Dr Christopher Teh, soil biologist and senior lecturer at Universiti Putra Malaysia answers gardening questions in Star2.com’s gardening column, Ready, Set, Grow!, which is brought to you by social enterprise Eats, Shoots & Roots. Go to questions.eatsshootsandroots.com to send in your queries.

I have two gardening problems and would be grateful if you can help me solve them.

Firstly, my soursop tree produce lots of flowers but hardly any fruit. The petals fall off leaving tiny bud-like fruit, the stems and fruit turn black, dry up, and drop off. The leaves turn yellow with brown patches and fall off. There is also a dieback of some branches. I have tried spraying with enzymes and a mixture of dishwashing liquid, baking soda, oil, and water but nothing works. In all these years, the tree has only produced three small fruits, though it has grown as tall as a two-storey house.

Secondly, my pomelo tree leaves are being attacked by mites, I think. Its bark appears dry and wrinkled, and parts of it look like they have separated from the trunk. The tree bears a few fruit but they are very small and tasteless. – Adeline Lee

First of all, stop using the home remedies of enzymes, baking soda, oil, etc. They are probably making things worse, not better. It is likely that you have over-watered your trees, causing good tree growth but also causing most flowers, if any form, to drop. You can rely totally on rain for water, but check once in a while whether the soil about 15cm deep is dry, moist, or wet. Water only when the soil is dry.

For mite control, you can use any mite-specific pesticide, but for more natural control, you can try blasting them off the tree with a powerful jet spray of water or spray them with neem oil mixed in water (see proportions above).

I have planted a few pomelo seeds, from pomelo imported from China. They are growing upwards only. How to make them grow like a small tree with branches that can bear fruits in the pots? Not to mention, the older leaves tend to grow yellow. Please advise how to prevent older leaves from growing yellow. – Siew Kwan Lee

You start pruning once the tree reaches the desired height, not before then. Prune the upright branches, so the tree grows laterally. Prune the branches where they meet the main trunk, but cut at a slight angle, not flush at the trunk.

Yellow old leaves are usually indicative of a lack of nitrogen fertiliser.

Plants growing on my balcony on the ninth floor tend to be attacked by aphids. I notice that they are attracted to certain plants only, like roselle. Why is this so, and how can one protect edible plants from aphid invasion in a natural and safe way without using insecticides? – Anonymous

Most aphids are host specific, meaning that the aphids feasting on your roselle will not readily attack another plant, even if it is right next to the roselle. This is true not just of aphids but of other plant (as well as animal) pests and diseases. This is because pests and diseases require a certain kind of environment and conditions that are only provided by a narrow range of potential hosts.

To naturally control for aphids, you can buy insecticides made with neem oil, or you can make your own: Mix two teaspoons of neem oil in one litre of water. Spray onto the infected plant parts. Additionally, you can grow garlic plants nearby as they have been known to prevent aphids in some cases.

If we over-fertilise a field, I’ve been told we would have to flood it to attempt to remove the excess fertiliser. What if we just add more organic matter/compost on top? But it seems that mulching makes it worse – why is that? I thought the further decomposition could mix in with the fertiliser? – Lamis Jamil

An over-fertilised field does not always mean that the plants in it will be damaged by the toxicity. This is because the plant may not take up the excessive nutrients. If the plants do show signs of toxicity, they can recover on their own. Some plants, especially trees, can be quite hardy and withstand toxicity, and can eventually, on their own and without our help, recover from excessive fertilisation. Depending on the severity of the toxicity and plant type, this recovery process can take a few weeks to a few months.

If the plants show signs of toxicity, do not compound the problem by adding more nutrients, either by composting or mulching, because they will simply add more nutrients to an already overloaded plant!