How To Maintain Plant health

Apart from a few insect pests and possibly mildew, ill health in house plants is almost always due to external conditions or cultivation faults. It is seldom possible to tell what is wrong with the plant merely by looking at it; one has to consider the conditions and pre-vious treatment. Below is given a list of possible symptoms and their alternative causes. Many of these symptoms may also show up on plants not properly acclimatised and, therefore, are most frequently seen in winter. Details of insect pests are given later, together with general remedies.


Weak, spindly growth Lack of light; excess winter heat.

A check or reduction in growth: production of small leaves A sudden change in conditions; overwatering; overfeeding; often exhaustion of soil or excessive dryness at roots. Dry spots, patches or margins on leaves Overwatering; overfeeding; draughts; sun scorch; water on leaves in strong sun; gas fumes; hot dry air; thrips or scale insects. Leaves yellowing (followed eventually by leaf fall) Leaves at base of stems will naturally yellow and fall occasionally, but when several or all do it something is clearly amiss. Over-watering; overfeeding; exhaustion of soil; dryness at roots; draughts; excessive cold; gas fumes; hot, dry air; red spider.

Leaves falling suddenly A sudden change in conditions; draughts; watering with cold water; gas fumes; lack of light in winter; excessive sun suddenly in spring. Leaves distorted or blistered, often yellowing Aphids.

Small raised waxy mounds Scale insects. White mealy insects Mealy bugs. General yellowing and poor appearance (in succulents, especially cacti) Exhaustion of soil; root mealy bug.

Complete collapse Frost; excessive cold; excessive sun, particularly on pot; gas fumes; complete drying out of soil. Wilting Dryness at roots; excessive sun; sometimes waterlogged soil. Rotting of leaves or stems Fungus attack due to damage or bad conditions; very rarely direct infection.

Whitish powdering on leaves Mildew, usually on overcrowded plants; overwatering ; excessive humidity. Bud and flower drop Overwatering, dryness at roots; air too dry; scorching sun; fluctuating temperatures; draughts; moving plants from one place to another.


Aphids (green or black fly) These common pests suck the sap, causing leaves to twist up, turn yellow and eventually fall, and may also damage growing tips. Young aphids are hardly visible, being pale and less than I mm. Long. Control: small colonies may be wiped out by hand, or spray with an insecticide such as malathion or dimethoate. (Do not use malathion on crassulas or ferns.) Mealy bug A kind of aphid, broad, white, woolly and about 2 in. long. Control: pick off individuals with a paint brush soaked in methylated spirit or malathion solution, or spray with malathion.

Red spiders Tiny reddish mites, which suck sap and multiply rapidly; encouraged by hot, dry air. Any mottling or yellowing of leaves should be regarded with suspicion and the undersides examined. While the mites are just visible to the practised eye, a lens is desirable to confirm the diagnosis. When a large colony is present, a white webbing is produced. Control: where possible, fairly strong syringing with water on leaf undersides. Spray plant with derris, malathion or dimethoate.

Root mealy bug Similar to mealy bug, infesting roots (mainly on succulents) where it makes a white woolly mass. Control: shake off soil, wash roots, remove pests with a paint brush dipped in methylated spirit, or water soil with liquid malathion. Scale insects A kind of aphid which settles down in one place, forming a tiny waxy mound, later followed by yellowing around the mound as the insect sucks the sap.

Control: wipe off with a soft cloth or paint brush soaked in warm soapy water or methy-lated spirit; spray badly infected plants with malathion or dimethoate. Thrips Small, thin insects, 2 in. long, which jump when disturbed and cause small brown or white marks on leaves and flowers. Control: spray with malathion or derris. General After dealing with an insect attack on one plant, keep an eye on it for several weeks, as very young insects or eggs may not be wiped out; also examine neighbouring plants carefully. If a bad attack occurs a general spraying should be carried out when the plants are being cleaned. It is important that all chemicals should be used strictly according to manufacturer’s instructions. Many insecticides have a harmful effect on fish and other livestock and to safeguard pets it is a good idea to treat plants in a sheltered position out of doors. Dimethoate is a systemic insecticide, which means that it passes into the plant’s sap and provides protection for several weeks.

Minute grubs which turn into very small flies may appear in the pot soil. They are almost always quite harmless but can be eradicated by watering with malathion.


The action to take for the majority of the complaints listed above is obvious: the most important thing being to observe the first signs of trouble, deduce the cause and act on this immediately. Many of these symptoms can be detected when the plants are given their regular cleaning, and some — including most insect attacks — will hardly occur if regular cleaning is given.

The remedy may not always be so easy. Mildew may be cleaned off, or a fungicide such as thiram or colloidal copper be applied. Rotting is, as stated, usually a secondary symptom. All rotten parts should be removed at once. If the base of the stem is involved, cut off the healthy top and treat this, if possible, as a cutting. The same advice applies when a plant has lost most of its basal leaves. When a plant is suffering from drought, the pot should be stood in water for some hours. A plant which has been thoroughly overwatered should be allowed to dry out completely, but severe damage has often been done before the cause of the symptoms has been realised. A frost-damaged plant may be rescued by gradually thawing it out in a dark, cool place.


If the following points are watched, little should go wrong with the plants, assuming that they have been chosen to suit the existing conditions.

1. Do not waterlog the soil or allow the pot to

stand in water too long.

2. Make sure the drainage is adequate and has not become clogged.

3. Do not use very cold water.

4. Do not let the soil dry right out.

5. Water sparingly in winter (in general).

6. Do not water by the calendar – check by feeling the soil.

7. Do not feed too often, with too much or too strong a solution.

8. Feed well-established plants only.

9. Clean plants regularly.

10. Re-pot plants as required; but do not keep looking at the roots.

11. Keep out draughts.

12. Make sure any gas appliance is not leaking.

13. Try to keep the air round the plants moist.

14. Avoid sun scorch and keep sun off the plant and pot.

15. Avoid direct hot air from any radiators reaching plants.

16. Avoid widely fluctuating temperatures and stuffy air.

17. Avoid shutting plants between curtain and window on cold nights.

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