If you search the internet you will find lots and lots of conflicting advice about how to grow and look after cacti. The truth is, it depends on your circumstances and where you live what will suit your plants best. The advice for someone in the north of Scotland is going to be different for someone in Italy, and different from someone in North America. However there are a few ground rules which apply to all.
There is a wide variety of “cactus” composts commercially available. Most if not all of these do not provide enough drainage except for use in hot countries. The best thing to do is to make up your own- it is easy. There are basically two ingredients which need to be mixed together well:
- A loam based compost, like John Innes (No. 2 or No. 3). Don’t use a peat-based compost as this may attract sciarid flies which can kill your plants.
- A drainage material. This can be fine chick grit (not sand, which retains water), perlite, non-clumping cat litter, pumice, anything like that which does not retain water and lets the compost dry out quickly. Many growers will mix up a few of these together and everyone has their own special recipe.
The proportion of these depends on where you live- how hot does it get? How quickly does it dry out? Here in the north of Scotland where it is frequently cloudy and cool in summer I use about 20% compost and 80% drainage. The more drainage to compost, the more you need to provide food with your watering as there is proportionately very little nutrient in the mix.
Because it is important for cacti to dry out quickly after watering, always use a pot which is only just slightly larger than the root mass. When it comes time to pot on to a larger pot, don’t be tempted to use a large bowl because it looks good. A small pot will dry off much more quickly and avoid the risk of rotting roots. Shallow pots are better than deep pots for the same reason; but bear in mind that some species of cacti have long tap roots.
Watering & Season
Cacti, like all green plants, need water to survive while they are growing. However, they don’t like being wet, especially around the roots. If the roots remain wet for too long, they tend to rot. Some species are worse for this than others. The growing season varies from one part of the world to another. In the tropics, it continues through pretty much all year round; in the north, the season runs from about March through to September. Outside of these times (October to February) cacti go into a winter dormancy where they grow fairly slowly and need no water.
The trick with watering is to water only when the compost is bone dry; and only water during the growing season. Here in the north of Scotland I start watering around mid-March, and I water every 3 to 4 weeks, with the last watering of the year in mid-September. I try to water when there will be a period of sunny warm weather afterwards, but it is difficult to predict.
Water quality also varies from place to place. Here in the north-east of Scotland we are very lucky to have very soft mains water (i.e. very little calcium is present). In many other places hard water from the mains will not be enjoyed by cacti, and it is better to collect rain water and use that instead.
Although cacti, like other green plants, make their own food using water, CO2 and sunlight, there are certain additional nutrients that they have to get to be healthy. Usually these are taken in through the roots from the surrounding soil. When cacti are potted in a compost which has very few nutrients (like my high-drainage mix) it becomes necessary to add a supplement to the water. I add a cactus fertiliser with every watering- there are various different manufacturers, such as Chempak, Kohres, etc. For cacti you need a low-Nitrogen fertiliser, preferably with added trace elements. Tomorite tomato fertiliser works quite well but needs to be applied at half the strength it says on the bottle.
There are different kinds of pests in different parts of the world. Where cacti are kept in glasshouses, there are a few quite serious pests which occur worldwide which can destroy plants if not kept in check. These are: mealy bug; root mealy bug; red spider mite and western flower thrip. We have had them all at one time or another here. Unfortunately the chemicals which are available to control these pests are not usually available to the amateur grower. Biological control by introducing predators can be useful, but it needs some care to provide the right conditions for the predators to survive, and of course will not eliminate the pest, just reduce the numbers. Always, always, put any new plants into a quarantine area until you are absolutely certain there are no pests on them. If you discover any, there are a few things you can do. Try and pick off any pests you can see. Look underneath the pot, and around the rims- sometimes pests like mealy bugs can hide there. Remove every sign of the pests you can find. In some cases this might require de-potting the cactus and cleaning the roots to remove root mealy bugs.
Horticultural soap will kill most pests by contact (but not eggs). Although frowned upon by other growers I’ve had some success in the past by using ordinary washing up liquid, by filling a bucket with water and about double the doze of washing up liquid that I would use to wash dishes (add the liquid after the water, to avoid too many bubbles) then dunking the entire plant, pot and all, underneath for a half hour or so, then letting it dry off. Horticultural soap is safer as it does not have the same additives as washing up liquid. Nevertheless I’ve never had a problem with my plants.
Check your garden centre or local nursery to find out what pesticides are available in your area. Here in the UK the selection available has become very limited indeed due to concerns about health & safety, and about the effect on other wildilfe like bees.
Whatever you do, check again in a week’s time as no doubt there will have been eggs that have hatched.
Hot and cold
Although cacti are all native to the Americas, this covers a vast range of different climates. Most of the cacti in our National Collection are mountain plants from the Andes; but we also have many plants from Mexico, Arizona, the Caribbean, etc. Needless to say different species are tolerant of different conditions.
You need to do your research 😉 Find out where your plants are from. What temperature ranges do they have in the wild, and what sort of places do they live in? Windy? Exposed? Open to bright sun? Cold at night? Never below freezing? Deep inside crevices? Hanging from cliffs? If you find out what your plants like, and can provide something similar, you will be much more successful.