Cold-hardy citrus!

Cold-hardy citrus One of my favourite groups among citrus plants. I see a lot of potential in them. Unfortunately, they cannot withstand our winters, so they are not winter-hardy, but rather only cold-tolerant. Fruit: The quality of their fruit is quite poor, so producing delicious, tasty fruit is not their main strength. If we consider that only the hardy lemon can survive here and what its fruit is like, then we are happy with almost any other citrus. The hardy lemon has a sour, terpene-like, poncirus flavour, the fruit is full of seeds, contains very little flesh and its resinous peel is inedible. Hardy lemon hybrids have much better fruit quality. The fruit peel is already more characteristic of citrus, so it can be used, while the flesh is juicy and contains seeds, but there are far fewer seeds. However, the flavour of the hardy lemon is still clearly noticeable in the fruit. In triple hybrids (e.g. US-119), the fruit quality is much better, and the poncirus flavour is barely noticeable or not noticeable at all. The fruit of Ichang papeda and Yuzu does not have the flavour of hardy lemon, although the flesh is dry and contains seeds. Satsuma mandarin and kumquat have the best flavour, and these are also sold in shops. Uses: The fruit peel can be grated and candied. The juice can be squeezed out. It can also be made into jam. The seeds can be used to grow rootstocks with very good properties. Cold tolerance: Cold-hardy citrus plants tolerate cold well. This means that the minimum tolerance limit is generally given in degrees Celsius (°C). The tolerance limit of the plant is divided into stages, so a single number does not necessarily give an accurate picture of the citrus plant's resistance. The limit may mean complete death, but it may also mean that the plant freezes back and survives, but will not bear fruit the following year. As the temperature drops, the citrus plant is subjected to stress. The plant does not require cold; for it, this is more a matter of survival. The order of damage caused by cold is: 1. Flowers and fruit 2. Leaves 3. Thin branches 4. Thick branches 5. Freezing back to the base 6. Complete dieback This is the general order, but it depends on many factors, and it can even be the case that number 6 comes first. A citrus plant can die for two reasons. One is dehydration. In this case, the leaves and thinner branches dry out due to lack of water. Most likely, the entire tree will dry out. The other is bark splitting. The bark can split on young branches, but cracks can also develop on several-year-old thick trunks. There is nothing that can be done with a crack on the trunk; the plant will die back to that point. Small damaged branches should be pruned off. At the site of the crack, the plant dries out, it is no longer able to produce scar tissue there, and the phloem will eventually die as well. Leaf drop is somewhat normal in poncirus hybrids, as one of their parents is deciduous. They can also drop their leaves when grafted onto a poncirus rootstock. If it is too cold, the entire plant may become bare. This is not ideal, but it has no harmful consequences. Overwintering: They have a chance of surviving the winter in a protected location. It is possible that even if a plant survives five winters in a row, it may freeze to death during the next winter. Nevertheless, in my opinion, it is worth trying, because there are places where the microclimate is so favourable that the plant can survive the winter without any problems. Places where they have a chance: where cold air can drain away, sheltered from the wind, with a high level of heat retention, on top of a hill, without frost pockets. These environmental effects and local conditions increase the plant's chances of survival. If we feel that we do not want to take such a risk, an unheated polytunnel may also be sufficient. In cases of extreme cold, an emergency heating option is recommended. A polytunnel collects heat and the plastic film provides good insulation. If the temperature inside is already 5 degrees higher and it cannot suddenly cool down, this provides a great deal of help to the plant. It is also possible to heat it; it is sufficient to maintain a minimum temperature of 0 to -5°C, so there is little risk and the heating bill also remains low. Rootstock: One important thing is the rootstock. In the case of cold-hardy citrus, we should not graft it onto a less cold-tolerant rootstock if the plan is for it to be exposed to cold. In this case, the rootstock