How much light do cactus need to grow well? The importance of sun and air and a summary table

Pediocactus knowltonii in fiore

Light is fundamental for every living being, starting with plants. This is obvious, but we cannot ignore it if we want to grow cacti and succulent plants in general. Plants, moreover, that often need many hours of light per day; in some cases direct or non-filtered light, in others not straight but equally intense daylight. In fact, there are succulent plants that, if placed in full sun, slow down growth, burn and even die. On the contrary, many succulent plants and many cacti, if they do not receive the right amount of direct light, will have stunted growth, with weak thorns and elongated stems.

In this article, we see what we need to know about the right exposure of cacti and succulent plants in general. Also, at the end of this article, you find a table summarizing the type of exposure needed by the main families of succulents.

General introduction
Alcune piante al sole
Some of my seedlings in full sun

If cactuses and succulents as a whole need a lot of light to properly grow, there is no shortage of genera (e.g. Ephiphyllum) that do not like much light, let alone direct sunlight, as they are “programmed” to grow sheltered from the foliage of trees in rainforests. As for watering (here the link dedicated to the topic), there are two elements to consider. The first is a minimum knowledge of the various species. There’s a big difference, for example, between the sunshine that Ferocactus receives in their habitat and the one that a Gymnocalycium gets. Here will be the experience and the deepening to come to us (at the end of this article you will find a useful summary table about the light needs of succulents).
The second element is the place where we grow. The hours of light and the sunlight’s intensity greatly vary between the North (where I live) and South of Italy, as well as between the French Riviera area in France and the regions of Southern Spain and the Scandinavian countries, just to stay in Europe. There’s no sense to compare it with Cactaceae’s area of origin, such as Mexico, Argentina, Bolivia or the United States of the South.

Ferocactus latispinus in terriccio sperimentale
F. latispinus, my seedling grown in full sun

The incidence of sun rays varies from latitude to latitude, and it is difficult in Europe to get the thorns that some Ferocactus develop in their habitats.
Giuseppe Lodi (Italian pioneer in the cultivation of succulent plants), for example, was rather sharp and suggested to give up growing plants if we see that “they are too far from their natural look”. This is a valid first suggestion that tells us to deal with the light we can give, the results we can achieve, the choices we can do. If we can offer direct light to our cacti only in the early morning hours, it is better to avoid genus like Ferocactus and Echinocactus that wants a lot of light and concentrate more, for example, on Gymnocalycium and Parodia, which grow well even with fewer hours of direct sunlight or with partially filtered light.

In any case, all cacti need light and lots of air. And if some like direct sun, others are satisfied with a few hours of full sun or filtered light. To be informed about the needs of the plant is, therefore, the first rule is to observe the plants. The experience then will help us to get to know the plants further and understand if we can give them the amount of light they need or not. The tests we can do are essential too, especially with equal plants obtained from the same sowing, to see how they grow in the greenhouse, on the balcony or terrace, and in other areas, maybe in full sun all day long.

Etiolation

If the light is not enough for the plant’s needs, the cacti will begin to “spin”, i.e. grow longer and narrower at the apex. The stem will have a light green colour, and the thorns will be weak or almost absent. Overall, the cactus will have an elongated and anything but natural appearance. This phenomenon is called etiolation and deforms the plants irremediably, because even if we can later give the plant the right light conditions, the etiolated part will remain, while the new section will resume properly growing, with the result that the overall shape of the plant will no longer be spherical (in the case of globular cacti) or will have bottlenecks (in the case of columnar cacti).

Greenhouse, windowsill, terrace?
Alcune succulente all'esterno della serra
Succulent plants outside my greenhouse

Some grow their plants by keeping them on windowsills or balconies, others are lucky enough to have a terrace, others have a garden or a greenhouse. The latter is, of course, the solution that gives you more freedom: it is usually exposed all day long and has a transparent cover that we can shade or not. Not only: the greenhouse also provides the possibility to transfer outside (unless it is placed in small spaces) a large number of plants during the growing season. This allows us to experiment with different solutions: sun filtered by sheets (even transparent sheets reduce the incidence of ultraviolet rays), the sun filtered by shading nets, or full sun if we relocate some plants outside.

Alcune piante al sole a lato della serra
Some of my cacti outside the greenhouse

In my case (North Italy), I chose not to shade my greenhouse, which receives light from early morning until sunset. I only use a net able to shield 30% of the sun’s rays along the exposed side during the hottest hours of the day, i.e. from about 13 to 15. The roof is covered by a double transparent sheet, so my plants receive a considerable amount of light. This is why I grow almost exclusively cacti and succulents that can withstand many hours of intense light-per-day.
The few species that I grow and that need a light shade I keep at the end of the pallets, at the corners of the greenhouse, which are made of polycarbonate, therefore able to filter the sun more. In some cases, I have placed plants that want less sun behind larger plants, so that the former are sheltered from direct sunlight.

