Houseplant Physiological disorders

Although you might imagine that pests and diseases are the cause of so many plant problems, it is, in fact, the physiological disorders or cultural faults which claim the largest toll of indoor plant failures.

Physiological disorders or cultural faults are by definition problems that affect the plant due to where it is grown, how it is grown and what we may do to the plant. This includes over-watering, under-watering, sun scald, cold water scald, physical damage, draughts, temperature fluctuation, low humidity and misuse of aerosols.

Over-watering, Under-watering?

Probably more plants die, or at the least suffer, from over-watering than from any other problem, and indeed it can be a little difficult to identify immediately whether the plant has been over- or under-watered.

The obvious symptoms of both are that the plant wilts, followed by leaf dehydration and thereafter rapid leaf drop. In both cases this is due to the fact that the plant is not absorbing water. In the dry plant there is obviously little or no water left in the mixture, but with an excessively wet plant, although there is a superabundance of water, there are usually no active roots.

The cause of an under-watered plant is obvious but with an over-watered plant the problem is a little more complex. Most people think that to keep a plant healthy, the addition of water to a pot is quite sufficient, with the occasional drop of liquid fertilizer, but the truth is that air is also required. Therefore, with the majority of plants frequent addition of water to an already moist soil mixture quickly displaces any surplus air and an araerobic or airless soil mix results.

The roots then suffocate, die and break down, and the plant above, ironically, dies from dehydration.

A plant that is over-watered and is showing primary signs of stress may be saved by removing it from its pot and letting it dry out. This should take 24-48 hours, after which it can be replaced in the pot and watered. It does help at this stage to water the plant with a ‘BenomyF solution as this fungicide applied to the soil kills parasitic fungi which may inhibit the growth of fresh roots.

Sun Scald

When certain plants are exposed to high light intensities, sun scald may result. The damage may take the appearance of dehydrated areas 8 on the leaf surface, for large irregular patches on the leaves usually mean that the plant has been exposed directly to the sun’s rays.

Cold Water Scorch

This more usually affects plants such as Saint-paulias and Gloxinias and occurs when a plant is watered with cold instead of tepid water, the water droplets being allowed to remain on the leaves. An unusual marbling effect results looking somewhat similar to the tracks left by the leaf miner in chrysanthemums and cineraria. It is better, therefore, to water these plants from below.

Physical Damage

It is surprising how much damage can be caused to a plant, the resultant effect not becoming apparent for some time. The actual visible result is extremely variable, but tears and splits in the leaf are most common, caused by rough handling.

Probably the most unusual effect of physical damage occurs when the growing point is damaged. Even if the damage to the terminal bud appears slight, as the leaf grows the damage looks more severe and subsequent leaves invisible in the terminal bud at the time of damage may also show some signs of marking if the effect was more than one leaf deep. Eventually the plant will grow through this although it may take a little while.

Draughts

The more delicate plants are susceptible to draughts and the usual effect is sudden and rapid leaf drop. On certain plants, however, the effect may be nothing more than drooping of the leaves. Quite obviously if the plant is reacting either way it is in a bad position and should be moved immediately.

Temperature Fluctuation

This is more of a problem in winter than at any other time as temperatures are more liable to sudden changes. The most usual effect of temperature fluctuation is rapid leaf drop: as much as one-third of the leaves may drop overnight if the fluctuation is great enough.

During the day the room tends to be warmer while it is occupied, but at night the heating is usually lower and the temperature drops. It is not the low temperature which is the causal factor but the temperature difference, I.e. relatively high to low temperature in a short time.

Plants allowed to stand on windowsills are more susceptible than those placed in the body of the room. It is more important to aim at a stable temperature of approximately 13-15°C/ 55-59°F constant than to aim at a high temperature of 24°C/75°F which will drop as soon as the heating is lowered.

Low Humidity

Plants that originate in warm humid conditions are particularly susceptible to dry arid conditions. Leaf tips and edges are the first to suffer, turning brown and dehydrating.

Plants continually lose water from their leaves by the process of transpiration in an effort to stop the leaf tissue from over-heating. If the atmosphere is excessively dry, the plant continues losing water, often at a higher level than it can compensate for by absorbing water through the roots in the process of osmosis. The result is that the last part of the plant to receive water is the first to lose it and thus dehydration occurs.

The brown tips and edges can be trimmed back but this does not solve the problem, only masks the effect.

In order to obviate the problem an attempt must be made to raise the humidity around the plant by plunging the pot into a tray of a moist medium such as peat or sphagnum moss. The micro-climate thus produced helps the plant to retain water within its tissue.

One other problem thought to be caused by excessively dry conditions is the loss of perforations and slits in the leaves of monstera – the Mexican Breadfruit Plant. In its natural habitat holes are produced in order to accelerate trans- piration under excessively humid conditions. However, in a dry arid condition the plant no longer has to adapt to lose water and so reverts, it is believed, to a simple leaf which is not as efficient as a perforated one.

Aerosols

Certain aerosols can damage plants, producing minute flecks on the leaf, sometimes with a silvery appearance. Avoid spraying aerosols such as hair lacquer, polish, etc. near plants.

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