Do cacti resist frost? In winter, do succulent plants have to be kept indoors or can they stay outside? And what are the minimum winter temperatures they can tolerate? Even among experienced growers, the minimum temperatures of cacti and succulents are still debated today. I tackle the topic starting, as always, from my personal experience, which is my only way to have accurate data, found in the field, related to my system and my growing conditions, here in North of Italy. Let’s say right away that during winter I keep most of my plants in the greenhouse. It is a large greenhouse of 60 square meters and with a height, at the top, of 4 meters. These dimensions guarantee a satisfying volume of air, which in turn prevents moisture stagnation, the first real enemy of cacti and succulent plants in winter.
Let’s see, in the following article, the various factors that influence the resistance of cacti and succulents to cold. At the end of the article, you will also find a schedule with the minimum temperatures tolerated by the various families of succulents.
Know your plants
The cold, if you know the needs of plants, is not a problem at all. I, for example, do not have any particular cover for the greenhouse: just the nylon sheets that cover the roof and that in winter I lower along the sides. Inside I have a hot air generator with thermostat that trips when temperatures fall below zero. Usually, on sunny days during the winter months, during the day in the greenhouse, the temperature rises well over 20 degrees; at night it can sporadically go up to two or three Celsius degrees below zero (this if outside you travel around -7 or so, to the point that the generator can not immediately bring the temperature inside to zero). However, even about the heat, many factors affect plant health and establishing precise rules is impossible. Thus, some “staples” may be fixed.
First of all, it is necessary to know the area of origin and the main characteristics of the succulents we have. There is a big difference, for example, between a Cactaceae like an Escobaria (whose range goes as far as Canada) and an African caudiciform like Adenium obesum. While the former can withstand temperatures down to -20 Celsius degrees, the latter already below 12 Celsius degrees will start to suffer. So to the classic question “can succulent plants stay out in winter?”, the only reasonable answer is: “it depends”. It depends on the plant, on its state of health, on how we have prepared it for winter, on where we are (in Italy, for example, the Southern winter and the Northern winter are not the same things). It also depends on what we mean by “staying out in winter”.
External or internal?
This is the second watershed: do we keep them outside the house or the greenhouse, but sheltered from rain and snow, or do we keep them outside, exposed to all the weather? This is not a trivial question, and a big difference, in this sense, the soil makes it. Some plants can spend the winter in the cold, under the rain and snow, with wet soil, without any problem. From sempervivum to many sedums, from many opuntias to Cactaceae particular as Pediocactus and Sclerocactus. Instead, some species can very well withstand low temperatures as long as the soil is dry.
Many good growers have been testing for a long time different kinds of cacti in conditions that until a few years ago manuals would have considered “criminal”: watering in the middle of winter, frozen soil, plants under the snow. There is nothing wrong with all this: you just need to choose the right plants, i.e. those that live in nature in situations of this kind.
In the last years I have left a lot of cacti and succulents outdoors: various Opuntia (including gracilis and azurea), Escobaria, Pediocactus, (simpsonii and knowltonii), Ancistrocactus (uncinatus and mathsonii), Ferocactus acanthodes, Neoporteria, Mammillaria bombycina, various Agave, various Echinopsis, Tephrocactus articulatus papyracanthus, Astrophytum myriostigma nudum. These are mostly plants from my sowing (except Pediocactus and Opuntia), which I keep outside the greenhouse for the whole year, sheltered only by a small polycarbonate canopy.
These plants absorb all the humidity (and here in the Po Valley we have so much that we could sell it!) and the frost of the coldest hours before morning. Over the years, I have lost a few of them, some have become stained. However, there is nothing to complain about since they have withstood temperatures as low as -10 Celsius degrees even for several nights. It must be said that the soil, thanks to the cover, remains dry. I only water the Pediocactus and leave Opuntia, Sempervivum, some Agave and some Echinopsis in the rain.
