Dr Christopher Teh, soil biologist and senior lecturer at Universiti Putra Malaysia, answers gardening questions in Star2’s gardening column, Ready, Set, Grow! To send him your questions, fill in the form at the website questions.eatsshootsandroots.com.

What causes my plants to have unhealthy leaves and how I can rectify the problem? These are photos of my rose (above) and  hibiscus (below) plants. Also, how can I keep bugs off my plants? – Mary John

Plants that are unhealthy might need a fertiliser boost

Note those unhealthy-looking spots on the leaves of this hibiscus plant. Photo: MARY JOHN

Both your plants need fertiliser. They appear to be deficient in magnesium and possibly also in potassium. Your hydrangea could additionally be lacking phosphorus. If possible, change the soils of the rose and hibiscus plants and add a fertiliser that includes phosphorus, potassium and magnesium. Follow instructions on the fertiliser package and do not over-fertilise. Some people use Epsom salts, which contain magnesium sulfate, as a source of magnesium.

To control pests, you can mix neem oil with water in a 1:100 ratio and spray onto the plants.

I have a soursop fruit tree that is two-and-a-half years old and 3m tall. It’s very healthy and leafy and bears beautiful yellow flowers. After the petals drop off, something is left behind – not sure if it’s the fruit or seed. After a few days, this seed or fruit dries up, turns brown, and falls off! Why does this happen? What treatment or fertiliser should I apply to get my soursop tree to bear fruit? Is my tree a “male” tree that bears only flowers and no fruit? Or is it still too young to fruit, and do I have to wait another four or five years to see any fruit?

Another problem plant is a lime tree which I have nursed since it was under a metre tall. Now it’s almost two years old and 2.5m tall. It’s also very healthy, green and leafy. When I squeeze a leaf, the aroma is like lime. But it has no flowers or fruit. Please let me know why, and what I can do to get it to fruit. – Patrick Leong

Plants that are unhealthy might need a fertiliser boost

Reader Patrick Leong is hoping to see his soursop tree with a fruit like this. Photo: Tatters/Visualhunt.com

Your soursop and lime tree are just about the right age to produce yields (three to six years old), but you should prune your soursop tree to about 1.8m to 2.5m (6 feet to 8 feet) high and encourage more lateral instead of vertical growth to facilitate easier harvesting.

For both lime and soursop trees, flower drops (or no flowers at all) are often indicative of over- or under-watering or applying too much fertiliser.

For the soursop tree, you should be applying about 300g of nitrogen, phosphorus, and potassium in a 10-10-10 ratio per tree, every three months. Fruit-bearing lime trees require more nitrogen and potassium, less phosphorus, and more trace elements. The simplest thing to do is get fertilisers formulated specifically for lime trees, and follow the instructions on the package. Do not add more than what is recommended.

For both trees, watering should only be done to keep the top soil moist. Your top soil should not be dry or too wet. If you have over-applied fertilisers already, stop all fertilisation and observe if your tree is able, over time, to bear fruits. You can resume fertilisation after that.

Dear Plant Doctor is part of Star2′s gardening column, Ready, Set, Grow!, which appears on the fourth Wednesday of every month in print in Star2. The column is brought to you by social enterprise Eats, Shoots & Roots, which champions urban farming.