Dear Plant Doc­tor

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Go to ques­tions.eatsshoot­san­d­ and fill in the form to have your gar­den­ing queries an­swered by an ex­pert.

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.

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.

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!

All ques­tions are an­swered by Dr Christo­pher Teh, soil bi­ol­o­gist and se­nior lec­turer at Univer­siti Pu­tra Malaysia.

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