Correct watering is of the utmost importance, and determines success or failure with house plants. There are no cheap and reliable instruments for measuring the water content of the soil in the pot, but gardeners and horticulturists have evolved some simple, practical methods. With experience, you can quickly tell from the weight of the pot whether the soil is wet or dry, but a sharp rap on the side of a clay pot with a small wooden mallet or a heavy wooden stick, such as a wooden trowel handle, will confirm this. If the sound is a hollow ringing tone, the soil is dry; if it is a dull thud, the soil is wet. If the plant is in a plastic pot, the difference in weight and the condition of the surface soil must be relied upon. When wet, the colour of the surface soil is black or dark; when the soil dries out, it becomes a greyish white. Also, by pressing the tips of the fingers into the topsoil one can learn whether it is wet and soggy, moist, or hard and dry, by the resistance offered by the soil. The two extremes of wet and dry soil should always be avoided, the ideal being an intermediate, evenly moist condition.

The amount of water required by most house plants changes with the season, and even from one house to another. All but desert or bog types of plants require generally the same care in watering. During the period of vigorous
growth in spring and summer, they require plenty of water, and the soil should not be allowed to dry out too much between waterings. In autumn, when growth slows down, the soil should be kept somewhat drier. In winter, particularly during very cold weather, the greatest care should be taken, as many plants then have almost ceased to grow. In this semi-dormant period, allow the soil almost to dry out before watering again. Give only sufficient water to maintain life, but never permit the soil to become so dry that the foliage wilts or the stems shrivel. At the usual room temperature of a heated house 21 C. or higher the air is comparatively dry and frequent watering of smaller pots even in winter may be needed.

Always use water which is at room temperature or slightly tepid. A good practice is to keep a filled watering-can, preferably an ornamental one, in the same room as the plants to ensure the right temperature. Water around the rims of the pots, avoiding direct application to the crown and leaves. Periodically immerse the plants in water to give a thorough soaking or place the pots in a tray of water until the topsoil is moist.

The golden rule, particularly in winter, is never water when the soil is wet. Make a practice of watering early in the day, and drain off all surplus water before replacing the pot in position. When water is applied, always give enough to saturate thoroughly the whole mass of soil. Never follow a period of dryness with repeated heavy watering, as this can cause the loss of the lower leaves, but return gradually to normal watering. Black or brown wet patches on the leaves or stem are often a sign of over-watering in winter. If the lower part of the stem is affected, the plant may die. If the leaves are affected, keep the plant almost dry for a while. Where the room temperature drops sharply at night during cold weather, always remove a plant that is moist or wet to a warmer place. Plants left on windowsills, between curtains and window, are in danger of being damaged or killed by frost.

Before going away for any lengthy period in spring or summer, water all house plants thoroughly. Insert two small stakes in each pot, slightly taller than the plant, one at each side of the pot. Slip a polyethylene bag over the stakes and fasten it to the sides of the pot with a rubber band. Place the plants in a position where they will not be exposed to strong sunlight. While you are away moisture will evaporate from the leaves, condense on the walls of the polyethylene bag and run down. The plants will be in an ideal atmosphere, with sufficient humidity.

We often hear advice to give of water per week to a plant. This can be unreliable. The amount of water a plant need depends on temperature, humidity and the size of the plant in relation to soil and light. A plant in a warm, well-lit position may use twice as much water as one in cool shade. If a plant is making new growth, its water demands will be much greater than those of a dormant one.

The only answer to watering house plants is to let the surface soil dry out to a stage where it feels dry then give a good soaking. The main exception to this is ferns, especially maidenhair, and to a lesser extent, flowering house plants. Water ferns daily unless they are in peaty soil that holds water well. If so, water them when the soil surface is drying out. Cold water can shock most house plants, retarding growth, stopping flowering and, on plants such as African violets, can cause unattractive marks on foliage.

Use water at room temperature. Fill your watering container after use and leave it indoors until needed again. Or add enough hot water to take the chill off the tap water.

Chlorine can affect many plants. It can cause brown tips on spider plants, dracaenas and can kill Venus fly traps. It also causes brown blotching on leaf margins of dracaenas. This problem can be overcome by using cooled water which has been boiled: what is left in the jug or kettle before refilling.

Fluoride is more difficult. About half a cup of lime water, made by adding a teaspoon of hydrated lime to a litre of water, every three months for each sensitive plant is the best solution. Keep the lime water well corked.