How to Look after Cacti
Created | Updated May 25, 2005
This entry is intended for the person who has one or two cacti on a windowsill, or a handful of plants in a greenhouse or conservatory. Most cacti will flourish with the instructions given below, although anyone looking to expand their interest in the family Cactacae should be aware that not all of them will thrive in the same growing conditions.
Why Grow Cacti?
Cacti have evolved to grow in one of the harshest environments on earth and are incredibly resilient to neglect, making them an attractive proposition to people who want to grow houseplants but who feel that they don't have a 'green thumb'. Cacti will also tolerate the well-meaning - but busy or forgetful - horticulturist. Furthermore, being rather slow-growing plants, they're ideal for anyone with no more space than a windowsill on which to place a few plants.
Wherever you may live, cacti1 can easily be grown, and many species will produce flowers - there's no special magic to bringing them into bloom2. What's more, it's a myth that they only flower every seven years.
Caring for Cacti
There are three main factors to consider when cultivating cacti - water, heat, and light.
This is by far the easiest one to get wrong. The two basic rules to follow are to let the soil dry out between watering and to refrain, mostly, from watering during the winter when the plants are dormant. In their natural habitat most cacti will have a long growing season and a short dormant period - winter. Like most perennials3, they take a rest during the cooler, drier months. Come the spring however, the growing season begins again.
The precise time that the growing season begins will depend on where you live and which species of cactus or cacti you have. If you see flowerbuds appearing or if you see any new growth on the plant, give it some water - a little at first, then gradually more over the coming weeks, remembering the golden rule - let the soil dry out completely between watering. Don't worry too much about the plant not getting enough to drink - they can go several months without water, although this isn't recommended, particularly if you want to get the best from your plant. A good rule of thumb to follow is to start watering two to four weeks after the spring equinox.
During the height of the growing season cacti can be given a good soaking about once a week. Watering your plants on the same day each week is a good routine to get into. It ensures that you don't forget to water them, and should prevent them from being over-watered.
The best way to water them is to use a shallow tray - the size and depth of the average roasting pan is ideal - partially filled with water. Stand your cacti in the water and leave them there until you can see that the top of the soil is wet, by which time the entire root ball and surrounding soil of each plant will have been thoroughly saturated. Remove the pot(s) from the tray and stand them where any excess water can drain freely. If your cacti are in a greenhouse4, you will most likely find that the soil has completely dried out after a day or two. If the plants are indoors, it may take a little longer, but should be dry again before seven days is up.
If you are unable to use this method, or if you only have one or two cacti, you can use an old dish or saucer to do the same job, as long as the water is allowed to drain afterwards.
As autumn approaches, your cacti will want to wind down for the year and take a rest, so you should gradually ease up on the watering. In north-western Europe for instance, this would be towards the end of August and into September. If you continue to water them on into the winter, they will eventually rot. Indeed, during the winter they will need little or no water at all. The exceptions to this are cacti that are kept indoors in a centrally-heated room. These plants don't slow down their processes to the same extent as those kept outside in a greenhouse, and should be given a light watering about once a month.
While it can get pretty chilly at night in the deserts where cacti grow, none of them will tolerate more than a degree or two of frost5, and some of them will give up the ghost if the temperature falls much below 5°C (42°F). If you are keeping your cacti indoors you probably have little to worry about - even if your home isn't centrally-heated, it's unlikely to experience indoor sub-zero temperatures. If your cacti are in a greenhouse or conservatory however, you will need to provide heat during the winter.
If the conservatory has electrical power outlets, you could plug in a small space heater to keep the frost away6. If, on the other hand, your plants are in a greenhouse, you will most likely be using a paraffin greenhouse heater - easily obtained from most garden centres and nurseries.
The ideal temperature for cacti during the winter is around 10°C (50°F). This is plenty warm enough to keep them from coming to any harm from the cold but cool enough to maintain a state of dormancy. It's advisable to keep them out of cold drafts.
One final word of warning regarding heat - cacti can easily get too hot! It's not a good idea to let their environment get much above 32°C (90°F). If you have your plants in a greenhouse, make sure that it's well ventilated during the summer. If your greenhouse has windows or ventilation that opens automatically at a pre-set temperature, great - you've little to worry about. If not and a hot day is forecast but you won't be around, open the windows before you leave for the day.
As soon as the last frost of the winter has occurred, there should be no problem leaving the greenhouse windows open throughout the summer, but don't be tempted to do this too early - one late season cold snap could put an end to some of your most cherished plants.
