Plant Doctor Archive

Cracked cacti

I LOVE and collect cacti. One of my cacti has been getting dry, cracked rotten bits around the base of the spines. What is this and is there a cure?

I CHECKED with the experts at Coromandel Cacti and we've decided that your plant is suffering from some sort of stress. Under extreme stress, cacti sometimes convert normally green tissue into a dry, woody bark-like substance, which often looks diseased.

Stress could be caused by the plant being pot bound, suffering root pests or diseases or a combination of these. Although the damaged areas won't recover, you can give the plant a new lease of life by repotting it. If it has dead roots, trim them off - in severe cases you may have to remove the whole root system and perhaps even cut into the base of the stem to remove rotten tissue. If you do trim the roots, leave the plant in a warm dry place for several days so the wounds properly dry off and new tissue starts to grow.

Cut stems need to dry for longer and the bigger the cut the longer they take - it pays to leave plants with large wounds for weeks before repotting. Use a specialist cacti mix and remember that although cacti can stand long periods of drought, most varieties grow better with regular watering.

Mealybugs can attack the roots of some cacti. These small, white, woolly-looking bugs could be contributing to your plant's condition. You can control them with an insecticide such as Carbaryl or Malathion. Thoroughly soak the whole root system with it (leave the roots to dry before repotting). Or leave the plant in its pot and water the solution into the mix. Follow the label's safety recommendations while doing this and dispose of any left over spray safely - the best way is to pour it on soil in an unused part of your garden, not down a drain.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.