may freeze before the scion and this can cause the entire tree to die. I recommend Poncirus trifoliata as a rootstock. It has many advantages if we want to place the grafted plant in the cold. Some characteristics: winter-hardy, resistant roots, slow growth, so the grafted scion will not produce soft, loose-tissue shoots, it does not grow as quickly, making it easier to protect, and it may perhaps improve cold tolerance by 1–2 degrees because it prepares itself better for the cold as winter approaches than other citrus plants. Relationship between cold tolerance and fruit: It is also an observation that the better a citrus plant tolerates the cold, the more the quality of its fruit decreases. If we cross it with a citrus plant that produces tasty fruit but is much less cold-tolerant, the two observed characteristics change. Cold tolerance decreases, while fruit quality increases. New varieties: We can create new varieties by pollinating flowers; this requires the stigma of one plant and the pollen of another plant. It is also important which plant the fruit develops on, as this also influences the new offspring. The seeds collected from the fruit will become the new hybrids. Since we know the parents of the hybrid, its cold tolerance can also be estimated. The more cold-sensitive one of the parents is, the more the offspring's cold tolerance will decrease. The characteristics of the offspring also vary. Several seeds can develop in a fruit produced through cross-pollination. These seeds have different characteristics and do not carry the characteristics of their parents in the same way, so cold tolerance will also vary. It is worth creating new varieties because this way we may obtain citrus plants with better cold tolerance. Varieties: I have collected the best-known ones that can still be obtained. There are several varieties within the hybrids, but I will not cover them individually here. The tolerance limit differs between various descriptions, so I will indicate the value that is mentioned most frequently. /-25°C/ Hardy lemon - Poncirus trifoliata /-14°C/ Citrange - (Citrus sinensis x Poncirus trifoliata) /-14°C/ Citrumelo - (Citrus paradisi x Poncirus trifoliata) /-16°C/ Citrandarin - (Citrus reticulata x Poncirus trifoliata) /-12°C/ Citrangequat - [Fortunella japonica x (Poncirus trifoliata x Citrus sinensis)] /-12°C/ US-119 - [(Citrus paradisi x Poncirus trifoliata) x Citrus sinensis] /-12°C/ Sanford Curafora - [(Citrus sinensis x Poncirus trifoliata) x Citrus reticulata] /-10°C/ Citrangedin - [(Citrus sinensis x Poncirus trifoliata) x Citrus mitis) /-15°C/ Ichang papeda - Citrus ichangensis /-12°C/ Yuzu - (Citrus ichangensis x Citrus reticulata) /-10°C/ Satsuma mandarin - Citrus unshiu /-10°C/ Kumquat - Fortunella japonica A little help for those who do not know the Latin names: Citrus sinensis - Sweet orange Citrus reticulata - Mandarin Citrus paradisi - Grapefruit Citrus mitis - Calamondin Fortunella japonica - Kumquat The rest are described above. In practice: The temperature limits are for guidance only. They may not necessarily withstand those temperatures. Therefore, I recommend keeping them in a place that is at least 5 degrees warmer than the stated minimum. If we would be sorry to lose our only plant, we should not plant that single specimen outdoors; instead, take grafting material from it and plant the other grafted plant outdoors. A young grafted plant tolerates cold less well than an older one, so pay particular attention to it during the first 1–2 years. In practice, Citrange may freeze at -10°C even though it is said to tolerate down to -14°C. Ichang papeda is one of the most cold-tolerant citrus plants, according to descriptions it can tolerate -15 to -16°C, but there have been cases where it froze to death in England at around -10°C. I have also read that Sanford Curafora (-12°C) survived a cold spell of -24°C; it is true that it froze back, but it produced 40 cm of new growth the following year. This is why it is important to remember that environmental factors influence the plant's resistance. For example, wind, how sheltered the location is, its orientation, etc. The safe temperature is -5°C. At this temperature there is no risk, and even if the temperature drops lower than this for a short period, there should still be no problem. Additional note: In an enclosed location protected from the wind, it can tolerate the expected minimum temperature, while in a windy location it can tolerate considerably less. Written by: Márton Petróczki