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Hardy citrus varieties

Hardy citrus plants Lubera

This is probably the dream of every citrus lover: hardy citrus plants that can withstand any cold, any climate, even north of the Alps. Must this remain a dream?

   
 
Citrus keraji

A frost hardy citrus variety

£37.40 *

Ichang Papeda, Switrus® ichangensis

A frost hardy citrus variety with an aromatic peel

£37.40 *

Trifoliate orange

Poncirus trifoliata

£27.90 *

Yuzu

Citrus junos

From £37.40 *

   
 

More information about frost hardy citrus plants

No, because many citrus plants already bring some frost hardiness from their areas of origin in China and India, at the foot of the Himalayas. Bitter oranges, for example, can withstand several sub-zero temperatures for a short time - and so can many other citrus varieties. In nature, however, it is always the case: if there is a bit of frost resistance, there is more of it. The plants adapt to their environmental conditions over long periods of time quite easily. In northern China, at the foot of the Himalayas, citrus varieties and species have developed that can survive a whole winter even when planted out north of the Alps.

 

 

The trifoliate orange - a hardy citrus plant for every garden

Here, Poncirus trifoliata, the trifoliate orange, which belongs botanically to a different genus, but is nevertheless very closely related to citrus plants and can also be crossed with most other citrus species without problems. Another source of frost hardiness is the Ichang papeda, Citrus ichangensis, which can easily withstand -15°C. In contrast to Poncirus trifoliata, it also keeps its leaves in the winter. From these two sources of frost resistance, most of the more tolerant, winter hardy citrus plant varieties have developed.

 

What do frost tolerant, frost hardy and winter hardy mean?Hardy citrus plants Lubera

Very many citrus species and varieties exhibit a certain frost tolerance. This means that they can withstand even a few degrees below zero for a short time without damage. Just recently, we experienced this at the garden festival at Ippenburg Castle. In the nights up to 30 April there was a surprisingly real and sharp spring frost, about -1 to -3°C, and this happened after a warm and pleasant day. All of the sweet potatoes, all of the chilli and pot dahlia varieties froze to death at the festival, but the citrus plants didn't at all. There was not even anything to be seen on the flowers. On the contrary, I had the impression that the next morning with the increasing sun, the citrus scent was even stronger than on the days before. So the plants wanted to announce the victory over the frost: 'Look here, you can't do anything to us! This frost tolerance can help us in the north to keep the hardy citrus plants in pots out a little longer and so we don't have to put them back in the house every time the weather settles in April or May. The same is true for the autumn in October: here it is even good if the plant can tolerate colder temperatures below 10°C, even 1-2° minus, because it can prepare itself for the winter.

However, it must also be clearly stated that this temporary frost tolerance has nothing to do with winter hardiness and long-term frost hardiness. To repeat it again very clearly: with the exception of the varieties in this category "hardy citrus varieties", all other citruses in our assortment are not hardy, meaning that they cannot stay outside in the winter and cannot be planted out.

 

Why do even frost hardy or winter hardy citrus plants sometimes suffer damage?

I know that this is difficult to explain now: how can a plant be frost hardy and at the same time suffer damage under certain circumstances? How does that work, isn't that a contradiction in terms? Of course, the contradiction cannot be resolved completely, but in the end, the point is that the plant is a living being, an organism that reacts actively to its environment. For example, the plant evaluates the weather data and decides on this basis (and based on the genetically determined 'experience values') which survival strategy is best. In the spring, it would be rather negligent not to grow out of the plant because then the plant would be overgrown by its competitors and it would not get enough light. On the other hand, an early start is dangerous because reserve substances are converted into sugar and the susceptibility to frost rises sharply. This is precisely the most common reason for frost damage to intrinsically frost hardy or winter hardy plants such as figs, kiwi, persimmon, vines and frost hardy citrus plants: the plant already feels the spring - and yet the hard winter comes back surprisingly, perhaps combined with intensive sunshine during the day and low temperatures at night. If you want to do something about it preventively, it is certainly most efficient to protect the plant against heat, i.e. to insulate it rather than against cold. This is because the actual reason for frost damage almost always lies in temporarily excessively warm temperatures and at the same time too much sunlight.

 

Should frost hardy or winter hardy citrus plants be cultivated in pots or planted out?

The consideration is short and simple: all non-winter-hardy citrus plants (i.e. 99%) must be cultivated in pots and overwintered in a cool place at 5-12°C. Why should I now also cultivate the frost-hardy exceptionals in a pot? It simply makes no sense! Winter-hardy citrus plant varieties basically belong in the garden soil - with a few exceptions. If they are already frost hardy, then we want to have something special and new so that we can enjoy them in the garden. And there is something else: the pot is an incredibly difficult matter in terms of climate technology. It warms up very quickly, especially in the winter, and could thus give the plants false spring signals if it were placed outside (because the roots are the most sensitive organs of our plants). The fact is that hardy citrus plants in a pot are often less frost hardy than when they are planted out.

