Spearmint, sawtooth coriander are culinary herbs

The leaves of both plants produce a strong fragrance. However, the leaves of one show discoloration. What are the names of the plants? What is the cause of the discolouration and what can I do to prevent it from happening again? Also, are the plants edible?

Gan Sing Hai

The plant (Photo 1) is the spearmint (Mentha spicata); the other (Photo 2) is the sawtooth coriander (Eryngium foetidum). Both are common aromatic culinary herbs used in cooking.

Both thrive with four to six hours of filtered sunlight daily and in moist, well-draining growing mixes.

Regular pruning of the spearmint is needed to keep the plant bushy and tidy.

The yellowing of leaves of the sawtooth coriander is in a few of the older leaves, which is normal. Older leaves may shed with age.

However, do check if there are wet mushy spots as these can indicate a disease due to the leaves' contact with moist soil. You can prune these to prevent the spread.

Basil may have nutrient deficiency

My basil was growing quite well but lately, light green/yellow streaks and patches have appeared on the leaves. Some leaves have also turned completely yellow. What is causing this and how do I prevent it from occurring again?

Hwang Wan Mei

The lower and older leaves of the basil plant appear yellow. This may be a sign that the plant has a nutrient deficiency, such as in nitrogen. You can fertilise the plant with processed organic fertilisers like chicken manure. For faster results, a water-soluble chemical fertiliser can also be used.

Make sure your plant is growing in a pot that is large enough to support its growth. It should not be in a pot-bound situation.

Also, the growing media should be well-drained and aerated as heavy and waterlogged media can impede nutrient uptake.

Plant is an epiphyllum cactus

I bought this plant recently, but I do not know its name. How do I care for it?

Cindy Chai

The potted plant is likely an Epiphyllum cultivar. It grows well in a semi-shaded location, with at least four to six hours of filtered sunlight.

It is an epiphytic cactus that thrives in a well-drained growing mix. Most people grow it in an aerated mix consisting of coconut husk chunks that will retain water yet permit air in the root zone.

Water this plant thoroughly and allow the roots to dry out slightly before watering again. Constant wetness at the root zone can cause rot.

Podocarpus may have transplant shock

About three months ago, I bought some bonsai podocarpus. The original pots were shallow, so I got the nursery to repot the plants into taller pots. The media was an equal mix of black volcanic sand and normal black soil. The plants were grown outdoors at the nursery. I moved them under the shade when I got home and they received three to four hours of sunlight on alternate days. Watering was every two to three days. The drainage was good - water flowed out at the bottom of the containers almost immediately upon watering.

But after about six weeks, some of the leaves turned either orange/brown or dark grey/ black. I watered more, but the problem did not go away. I then moved the plants outdoors, but protected them from the rain. With the plants getting almost full sunlight, I water them daily now. It has been about three weeks, but the problem persists.

Lawrence Lum

The issue appears to be transplant shock. Do you know how much soil was removed during the transplanting process? The podocarpus is quite sensitive to root disturbance and it can take a long time for the tree to recover. In severe cases, plants never recover and they decline and die eventually.

For now, move the plants to a cool and bright location and keep the root zone moist, but not wet.

Excessive drying out of the root zone can cause new roots to die. Apply a thin layer of good quality compost to help retain moisture and keep the root zone cool.

You can try to further reduce the stress of the plant by applying seaweed extract on the leaves as a foliar spray as well as a soil drench.

Shrub is mulberry plant

Is this the perilla and is it edible? Its stem has a white substance and it is now spreading to the other stems as well. Also, the leaves turned yellow and I had to prune them.

Tan Mei Choo

The plant is not the perilla, although the leaf shapes of both plants may look similar.

What you have is a mulberry plant (Morus alba). The plant grows as a shrub and becomes a small tree. It produces fruit that are edible when they are ripe and red. The leaves are harvested and made into a tea.

The white spots on the stems may be the lenticels, which are breathing holes of the shrub that occur in the woodier part of the plant.

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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