Cacti grow in deserts, and deserts are generally sunny places, so cacti will thrive where they can get the most light. If you are growing them indoors, a south-facing window in the northern hemisphere and north-facing window in the southern hemisphere is ideal. Any aspect where they get some direct sunlight will do, and they'll even tolerate a north-facing window (or south-facing, in the southern hemisphere), but don't expect as much growth or as many flowers as those plants bathed in the warmth of the sun.
In a greenhouse of course, your cacti will get all the light they need. If you are growing them in a conservatory or lean-to, the same directional principles apply as to those grown on a windowsill.
Soil and Re-potting
Cacti will easily rot if their roots are kept wet for too long, so a free-draining, sandy soil is essential. Prepared cactus soil is sold at most garden centres and nurseries, but if you are unable to find any, simply mix two parts of good potting compost with one part one part of sharp sand (not builder's sand - buy your sand from a garden centre or nursery).
When re-potting, you should try to disturb the roots as little as possible so as to minimise any damage - broken roots are very susceptible to rotting, and a newly re-potted plant should not be watered for a few weeks. Clay pots are better for cacti than plastic because water can soak through them and evaporate, encouraging the soil to dry out quickly after watering. Use a pot one size larger than the previous pot - putting your cactus into a much larger pot will encourage the plant to quickly expand its root system, at the expense of any growth on top. A dressing of gravel or small stones on top of the soil will show off the plant at its best.
When to Re-pot
With globular cacti and those which grow as a cluster it's very easy to judge - you do it before the plant starts pushing against the sides of the pot and you can no longer see any soil or top dressing. With columnar cacti it's a little more difficult to know when to re-pot. If you find that plant is becoming top-heavy and is easily knocked or blown over, that's a sure sign that you need a bigger pot.
Cacti bought from non-specialist nurseries and garden centres will often be pot-bound, and the cheap growers' pots which they come in are not the most aesthetically pleasing, so give them a new pot and some fresh soil as soon as you get them home.
It's safe to re-pot at any time during the growing season, although the serious collector might insist that you only re-pot in the spring when the plant is waking up from its dormant period. Re-potting in winter is not recommended, but also bear in mind that re-potting a plant with flowerbuds will most likely cause the buds to die.
Handling Cacti During Re-potting
If your cactus has particularly sharp or ferocious-looking spines, there are ways to re-pot it without suffering injury to yourself or the plant. A good pair of gardening gloves will give you protection when transplanting cacti that can be held in one hand. Larger plants can be handled by cradling them in strips of woollen blanket, sponge foam7, or better yet, both. A columnar cactus, three or four feet (a metre) in height, can easily be re-potted by two people using this method.
Warning One - If you are re-potting what looks like a cactus and you see milky sap oozing from anywhere on the plant, what you have is a succulent of the genus Euphorbia. The sap can cause nasty blisters, and in some species is toxic. Any contact with the skin should immediately be washed off under running water.
Warning Two - There is a variety of Opuntia (Opuntia Microdasys), or prickly pear, which has small hair-like spines, called glochids. These can work their way into your skin and cause irritation, rather like strands of glass fibre. Wearing gloves during re-potting will prevent several hours of itching.
The kind of soil which cacti naturally grow in is generally of a very poor quality, and they have evolved to be efficient at extracting nutrients from it. When grown as a houseplant and re-potted before they outgrow their pot, cacti will require little in the way of fertiliser. A proprietary tomato feed mixed at about one-third normal strength and given once a month during the growing season will be all they need. If you keep fish, use water from the tank to water your cacti - the waste products of the fish contain a lot of nutrients.
Over-feeding a cactus will make it grow too quickly and become spindly. Cacti which get too many nutrients will put a lot of effort into growing but produce few flowers, and tall cacti which have grown too fast will not be able to support their own weight.
Cacti are susceptible to a number of pests, particularly mealy bug and red spider. It's up to you whether you want to treat them with chemicals or organically. There are plenty of proprietary treatments on the market if you have no objection to using chemicals, and there's no doubt that they will efficiently rid your cactus of the pest. There are other more organic methods of fighting pests, such as quarantining the affected plant and, in the case of mealy bug for instance, using a small artists' brush to remove the insects whenever you spot one, thus ridding the cactus of them over a period of time. Spraying with a nicotine solution is another way of killing or preventing most pests without the use of harsh chemicals.
Your local garden centre or nursery should be able to give you help and advice about other non-chemical treatments, and you could even try your local radio station - many of them have weekend gardening phone-ins.
By following these few simple rules, there's no reason why you shouldn't be able to grow healthy, long-living cacti, and persuade some of them to put on a marvellous show of flowers each year. Incidentally, the seed pods of some cacti (Mammillaria and Epiphytum for instance) are edible and quite delicious, but as with wild mushrooms, don't eat one until you see someone who knows about such things take a bite themselves and live to tell the tale.