Cacti have a root system of modest dimensions, and are generally quite slow growing, but in any case it is advisable to repot these plants at least every 2-3 years, preferably in the autumn. They do not like to have large spaces to grow into, so always choose an earthenware container not much bigger than the one they were placed in before. Also avoid putting several species of cacti in the same pot, because they don’t all have identical needs and often tend to compete, usually leading to the death of some individuals for the benefit of others. Prepare a growing substrate that is well drained and a bit rough in texture, using one part loam, one part sand and one part lapillus, perlite or pozzolan. Take the cactus out of its container from the previous year, doing your best to avoid permanent damage to the spines. To avoid stings and also ruining the spines you can cover the plant in polystyrene before you take hold of it. You can also prepare a sturdy fabric or cardboard strip which you can use by wrapping it around the plant and joining the ends: that way you can make a real handle which you can use to pick up the plant without damaging it and without hurting yourself. Then insert the plant in a pot and cover the root system well with the substrate which you have prepared in advance, trying to compress the soil enough to allow the plant to stand up. Then place the pot in a sunny place, sheltered from the cold, and avoid watering for at least 10-12 days after repotting.