 

So who should buy hardy citrus plants and where can they be planted?

In principle, every garden owner can be considered for these hardy varieties. Even small gardens can easily accommodate citrus trees because they grow slowly and can be cut back without any problems. Ultimately, a garden with planted citrus varieties will have a completely different, Mediterranean character. But where to put the frost-proof lemon or orange tree? While Poncirus trifoliata, the hardy bitter orange, can be planted just about anywhere in Central Europe, the Yuzu lemon is more suited to a milder location. In principle, vineyard sites are suitable, but also particularly mild and sheltered places in almost every home garden. It is important to ensure that the future location is exposed to as much sunlight as possible and that no "pool" of cold air is formed. Instead, the cold air should be able to flow down a slope or embankment.

 

Do hardy citrus plants need winter protection?

Here you have to make a clear distinction: the leaf-losing Poncirus trifoliata and the more compact and bizarrely twisted Poncirus trifoliata variety 'Flying Dragon' do not need winter protection. They have acquired the most important winter protection mechanism in their homeland, in the climatic sphere of influence of the Himalayas, themselves: they lose their leaves during the winter and are therefore less threatened by cold and frost. Yes, falling leaves are better than evergreen leaves because they prevent water loss through the leaves in winter sunlight and frozen soil. Conversely, we recommend that the hardy, but leaf-bearing citrus varieties (Poncirus trifoliata hybrids, Ichang Papeda and Citrus Ichangensis hybrids) should be given a fleece or similar to protect them from the threatening sunlight during severe frost. It is also worthwhile to cover the ground around the trunk tightly with leaves or straw, in order to avoid or at least reduce the risk of ground frost.

 

Breeding

As a fruit grower I can't hide my disappointment: if I look at the initial situation (with the temporary frost tolerance in many citrus varieties and with the absolute winter hardiness in deciduous citrus varieties like Poncirus trifoliat and frost tolerance down to -15°C in Citrus Ichangensis...), the breeding yield of really tested and working frost hardy or frost harder varieties is quite small. Although citrus breeding was started systematically in the 19th century and then in the early 20th century, it always took place in subtropical or tropical regions. And if cold resistance was an issue, it was more about temporary frost resistance than real winter hardiness. Nevertheless, at the beginning of the 20th century, Swingle and others selected many hybrids between various citrus varieties and Poncirus trifoliata, and later crossed back with fruit citrus varieties to improve fruit quality. But the breeding and selection took place in Florida, and whether this is the right environment for such a breeding project is very questionable, with all due respect to the work of these pioneers. But of course, I have to take back my criticism immediately: Swingle and his successors had to solve the problems of citrus cultivation in Florida and California; I speak from the perspective of horticulture in Northern Europe! Actually, such a project should be restarted immediately...but does Lubera really need a new crazy breeding programme? In any case, dozens of frost-tolerant numbered varieties are buzzing around in collections, botanical gardens and among collectors, but a proper overview and reliable knowledge are still not available. We have set ourselves the goal of collecting as many of the frost hardy and frost hardier selections as possible and testing them under our conditions, in an alpine valley in Switzerland and in northern Germany. Currently, however, we limit our sales to the safe values: Poncirus trifoliata, in the normal growth form and also its twisted form, and Yuzu, a hybrid between Ichang Papeda and the mandarin, which has been cultivated in China for centuries and which has excellent fruit characteristics and winter hardiness down to -12° Celsius. In many areas of Central Europe and England, this is sufficient for planting out in mild locations, in a winegrowing climate, near a protective house wall, etc. However, if there is persistent ground frost, additional winter protection is necessary, on the one hand protecting the leaves from direct sunlight and on the other hand covering the root area with straw or leaves. Poncirus trifoliata, for instance, can be planted fresh and freely in any garden without any precautions.

 

The use of frost hardy citrus species as rootstocks

In the course of the last 100 years, the frost hardy citrus species have also largely established themselves as rootstocks, as root parts of citrus trees. Although a frost hardy rootstock cannot transfer this frost hardiness to the noble variety grafted above, it secures the basis of the "tree" system, the root, so to speak, which in turn can deal well with ground frost. In addition, frost-hardened rootstocks react less strongly to climatic fluctuations, for example to warming in the winter months, and thus provide additional protection for the plant. Specifically, bitter oranges are often used as rootstocks (which are also frost hardier than many other citrus fruits), but then above all Poncirus trifoliata and in Japan Yuzu, the hybrid between Ichang Papeda and the mandarin, is very popular. Our sales plants at Lubera are almost exclusively grafted onto Poncirus trifoliata.

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