Repotting a cactus is different from shifting an ordinary leafy houseplant into a bigger pot. You need to protect your hands from spines, keep fragile roots intact and choose compost that drains quickly. A large, deep container filled with rich, moisture-retentive compost can make life harder for a small cactus rather than giving it more room to grow.
Most cacti do not need annual repotting. If the plant is healthy and the roots are not overcrowded, leaving it alone may be the better option. When a repot is necessary, spring is generally a sensible time.
Decide whether repotting is needed
Look for roots emerging from the drainage holes, a rootball that has become densely packed or compost that has broken down and no longer drains properly. A cactus leaning because of a heavy top may also benefit from a better-balanced container.
Some slow-growing cacti naturally prefer fairly snug pots. Don't repot purely because the plant hasn't grown much in one winter; slow growth may reflect low light or the species' normal pace.
A rotting cactus is a separate problem. If the stem is soft or the roots are black and mushy, simply moving it to a bigger pot will not cure the underlying issue. Inspect and address the damage before treating it as routine repotting.
Choose a pot with proper drainage
Use a container with at least one open drainage hole. A decorative bowl without drainage is risky, especially for a cactus that takes a long time to use water.
Select a pot only modestly larger than the rootball. RHS advice on repotting generally cautions against moving small plants into containers far larger than the old pot, because excess compost can stay wet.
Terracotta can be useful where pots otherwise remain damp; plastic also works with appropriate watering and a good medium. Consider stability for tall or heavy specimens. A cactus that repeatedly topples can snap roots and create a hazard.
Choose compost that dries appropriately
Most desert cacti benefit from an open, free-draining mix. Use a suitable peat-free cactus and succulent compost or follow a reliable horticultural recipe with appropriate mineral grit.
Ordinary heavy garden soil in a pot may compact and remain wet. An excessively fine, moisture-retentive potting mix can also stay damp around roots for too long. The exact recipe depends on species, climate and pot material.
Forest cacti, such as Christmas cactus, are different from many desert cacti. They may prefer a more organic, airy mix that retains some moisture. Identify the cactus before choosing a very gritty desert mix for every species.
Protect yourself from spines
Wear suitable thick gloves and long sleeves where necessary. Wrap a newspaper strip, folded cardboard or several layers of paper towel around a small cactus to form a temporary handle.
Tongs may help with a small plant, but squeeze gently. Crushing a stem or pulling a cactus by its spines can cause damage. For larger specimens, ask another person to help support the plant and avoid lifting it alone if it is heavy.
Some cacti have tiny barbed glochids that lodge in skin despite looking harmless. Handle those with particular care. Keep children and animals out of the working area and sweep up fallen spines afterwards.
Remove the old pot carefully
Allow the compost to approach a sensible level of dryness so the rootball is less messy to handle. Support the cactus while gently loosening the pot. Flexible plastic pots can be squeezed around the sides; rigid pots may need careful tapping.
Tip the cactus out while holding the wrapped stem. Never yank hard on the top if the roots are stuck. If the plant is severely pot bound, loosen the edges patiently or sacrifice a cheap plastic pot rather than tearing off healthy roots.
Brush away loose exhausted compost where it falls away naturally. Do not wash every root clean for an ordinary repot, because unnecessary handling can damage delicate feeding roots.
Inspect the roots before planting
Healthy cactus roots vary by species but should be firm rather than slimy or collapsing. Look for damage, pests and areas of obvious rot.
Trim dead, mushy material with clean tools where necessary. If substantial roots are removed or injured, allow wounds to dry appropriately before watering according to cactus-specific guidance. A heavily damaged root system may need a smaller pot than you originally planned.
Don't trim healthy roots indiscriminately merely to force the plant back into a tiny container. The goal is to improve its growing conditions while preserving as much healthy root function as possible.
Put the cactus at its former level
Place a little fresh compost at the bottom of the new pot, then position the cactus so the base sits at approximately the same depth as before. Burying the lower stem deeper can leave vulnerable tissue in contact with damp compost.
Backfill carefully around the rootball without crushing it. Tap the pot gently to settle the mix into gaps. Avoid pressing down so hard that the medium becomes compacted.
If you use a decorative top dressing of mineral grit, keep it from masking poor drainage or trapping moisture against a sensitive stem base. A top dressing is optional, not a substitute for the right compost.
Watering after repotting
After repotting a cactus with undamaged roots, follow its species-specific watering needs and the condition of the new mix. If roots were injured, allowing a short dry settling period before watering can reduce the risk of decay.
Do not automatically drench an injured desert cactus in the same way you might water a freshly transplanted bedding plant. Some standard advice for leafy houseplants needs adapting to the drought-tolerant species.
When watering, soak the medium appropriately and allow it to drain completely. Don't leave the pot sitting in water. Wait until the compost has dried to the level appropriate for that cactus before the next watering.
Position the cactus during recovery
Return it to a bright place suited to the species, but shield a recently handled specimen from sudden intense sun if it was previously grown in shade. Abrupt changes in light can scorch tissue.
Avoid placing a freshly repotted plant against a cold pane during winter or right above a very hot radiator. Stable conditions make recovery easier.
Expect some species to sit apparently unchanged for weeks. Check for stability, firm tissue and the absence of spreading soft patches. Resist digging the plant up repeatedly to see if new roots have grown.
Fixing common repotting errors
A cactus that becomes soft at the base may be suffering from rot associated with excessive moisture or existing damage. Check drainage and root condition promptly rather than assuming it needs more water.
If the new pot is enormous compared with the rootball, consider whether the medium will remain damp for too long. A more modest pot can be safer for a slow-growing plant.
If a tall cactus wobbles, support it temporarily and check the rootball is adequately surrounded by compost. Don't bury extra stem to anchor it. Above all, identify the species: a desert cactus and a tropical epiphytic cactus can need substantially different care.