Cultivation tips for cacti

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:

  1. 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. “John Innes” does traditionally contain some peat, but this is gradually being replaced by alternatives as we phase out the use of peat for environmental reasons. “Peat-free John Innes” is something of an oxymoron but they do exist; if you find it, give it a try. I am now making up my own “John Innes”, using the traditional recipe, but replacing the peat part with Sylvagrow Ericacous Compost (mainly composted bark and coir).
  2. 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 much and lets the compost dry out quickly. Many growers will mix up a few of these together and everyone has their own special recipe. If buying chick grit, make sure it is the type that is sold for young chicks. Grit sold for older hens has eggshell and / or seashell added which is not good for some cacti.

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.

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 2 to 3 weeks, less if it is cold and damp, 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 used to be very lucky to have very soft mains water (i.e. very little calcium present) but now, following mains works and diversion, it is harder water and higher pH. 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, Koehres, etc. For cacti a low-Nitrogen fertiliser is best, 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. Following a recommendation from Prof. Ralph Martin I have started using Miraclegro Ericaceous plant food for the last few years and it works well.

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. Finding pests means hard work- you have to keep at it again and again and again to keep your plants healthy. In a warm greenhouse in summer, an entire generation of pests can be born and reproduce in one week so it becomes necessary to repeat treatment within a few days, in order to catch newly hatched pests.

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.

You will read all sorts of conflicting advice about growing cacti from seed. Everybody has their own tried and tested methods.

Some cacti are easy to grow from seed, and some take a lot of work. Opuntia, for example, can be very difficult to germinate. I describe here my method for growing most of the plants in the National Collection, which, by and large, are quite easy.

You can sow cactus seeds at any time of year, but early spring is best, as the plants then have a whole growing season to get bigger before the trials of the winter dormancy period start.

I sow seeds sparsely onto the surface of John Innes Seed Compost, straight out of the bag with nothing added, in 6cm or 2″ plant pots. Tamp down the compost slightly before sowing, to get a flat surface. The seeds are not covered up as many of them need light to germinate. I don’t sterilise the compost, and I don’t do anything else in particular. I believe many of the problems with fungus come from handling, and I try not to touch the compost, seeds or inside the pot with my fingers. Simple, quick and easy. The pots are then stood in a tray of tap water until the surface is visibly starting to get wet.  Don’t use rain water collected from a roof- it will have lots of invisible moss spores in it which may become a problem later. If your tap water is very hard, add an espresso-sized amount of vinegar to your watering can. The pot (with its newly written label!) is placed in a clear small polythene freezer bag. I use the blue tinted type with tie-handles, bought from the supermarket. The handles are tied (or the bags are sealed in some other way) and then they go into a bottom-heat propagator set to a constant 18 degrees C.

Then (and this is the most important part) they are forgotten about for at least a couple of months. I usually peer through the bags at this stage to make sure that mould, moss, or other unwanted growth isn’t happening, and that the inside of the bags is still nice and moist. All being well, I do not open the bags and they get left alone for another few months.

It is usually the following spring before I open the bags and take the pots out to dry off. The exception is the aforementioned moss or mould, etc. In which case the pot is taken out of the bag and the offending material removed; the pot is then dried off completely and from then on treated the same way as other cacti, watered occasionally and dried off in between. But I have to say this doesn’t happen very often.

Once they’ve had their year in the “baggies”, the pots are removed and allowed to dry, and then watered with the rest of the cacti. I feed them with the usual cactus feed with every watering. After a few months of this, some of them will be big enough to transplant into their own individual pots, but many will still be too small and for those, I leave in their same pots until the 2nd spring.

Growing plants from cuttings is less productive than growing from seed, but if you want to grow plants which are true to a particular field collection number, this is the way to do it.

It is relatively easy, but is a bit different from taking cuttings from non-cacti. Cuttings can be taken at any time of year but are best taken in spring to allow the maximum amount of growing season for roots to get established.

Using a very sharp, sterilised, knife (you can sterilise it by heating with a lit match), cut off a branch or offset of your parent plant at the narrowest point where it joins.

Dip the cut end in some hormone rooting powder; or, if none is to hand, some ground cinnamon will help stop infection. Then leave the cutting to completely dry off for at least 24 hours. If there is a wound on the parent plant, dust it with flowers of sulfur or cinnamon to help prevent infection.

Once dry and calloused over, the cutting can be put into a tray or shallow pot, or propagator, which has been filled with a mixture of grit, sand, and a little compost. Most will grow roots without any bottom heat but putting into a heated propagator at about 20oC will make them root faster. Leave them for a month or so before starting to water, and thereafter treat the same way as other cacti.

Note that this is a slow process! It will be a few weeks, possibly months, before the cuttings are rooted well enough to pot up in their own pots. You can tell how well the roots have grown by giving them a little nudge- those which have rooted well will feel solidly firm in the substrate.

If any of this is not clear or you have questions, feel free to ask using the contact form.