Growing Guide
Cacti are among the most intriguing houseplants, and are quite low-maintenance. While they don't demand excessive attention, they still require proper care and the right growing conditions to thrive.
Light & Temperature
The most important part of looking after cacti is light. Healthy growth, spine development and the number of flowers the plant produce are all determined by the amount of light they are given. Ideally they should be grown in a greenhouse, giving them all round light. However, a sunny windowsill will do very well for most cacti. If grown on a windowsill, they should be removed at night if you close the curtains, especially during colder months. In a greenhouse the temperature should not drop below 40°F (5°C). If your greenhouse is exposed to full sun, it may be necessary to use shading on the glass to prevent the plants becoming scorched. Cacti are particularly prone to scorching in Spring, when the sun suddenly becomes stronger following the more gloomy Winter months. Good air circulation will also help to prevent scorching. Try to keep the Summer temperature in the greenhouse below 100°F (38°C).
Watering
In their natural habitat, cacti only receive water at certain times of the year. In some cases, flash floods wash over them and the plants become submerged for hours at a time. Cacti have adapted to store water for long periods of time. This is done by their concertina-like tissues which swell or contract, and their often waxy exterior helps to minimize water loss through evaporation. When growing cacti indoors, you need to try and imitate their natural growing conditions as closely as possible, During their growing period (Spring to early Autumn), water once a week. It's important to start slowly in Spring after their dry Winter spell. It's best to let the soil dry out completely between watering. By early Summer, when the sun is much stronger, your cacti can be watered more frequently, ensuring the soil is either almost or completely dry from the previous watering. Never over-water your cactus, and never leave it standing in water. The best way to check if the soil is dry is to stick a plant label or even a knife into the soil. If it comes out clean, the soil is dry, and ready for the next watering. In Autumn, the gaps between watering should become longer, especially for plants in small pots. I stop watering all of my cacti towards the end of September. Don't be tempted to give a little bit of water here and there during their Winter rest. This can lead to poor and uncharacteristic growth, or even the loss of the plant altogether.
Feeding
Cacti require a high potash fertilizer, as used for tomatoes and roses etc. Regular feeding encourages strong growth of the stems and spines, and also increases the chances of flowers being produced. It's important to always dilute the fertilizer as per the manufacturers instructions, otherwise the plants roots may become burned. It's worth noting that some fertilizers can leave unsightly stains on the body of the plant if watered from above, so it's always best to try and water around he base of the plant.
Compost
Cacti need a well drained compost mix to aid good healthy root development, and also to help stop the soil staying too soggy. Do not use unsterile garden soil. As a general rule, we use a mix of John Innes number 2 with lime free horticultural grit, mixed at a ratio of 70:30. Several special cactus composts are available from garden centres, but these are often worse than a good home-made mixture.
Repotting
Many cacti are shallow-rooted, and as such will do better in half-pots or pans. Cacti with thick, tuberous roots require a deeper pot to avoid the roots becoming too cramped. Plastic pots seem to be more popular these days than clay ones. However, for some plants, such as Ariocarpus, which come from very arid areas, clay pots have advantages, such as allowing the moisture to evaporate very quickly. For these more difficult plants, if you are using plastic pots, you can increase the grit content of the soil mixture up to 60 or even 70%. If you are short on space, consider using square pots. This makes the maximum use of space, and also gives more root room. The best time for repotting is in late Winter or early Spring, before any fresh roots have developed. Always check the roots when repotting. If there are plenty of new roots and the root ball is solid, repot the cactus into the next sized pot. It may be a good idea just to gently tease a few roots out of the compacted root ball. If you do this, do not give the plant any water at all for two weeks. This will allow any damage to the roots to heal over. If water is applied and the plant has any root damage, rot can set in. However, if the compost falls away from the plant when you take it out of it's pot, and the root system looks good and healthy, you can repot the plant back into the same pot using fresh compost. Always plant your cactus back at the same level it was prior to repotting. Add a top dressing of lime free horticultural grit. This will help stop the soil from drying out too quickly in hot weather, it'll help prevent the compost becoming too compacted into a hard layer following watering, and it also helps to stop water splashing up onto the lower body of the plant. We use a layer of pigeon grit on our plants, and have not suffered any problems at all.
Propagation
Most cacti can be propagated from either seeds or cuttings.
Seeds: In the wild. cactus seeds only survive in damp and sheltered places, needing protection from the scorching hot sun. The best time to sow seed is in Spring, as long as a steady temperature of 70°F (21°C) can be maintained until the seeds have germinated. To help combat algae, add a thin layer of fine grit to the surface of the compost. To aid germination, cover the pot or tray with a clear plastic bag, secured around the bottom. As soon as the seedling begin to sprout, the clear plastic bag can be removed,
Cuttings: It's important to remember that, unlike any other kind of plant, cacti must never be placed into fresh compost and watered. The cut surface must be allowed to dry out for at least 1 week, or harmful bacteria may invade the tissue. Do not water them until fresh new roots have started to appear.
Grafting: Some cacti are difficult to grow on their own roots, or are very slow. Some variegated cacti cannot survive on their own roots. These plants are therefore grafted onto a rootstock which is much easier to grow. However, unlike cuttings (detailed above), the cut ends must not be allowed to dry out, and they should be pressed together immediately. The most important thing to do is ensure the rings near the centre of the stems (the vascular bundles), overlap when both cut surfaces are pressed together. New growth of the upper plant should be seen after around 3 months. If the rootstock is a more demanding plant (care wise) than the actual grated plant on top, ensure you provide the correct growing conditions for the rootstock. The grafted plant will then show some growth as it takes on the vigour of the rootstock.