If you have a windowsill or a balcony, you should check that it is exposed to the South or South/East. In these cases, the plants will receive light for a good part of the day. Exposure to the North and West should be avoided, unless you grow crops that are content with little light, such as Haworthia, Epiphyllum, some Crassula and some Euphorbia, for example.

Direct sun or filtered sun?
Copiapoa cinerea fiorita
Copiapoa cinerea

In my greenhouse, I grow a large amount of Gymnocalycium (a genus I particularly love, but only for very thorny species) and I have never had any problems, although it is said that this genus wants less light. Maybe the light-shielding given by the sheets is enough to make my specimens grow without problems. I keep outdoors during the growing season many plants of other genera. These are mainly plants that like full sun, such as Ferocactus, Echinocactus, several Thelocactus, Ariocarpus, Ancistrocactus, Opuntia, Tephrocactus, Astrophytum, Echinopsis, Agavi, Aloe.

A genre that deserves a separate speech is that one of the Copiapoa. I kept some of them in full light, and they got stuck. I think it depends on the fact that in nature they live in areas of Chile where for several hours of the day the sun is filtered by fog and the fact that with these plants it is better to force the growth to “reverse seasons”.

Getting plants used to the sun
Ferocactus acanthodes, mia semina in marna
F.acanthodes, my seedling growing in full sun

What is essential, especially if in winter you keep the plants in sheltered places and less exposed to the sun (which is weaker in the short days of the winter months), is to gradually get them used to the exposure we will give them during the growing season. Never put in direct sunlight a cactus that has been kept for months in a basement, in a garage, etc… From the end of March, it will be better to place the plants in areas where the direct sun only arrives in the early hours of the morning and then move them gradually, for twenty days, in areas with more sunshine.

Once the plants are accustomed to direct sunlight, we will no longer run the risk of scalding the skin of the stem. In case of sunburn, the exposed part will discolour and then dry, and leave a yellowish or light brown area on the plant. With the new growth, the disfigured part tends to be less visible, but on young specimens, a heavy burn may block the growth or, in the worst cases, lead to death. Similarly, direct sunshine on young specimens can slow down or inhibit growth altogether. In nature, in fact, seedlings and young plants grow in the shelter of mother plants, rocks or shrubs. These are also factors to consider, especially if you grow succulents on a fully exposed terrace.

On adult specimens or specimens accustomed to the light, obviously for those genera that live in full sun, direct exposure is a blessing: it strengthens and fortifies the plant, keeps compact the stem and makes thorns growing bigger and stronger.

The importance of air
Echinocactus texensis wild
Echinocactus texensis under the sun

Closely related to the exposure is the amount of air that cacti and succulents need. As for all plants (and this is obvious…) air is essential and I can assure you that it will never be too much. Growing cactus and succulent plants in narrow areas, where there’s no air circulation, where the air (and the humidity) is stagnant, not only does not help growth but risks exposing plants to rot or fungal diseases.

Both in the hot season and in winter we should try to give the plants as much air as possible. In summer, the air is essential because if it is true that cacti can withstand high temperatures (even over 40 Celsius degrees), it is equally true that in nature they have air available in quantity. The wind and breeze can lower the incidence of heat on the plant and avoid “boiling”. From the end of February, I begin to open the sides of the greenhouse by raising the side sheets, to open them at the turn of mid-March and keep them open until the end of November. In winter, on sunny days, I open the front and rear doors of the greenhouse for a few hours to allow air exchange.

Echinocactus parryi
Echinocactus parryi in bloom

During the growing season, the wind and air also help the soil to dry quickly and prevent rot. As in nature, the recirculation of air and wind help the plants to withstand high temperatures and the sun during the most intense hours. In cramped environments, such as small greenhouses with plastic cover, plants literally risk “boiling” in the heat and stagnant air. It is definitely better to gradually get them used to the sun and remove all the cover (or you can replace it with a light shading net).

The air is quite important, also, during the winter. During this period it’s useful to lower the humidity level and to keep the environment clean, avoiding that mushrooms such as the one emblematically called “rust” (that attack some cacti grown in sub-optimal conditions) can thrive. As soon as possible, it’s advisable to ventilate, open the greenhouse or lift TNT (non-woven fabric) covers to facilitate air exchange.

Summary table

The excellent PDF table below was created by Marcello Ceccucci, passionate grower and administrator of the Facebook group “Not only cacti … succulent plants and beyond“. The table shows roughly the type of exposure suitable for the main kinds of cacti and many other succulent plants. Marcello has compiled it consulting the Encyclopedia of cacti by Italian publisher Zanichelli. I thank him for having made it available on Facebook and for having given me permission to publish it on this site. Please note: the table is purely indicative and does not contain absolute values.

PDF TABLE – EXPOSURE OF SUCCULENT PLANTS AND CACTUS

You may also be interested in “How to repot cactus“, or “Watering cactus and succulent plants“.

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