We already have an article about succulents and rain with tips on how to keep succulents happy outside while it rains as well as listing species that we know do not like a lot of rain at all.
While Cacti are also succulents and much in the article applies directly to this intriguing group of plants they can be a bit more sensitive. They are true desert plants and are well adapted to arid and dry regions.
But are all Cacti super sensitive to water? Will they die if they get left out in the rain or watered too much?
Many cactus species can survive the rain, even heavy and prolonged falls while others can rot very quickly when exposed to too much rain. A small sprinkle or being rained on for a day should, however, not kill any cactus.
In this article we will have a look at what cacti are, how to care for them and which ones are very sensitive to rain and over-watering.
What is a Cactus?
A cactus is a succulent plant from family Cactaceae with thick, fleshy parts adapted to store water. On the outside cactus plants have spines which, once upon a time, used to be leaves or branches. These sharp parts serve multiple functions. They mostly act as a deterrent against being eaten by herbivore animals but can also provide the plant with shade from the sun, insulation from the cold or aid in spreading of the plant as they will attach to animals walking past and then drop and set root in a new location.
Most cacti are equipped with ribs which allow the plant expand when water is plentiful and then shrink in times of drought.
The roots system is usually shallow to allow for quick water absorption from the surface. Having said that, in cultivation where cacti are likely to get more water, the roots will spread deeper.
Flowers appear solitary or in clumps and when pollinated grow into seed pods. Cacti flowers are renowned for their beauty and come in an amazing array of colours.
There are many cacti genera (some 127) and combined as many as 1750 cactus species exist in the world. Some cacti can also be grafted putting together two different plants growing as one. This is done by slicing the top off host cactus ('stalk) and bottom off another, compatible catcus and they are put together. When done right the wound heals and the top and bottom cacti will grow as one plant.
Cacti are mainly native to the Americas but there are species native to Africa as well. They appear in the driest parts of America but also in forests and jungles where they grow as epiphytes and thrive in humid and wet environments.
It is difficult to give advice on how to care for all the Cactus species that would apply to every climate, but the fact that these tough plants have now been distributed all around the world thanks to succulent enthusiasts and collectors, proves that with the right care they can live in any part of the globe.
The most important thing to these plants, in our opinion, is light. All cacti need lots of bright light with at least some sun exposure/ filtered light, though Epiphytic cacti will survive in bright shade without any sun exposure.
A greenhouse is an ideal environment but a sunny windowsill or a balcony/veranda with some sun coming through will also suffice. As mentioned above, many cacti will survive outdoors exposed to the rain. A sunny or partly shaded position in the garden either in pot or the ground will be just as good so long a gritty potting mix is used.
Cacti are mostly not frost tolerant. In cold climates they will need to be brought indoors for winter until the danger of frost and snow passes.
A universal advice would be to wait until the potting mix dries out completely plus a few more days between waterings. In winter most Cacti are dormant and so they will need even less water, though we’d still recommend to give them a drink every few weeks (3 or so), especially if they start shrivelling a bit.
One problem that might be encountered with such sporadic watering is hydrophobic potting mix. It means that the potting mix will becomes so dry it will repel water and will not actually get wet. When this happens it is likely the plant is not getting any water as it will just run through the pot, leaving the potting mix and roots bone dry. To fix hydrophobic potting mix, the pot will need to submerged in a dish of water for a few minutes.
In my nursery, we leave the majority of cacti out where they get rained on and are watered with all the other succulents. We even have a few growing in the garden. To help them along we have either raised the garden beds or planted on a slope so when it rains the water can easily run off, not suffocating the roots.
The best growing medium for cacti is undoubtedly a succulent and cacti potting mix. It is always a good idea to add a little extra drainage agent such as perlite, coarse sand (NEVER use ordinary sand) or pumice. These can be mixed in with the potting mix and will help the water drain quicker and roots stay dry during periods of rain.
Another thing that will greatly help with water issues is a simple terracotta pot. Because terracotta is porous it allows water to escape through its walls, drying the potting mix much quicker than if the plant is left in a plastic or glazed pot.
In the garden many cacti should be quite happy in ordinary soil, though it won’t hurt to improve drainage with perlite etc. around the root area, especially if you live in a climate with regular rain.
Cacti That Typically Do Not Mind Being Rained On
Although our nursery is based in sunny Australia, we can get some heavy downpours and rain lasting weeks. There is a number of cactus species we keep in a plastic greenhouse, but find the majority grow quite happily outdoors with only shade-cloth for protection during the worst of summer when we get temperatures north of 40C (104F).
Please keep in mind that if you are not sure whether your cactus is able to survive being outdoors and you do not want it to die, keep it in a bright sheltered spot to be on the safe side.
In our experience, these cacti are happy to live outdoors and do not mind being watered. However, this may not apply in truly tropical climates or in places where it rains often- ie Ireland and also in cold climates with snow in winter. We are only going to list species we have been growing for quite some time and have experience with (there are loads more cactus species that can grow outdoors and won’t die in the rain)
Opuntia species (please note that opuntia is invasive and considered a noxious weed in many places- yes this includes the cute Bunny’s Ears too)
Cacti That Do Not Like The Rain
Again, this is our experience only and it is very possible that someone out there has a different one. We have previously managed to kill these when we left them out in the rain, but they thrive in the plastic greenhouse where watering is controlled and kept to a minimum.
Mammillaria (many but not all, also more mature plants that are several years old seem not to mind the rain all that much)
While this is not always a rule, we found that cacti that have fine hair as well as spines (espostoa species or similar) are a little more prone to rotting as the wet hair will keep water on the surface of the plant longer and will create a bit of humidity as well.
Most cacti available in nurseries or garden centres have labels. They can be quite generic and not always 100% right (the same label is used for all the plants, even if they are different), but it is a good place to start when establishing what kind of environment your new cactus will like best.
The name can also be searched on the internet. Read through a few articles as many blogs are just copies of each other, reworded.
In conclusion, cacti should not be a difficult plants to grow and many will survive the rain given they have a free draining potting mix and a pot with a drainage hole.
In frost free gardens, plant on a slope and mix a bit of gritty stuff through to allow the water to drain faster.
Happy cactus growing.