Tomato plant may have nutrient deficiency

I have three tomato plants grown from seeds given by the National Parks Board. All have curled leaves. On separate occasions, I treated them with sulphur soap (dissolved in water) and potassium bicarbonate according to the specified concentration, but there has been no improvement.

Tan Peng Boon

The symptoms in the new leaves do not appear to be pest related. Instead, the leaves appear chlorotic, where the veins are darker green than the rest of the leaves and, as such, the plant may be suffering from a nutrient deficiency.

Check the soil pH level and ensure it is in the range of 6 to 6.8. Make sure the soil is well drained and not too wet as wet feet can impede nutrient uptake.

Shelter the plants from excessive rain to prevent the root zone from being flooded.

Use fertiliser with trace elements. It is not uncommon for gardeners to feed plants only with organic fertiliser such as chicken manure, which does not contain all the nutrients plants need for healthy growth.

Plants are Plectranthus species

I was given these mint plants. What are their names and are they edible?

Gan Sing Hai

The plant on the left is botanically known as Plectranthus zeylanicus. It is not to be mistaken for its close relative, shown on the right, the Indian borage (Plectranthus amboinicus), though the two species look very similar.

Locally, this plant is sometimes called "lemon balm" due to its citrusy scent.

However, the true Lemon Balm (Melissa officinalis) is another plant species altogether. This plant is used in ayurvedic medicine.

The plant on the right is the variegated version of the Indian borage. Its leaves are sometimes used as an oregano substitute and, locally, they are boiled and the tea drunk to treat coughs.

Consider repotting fern pine

Why are some of the plant's leaf tips yellow with brown spots? Also, the colour of the leaves looks lighter than that of similar plants I see along the road and at nurseries. I live by the sea and the pot is on my balcony, which receives almost full sunlight all day. I water it once a day.

Jocelyn Toh

The common names of your plant include yew plum pine, Buddhist pine and fern pine. Its botanical name is Podocarpus macrophyllus. It is documented to be somewhat tolerant to salt spray.

How long has your plant been grown in its current pot? From the appearance of the leaves, the plant seems to be suffering from a nutrient deficiency. If a plant has been grown in the same pot for a long time, nutrients required for healthy growth could have been exhausted and the soil properties could have also changed, resulting in poor health of the plant.

It could also be a case of fertiliser salt building up over time and affecting the uptake of nutrients.

Check your soil's pH level to ensure it is in the slightly acidic range, which is optimal for growing this plant.

If the plant has been grown in the same pot for a long time, you may want to move it into a larger pot with the root ball intact and backfill it with a potting media that has quality organic matter like compost. Also, mulch the soil surface with compost too. Keep the root zone moist at all times.

Hibiscus needs sunlight, feeding, regular watering

My hibiscus used to bloom very often. However, for the past two months, I see flower buds but they do not bloom. What is wrong?

Linda Lim

Hibiscus are sun-lovers and the lack of growth in your plant may indicate it is not receiving sufficient sunlight. You will need to grow it in a place where it can get at least six hours of direct sunlight.

If the sun has shifted its path at this time of the year, you may want to move the plant - it is grown in a pot which makes moving easier.

The plants are also heavy feeders and you will need to regularly feed yours with fertiliser to boost growth and promote flowering.

The dropping of flowers could be due to plant stress. Ensure that the plant is adequately watered. Do not let the root zone dry out totally or be wet all the time. Also, avoid growing it in a constantly windy place which can dry out the plant.

Bud drop could also be due to an infestation by thrips. These pests can be eradicated with application of a pesticide called spinosad.

Chinese Wormwood with attractive foliage

I bought two of these succulents when there were four to five florets a pot. They have grown so wonderfully. What are these plants and how do I propagate them?

Laura Hwang

The plant is commonly called the Chinese Wormwood (botanical name: Crossostephium chinense) and is a shrub with highly ornamental, silver leaves. It can be propagated via stem-cuttings.

The Chinese Wormwood can be difficult to cultivate in Singapore as it requires well-drained soil that dries out slightly between each watering. It should also be protected from the rain with a shelter over it for the best results. The plant is best grown under direct sunlight for at least six hours daily.

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

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