What’s the problem?
I’ve had this cactus for many years, but it keeps getting taller and soon it will hit the ceiling. How can I stop the plant growing without doing it harm?
Diagnosis
The plant in question isn’t a true cactus at all, but a succulent called Euphorbia trigona, also known as the African milk tree. Like many columnar euphorbias, it can shoot up rapidly indoors if it’s happy, often outgrowing its space. Luckily, the plant responds well to pruning if done carefully.
Prescription
The most effective way to manage its height is to “top” the plant. This means cutting off the growing tip of your tallest stem. Use a sharp, clean knife or pruning saw, and wear thick gloves as euphorbia sap is highly irritating to skin and toxic if ingested. Once cut, the plant will often branch out lower down, creating a more compact, sculptural shape. You can also root the cutting: allow the end to callus for several days, then pot into dry, gritty compost.
Prevention
Periodic pruning is the only long-term solution indoors. Keep it in the brightest spot and rotate so it grows evenly. In time, your plant will adapt and may even look more attractive.
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