Remove cactus’ infestation with soft toothbrush

I have had this Gymnocalycium cactus since May 2022. When white spots started appearing, I treated them with neem oil and then with 75 per cent alcohol spray, but the spots seem to have overwhelmed the plant. I water the cactus once a week, and keep it away from rain and direct sunlight. What is wrong?

Eug Chan

Your cactus appears to have a scale infestation. First, try to remove the pests by using a soft toothbrush, but do note that their feeding habits may have permanently scarred your plant.

The contact pesticides you used work by suffocating existing pests. For a more lasting effect, use a systemic pesticide such as dinotefuran granules, which are applied around the root zone.

Once the granules dissolve, the pesticide within them is taken up by the plant, enters the sap and kills pests that feed on the plant. Replenish the granules according to the manufacturer’s recommendation.

Calathea plants need humid environment; may be sensitive to salts in tap water

The leaves of my Calathea plants seem to be drying up. The plants are in a corridor which gets only indirect sunlight. I water them on alternate days and make sure the soil is not too damp before doing so. What is wrong?

Karl Ng

Calathea plants are difficult to grow in high-rise environments. They do not thrive individually and need to be grown among other plants that create a humid microclimate. Strong winds and low humidity dry the plants out, which can give the leaves a rolled-up appearance.

Parts of the leaves appear burnt. This is because plants can be sensitive to dissolved fluoride in tap water or build-up from chemical fertilisers. Try flushing out the growing media and use rain water to irrigate your plants instead. Reduce the frequency and amount of chemical fertiliser you are applying, or use slow-release organic fertiliser pellets.

Tall plant is a citrus species