Root Awakening


Intense sunlight, insufficient nutrients cause ZZ plant's yellowing leaves

I place this plant (above) outdoors but under the shade of my balcony. Its leaves are turning yellow. Why is this so?

Madelene Chua

The plant is commonly called the ZZ plant and its botanical name is Zamioculcas zamiifolia. It grows best with about six hours of filtered sunlight.

Does your plant get any exposure to intense sunlight, even for a few hours? Coupled with the lack of watering and intense sunlight, the leaves of plants, which are accustomed to lower light conditions, can become bleached. Leaves that are damaged will not revert to the original condition.

New green growth produced under shadier conditions will gradually replace the damaged foliage, which can be seen on the plant pictured.

Another possibility is nutrient deficiency. How long has the plant been growing in its pot? A plant that has been grown in a growing media for a long period may have exhausted the nutrient supply.

The yellowing of older leaves points towards the lack of mobile nutrients, such as nitrogen. If this is the case, it is recommended that you move the plant to a larger pot with fresh potting media.

You can feed using slow-release fertiliser pellets to provide nutrients that this slow-growing plant needs.

Sucking pests infesting custard apple tree


Lately, my custard apples (above) have ants and a white substance. What is causing this and how can I treat it?

Teo Kim Meng

It appears the fruit of your custard apple (botanical name - Annona reticulata) is being infested with sucking pests such as mealy bugs or scale insects. The infestation looks severe and can be difficult to manage.

If there are not too many fruit and the plant is at a manageable height, you can attempt to remove the pests using a soft toothbrush.

The tree is probably being infested with these sucking insects and they have spread to the fruit. When infestation is very severe, it is recommended you prune badly infested portions of the plant and discard them.

As you are growing edible plants, you should spray the plants thoroughly on a regular basis using organic pesticides such as neem oil or summer oil, which suffocates the pests. The pesticides can be rotated with another organic pesticide called pyrethrin, which is derived from chrysanthemum and helps to rapidly reduce the population of the pests.

Sand Rose lacks sufficient light


Why does the succulent plant (above) keep growing upwards? Is it due to excessive sunlight or overwatering?

Rachel Lim

The succulent is likely Anacampseros rufescens, commonly known as the Sand Rose. The new growth looks very stretched and pale, which is a sign of etiolation, resulting from the lack of light.

The stretched portions will not revert to the original form and you may want to prune them.

If you live in an apartment, move the plant to a brighter place, say, in a location where it can get four to six hours of direct sunlight. Under more intense sunlight, new growth will likely be more compact and leaves may take on some colour.

Guiana Chestnut needs more light to thrive


I bought this plant (above) from a nursery to enhance the environment of my office in November last year. What plant is this and is it suitable for the office environment? The office does not have windows and my staff have been watering the plant regularly, despite the nursery's advice to water twice a week. Now the plant looks like this. How can I save it?

Tan Kok Keng

The plant is commonly known as the Guiana Chestnut. Its botanical name is Pachira aquatica. It is often sold as a braided form as an auspicious display plant for Chinese New Year.

Your plant appears to be declining due to the lack of light. Many plants are mistakenly sold as "indoor plants", which are thought to be shade-tolerant. However, even the most shade-tolerant plant needs sunlight to grow and the indoor environment is often too dim to support plant growth.

This plant can be grown under direct sunlight, but can adapt to grow under filtered sunlight.

For it to recuperate, it should be moved to a place where it can get at least four hours of filtered sunlight.

African Milk Bush's sap is highly toxic


I was told this cactus-like plant (above) can help get rid of white ants and is not found in Singapore. What plant is this?

Paul Chan

The plant is botanically known as Euphorbia tirucalli. Its common names include African Milk Bush, Pencil Cactus and Milk Bush.

By white ants, you are probably referring to termites. It appears that the milky sap of this plant can have an anti-termite effect as reported in scientific literature.

This plant is not rare in Singapore. It has been grown in community and home gardens as an ornamental or medicinal plant.

Do note that the sap of this plant is highly toxic. It can cause blindness and severe skin irritation, so handle it with care.

• Answers by Dr Wilson Wong, an NParks-certified practising horticulturist and park manager. He is the founder of Green Culture Singapore and an adjunct assistant professor (Food Science & Technology) at the National University of Singapore.

• Have a gardening query? E-mail it with clear, high-resolution pictures of at least 1MB, if any, and your full name to

A version of this article appeared in the print edition of The Straits Times on March 30, 2019, with the headline 'Root Awakening'. Subscribe