Ask the Plant Doctor! How to care for cacti, when to harvest papayas, and weed control

Cacti require ample sunlight and little water.

Do you have a question about plants or how to maintain your garden? Send your gardening questions to the Plant Doctor!

Email your questions to [e-mail] with "Plant Doctor" in the subject field. Questions may be edited for brevity and clarity.

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Dear Plant Doctor,

How much light and water do cacti need? And what kind of soil is suitable for cacti?

- Dr Chaw and Chris Cao

Cacti are easy plants to care for, provided two rules are strictly followed: that they get enough light and they are not over-watered.

Cacti are not low-light plants, regardless of whether they are indoor or outdoor plants. They require high light, with bright and long exposures to light.

For indoor cacti, they need to be placed away from direct sunlight yet still be able to get sufficient bright light, such as near windows, or if placed under the porch, they are placed close to the edges of the porch. Indoor cacti include Gymnocalycium mihanovichii, Euphorbia lactea, and my favourite, Euphorbia trigona.

Cacti are also very sensitive to excess water. Therefore, it is very important that cacti be planted in free-draining soils so that these soils do not turn into a water pond or become easily saturated when over-watered. Cacti require little water, so watering can be done once every four to even six weeks.

When can papayas on a tree be picked? Is it a myth that unripe fruits, once picked, cannot ripen on their own?

– Sally Teh

Harvest papaya when it is not fully ripened.

The skin of a papaya fruit turns gradually from dark green (immature) to dark yellow (fully ripe) as it ripens. Papaya fruits are often picked when they are between 50% and 75% ripe.

This harvest stage is denoted by the colour yellow covering between half and three quarters of the whole fruit’s exterior.

Although fully ripened fruits can still be picked, this is often not done to avoid skin blemishes or black spots from appearing on the fruits.

Once picked, the fruits can be wrapped in newspaper and left to ripen at room temperature indoors.

What is the best way to control weeds in a garden without using harmful chemicals?

– Kamila R.D.

You can use household vinegar, which typically contains only 5% acetic acid. Acetic acid destroys plant cell membranes by desiccating the cells and finally killing them.

Acetic acid is also safe for the environment because it breaks down easily into water in the soil, without leaving behind harmful residues. Vinegar can be directly sprayed, undiluted, onto the weed’s leaves.

Within a few hours, the weeds will wilt, turn brown, and die within a day. Vinegar is more effective against small, leafy weeds but is less effective against large, woody weeds.

For the latter, a stronger vinegar is required, but care must be taken when handling such vinegar, especially those containing 20% or more acetic acid.

Lastly, never use salt as weed control. Salt has no place in gardening, as it reduces soil health.