The winter stasis
The succulent plants that I keep in the greenhouse, including a lot of cactus obtained from my sowing, spend the winter completely dry from the end of September to the end of March. I water until November only the Lithops, Crassula and some other succulent plants. For the rest, stop watering from the end of September, even if October tends to be quite a hot month. The reason for the suspension well in advance of the arrival of the cold is twofold: on the one hand, I am sure that by November/December the soil will be dry; on the other hand, the plants have time to reduce the fluids stored in the stem or taproot. In this way the succulents will gradually dehydrate, the concentration of salts contained in them will increase, and the freezing point will drop. This process, in essence, stimulates the plant to lose liquids and at the same time to produce substances that lower the freezing point of the water that the plants themselves contain. We can say that cacti develop a sort of natural “antifreeze”.
In these conditions, they go without problems from 24 Celsius degrees of sunny days to -5, -6 Celsius degrees of the coldest nights (I don’t use any particular cover beside the sheets of the greenhouse). Last year they went to -9 (the diesel ran out, and I was away): only a few young crassulas died. Of course, I take at home genera like Melocactus, Discocactus, Adenium obesum etc. that are particularly sensitive to temperatures. For the rest, in the greenhouse, I have a discreet variety originating from North to South of the Americas: from Ariocarpus, Echinocactus, Ferocactus, Lobivia, Gymnocalycium, Mammillaria, Copiapoa, Sulcorebutia, up to Uebelmannia (which are always at the limit, in terms of temperature). I also have some Euphorbia (principally that obese) and several crassulaceae. I admit that it’s mainly for the succulent not-cactaceae that I keep the generator on.
To break down the high humidity levels that characterize the Po Valley winter, I open the greenhouse for a few hours on warm days. This is more than enough to encourage the recirculation of air, keeping the environment healthy (I don’t use hygroscopic salts or dehumidifiers).
In this regard, it’s also useful to remember prevention through specific products (copper oxychloride), which, together with optimal air recirculation, can help plants to stay healthy during the winter. During the winter months, however, I do not carry out any treatment: it is better to do it in spring, just before the vegetative recovery, and in autumn, just before the plants enter into stasis.
How plants change with cold
Another consideration related to winter concerns the chromatic changes and the “morphological” changes of plants. Let’s not worry about the transformations that plants undergo due to the cold and prolonged dryness! Many plants will take on a reddish colour, some will simply lose their dark green to lighten up a bit. Almost all will eventually tend to deflate, and some (especially those with taproot roots) will do so markedly until they are partially buried in the soil. There is no need to worry: just two waterings after the vegetative regrowth, and the plants will swell and regain colour. In the photo on the left, for example, you can see a Stenocereus during winter: the stem becomes reddish.
The advantages of rest
A good winter in the cold and dry, possibly with large temperature swings between day and night, for most succulents, is the ideal antechamber to produce abundant blooms. In winter most succulents go into stasis, recover their energy and start working for the next flowering. It is said that if the plants are perfectly still (in vegetative stasis), they can perfectly withstand even low light conditions and can therefore be moved on internal stairs, in garages and even in cellars.
I am not so sure. Apart from the high humidity that a garage and a cellar can have, I have noticed that many species (e.g. Notocactus/Parodia) produce more abundant blooms if they spend the winter in the light. Obviously, for those who have a greenhouse, the problem does not arise because they don’t need to transfer the plants from one place to another. For those who grow on a balcony or terrace, it may be useful to try (maybe starting with some specimens and not with the whole collection!) to protect the plants from the rain and use, if necessary, some non-woven fabric to give some more degrees. Of course, it’s good to know the plants and, if you don’t have a greenhouse to warm up, you should consider genera that can bear well the cold.
List of minimum temperatures
Below, in pdf format, you will find a useful list drawn up by the Italian Association of Succulent Plant Lovers (AIAS). It’s included in a booklet published by AIAS in 2005, that I bought as soon as it came out. Like all the lists, it must be taken as a rough indication and not in an absolute sense. At an indicative level, it can be an excellent “map” to orient oneself among the various genres. The minimums in the list are expressed in Celsius degrees.
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