Orchid infested by spider mites
The bottoms of my orchid leaves are infected (above) and I am wondering what the cause is. I use goat excrement pellets and liquid fertiliser, alternating them every two weeks. I also use insecticide about once a month. Could these be the cause of the infection?
Tong Ai Leng
Your orchid plant appears to be showing symptoms of a spider mite infestation - a common occurrence in plants that are grown in dry, windy and high-rise conditions.
To reduce the population of spider mites, you may want to increase the ambient humidity of your growing area by adding foliage plants around your orchid and regularly spraying your plants to wash the pests and their eggs away.
You may also find it beneficial to regularly spray organic pesticides like insecticidal soap solution, neem oil or summer oil, which can kill spider mites by suffocating them.
This can be done weekly to keep the spider mite population manageable.
Cover all parts of your orchid plant with the pesticide for it to be effective.
Sunflowers require full sun to grow well
What are these plants (above)? I grew them from seeds I bought overseas, but cannot recall the type of seeds.
It is likely you have planted sunflowers (botanical name is Helianthus annuus).
Moving forward, you should note down plant names and the dates the seeds were sown as soon as you have planted them. Such details will help you recall what has been planted, whether the seeds sown are viable and if the plants are growing at a rate that is expected of the cultivar.
Do note that sunflowers require full sun to grow well. If you are growing the larger cultivars, it is recommended to plant them individually in large pots. They need to be fertilised regularly and heavily to promote robust growth, which will lead to large flowers at maturity.
Pink Rain Lily a common landscaping choice
I spotted this flowering plant (above) growing on the turf near a carpark in Ang Mo Kio. What is its name and is it available at nurseries?
Kng Poh Eng
The plant is a common landscaping candidate and it is readily available for sale in local plant nurseries.
It is commonly called the Pink Rain Lily and its botanical name is Zephyranthes rosea. Rain lilies are so named as they flower after a rainy period. Recent plant breeding has resulted in a range of floral colours.
This plant is best grown in a sunny spot with well-drained soil. It also makes an ideal potted plant and can be propagated via division.
Withering rosemary common in lowland tropics
I have been trying to grow rosemary (above) to no avail. A few weeks after buying the plants from a nursery, they wither. I water them moderately and place them on the porch, where they get two to three hours of direct sunlight a day. Why is this happening?
Your rosemary plant (Rosmarinus officinalis) is infested by a sucking insect called a lacebug. It sucks sap from the leaves of the plant and leaves behind many yellow spots in the areas it has fed on.
It is difficult to eradicate using organic pesticides, so you may need a chemical pesticide such as cypermethrin. Do observe the withholding period - the time that needs to elapse after application of the pesticide before the plant can be harvested for consumption.
As for the withering, it is a common occurrence in the lowland tropics, due to soil-borne fungal or bacterial diseases.
Gardeners in Singapore have overcome this issue by growing rosemary using hydroponics, which is a "cleaner" cultivation method without soil.
Tip cuttings can be taken from existing healthy plants and rooted in water before being grown hydroponically in a nutrient solution.
• 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.
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