fruits similar to grapefruit or oranges, but inedible. For you have edible fruits we can instead choose the citrangequat ‘Thomasville’, born from the union of ponciro and kumquat, which is also remontant.

Watch out for the iuzu, the yellow mandarin for the Po Valley

“Among the varieties that resist frost without problems there is also the yuzu (Citrus junos), which tolerates -12°C for prolonged periods and is certainly the citrus fruit to be planted on the ground in the Po Valley without problems”, explains the collector Davide Chiaravalli, which reproduces 150 particular citrus fruits in its nursery in Monza. “Iuzu is the most cultivated citrus fruit in Japan due to the highly scented rind of the fruit, which looks like yellow mandarins and is used to flavor juices and many savory recipes. It is an evergreen bush with elongated leaves, also scented, and with very thorny branches”. Equally resistant is the Ichang lemona hybrid between yuzu and grapefruit, with lemon-scented leaves and fruits, while the curious Kinkoji grapefruit (Citrus obovoid), with sweet fruits, survives at -10°C. These particular plants are not found in all nurseries, but by searching on the internet “frost-resistant citrus fruit” we will be able to locate the farmers closest to our home who ship.

Chinotto and other great cold-proof classics

Even several citrus fruits among those easily available boast excellent resistance to cold. The best known is thebitter orange or melangle (Citrus aurantium), used as an ornamental tree in many cities. It is completely similar to a sweet orange, but has bitter fruits, good only for jams, and resists down to -9°C. Up to 9 degrees below zero even three very suitable plants for pots survive, such as the chinotto (Citrus myrtifolia), with fruits similar to small tangerines from which the drink of the same name is obtained, the kumquats (Lucky margarita), from mini elongated fruits and the calamondin, Citrus mitis. The common mandarin, Citrus reticulatacan survive at -11°C, always referring to adult plants at rest, as explained before.

The grafting trick, to have frost-free roots

Almost all citrus fruits are grafted on a wild citrus fruit, which increases its resistance to disease. Basically, there is a strong plant – the rootstock – which lends its roots to a more valuable variety. “If the citrus fruits are grafted onto poncirus plants, their resistance to cold increases by several degrees”, explains the nurseryman Davide Chiaravalli, who by applying this technique has recently reconstituted the collection of Habsburg citrus fruits grown in the Orangery of the Royal Villa of Monza in the early 19th century for the project Citrus fruits return to Villa Reale. The reason for this rusticity is easily explained: “in addition to having completely frost-resistant roots, the poncirus goes dormant in winter, therefore it forces the citrus fruit that is grafted on top of it to rest, making it less susceptible to frost damage. Furthermore , the plants grafted onto poncirus remain compact, and are therefore suitable for pots”, explains the expert. Conversely, many rootstocks used for industrially grown citrus in warm regions and in Spain cause the plants to grow continuously, making them brittle in winter. For cold areas, therefore, it is a good idea to order citrus fruits grafted on the poncirus.

Advice for those who love lemons

The lemon it is among the most loved citrus fruits, but it is not a champion of resistance to cold, on the contrary: below zero it suffers serious damage. For fans of the genre, the lemon most resistant to frost, down to -6°C, is that of variety ‘Meyer’, identical to the others in appearance, but with a slightly sweeter taste, because it is crossed with orange. With very fragrant orange blossoms, it is re-flowering, compact and very decorative, and is certainly the best lemon for those who want to try outdoor cultivation on a balcony in Northern Italy, being careful to place the pot in a warm corner sheltered from the wind .

Two tricks to increase plant resistance

We must ensure that the plants are not unprepared when the frost arrives, therefore there must be no tender branches with still young leaves and tissues (a rule that applies to all plants, because recently born stems have a resistance to cold of about 5-6 degrees lower than the rest of the canopy). For this reason, we pay close attention to fertilizations. Those who live in cold areas must feed citrus fruits only at the end of winter, in spring and then at the beginning of summer, without going beyond mid-July. From the summer, then, you have to suspend the fertilizer applications until the following March, because the nourishment stimulates the growth of new shoots. Another trick to increase the cold resistance of citrus fruits is to treat the foliage well with the verdigris (a fungicide allowed in organic farming) in autumn, because this product hardens the tissues. Also pay attention to the weight of the snow, which can break the branches even in frost-proof varieties; in the event of heavy snowfalls, we must intervene by promptly shaking off the snow with a broom, starting from the outermost parts of the crown and gradually proceeding inwards, otherwise the stems will break.

Where to place the citrus pots

In the North and in cold areas, the ideal is to plant citrus fruits close to a wall facing south or west. In fact, the wall accumulates heat during the day and returns it at night. In these cases, we can also decide to cultivate espaliered lemons by accompanying the branches against the wall and fixing them to a grid, like a climber (a tradition in use in the Pescia area, as taught by the history of the Oscar Tintori nursery, which other has activated a winter shelter service for citrus fruits in greenhouses, but also a Citrus Line for advice on these plants, please call 0572429191 or via Whatsapp messages on +39 393 8399488). Corners sheltered from wind and currents are the best, because the air greatly amplifies the effects of the cold on the plants. Indoors in courtyards and between houses, therefore, citrus fruits grow very well, as happens in the “citrus village”, Buggiano Castello (PT). Finally, the balcony cover it protects the plants a lot, maintaining a mild temperature at night.

The virtues of non-woven fabric

Assuming that young plants and delicate varieties must be protected from frost from late autumn to the end of winter, we can move the pots with citrus fruits to a bright but unheated place, for example a veranda or a landing. Alternatively, we can wrap the plant with del non-woven fabric, a breathable material that lets air and light pass through it. We can buy it in garden centers in the form of bags or in rolls to use it in one or more layers in order to cover the entire plant, starting from the top and extending the coverage also to the trunk and the pot, because even the roots fear the frost. Attention, however, to do not cover the plants with clear plastic bagsbecause under the cellophane the temperature rises a lot in the hours of sunshine, stimulating growth, plus a condensation is created which freezes at night.

Not to make mistakes

Only healthy plants do well waves of frost, reporting at most some damage to the outermost branches. To have healthy citrus fruits, let’s assure them at least half a sunny day it’s a specific soil for citrus fruits, which is slightly acidic. Let’s also remember to water them even in winter because, being evergreen, they drink a lot even in this season: let’s water them when the earth is dryfor example once a week, but only in the morning, so that the plant can absorb water during the day before freezing at night, and without spraying the leaves. In the end, pay attention to pruningwhich in cold areas must be done strictly at the end of winter when the minimum temperatures are stable above 10°C, bringing the foliage back to the desired shape and eliminating any dry branches.