Adequate supplies of water are essential for plant health, wilting of the foliage being the first sign of water shortage. The obvious reaction to this situation therefore is to water the plant. Unfortunately, wilting can be brought about not only by dryness at the root but also, somewhat surprisingly, by waterlogging of the growing compost. Consequently the application of more water may only exaggerate the problem. When the plant is seen to be wilting, therefore, you must first check on the condition before attempting remedial action.
How much to water
Too Little water
Modem peat-based composts shrink as they dry out and also become very lightweight. So first of all you should check to see if there is any sign of the compost shrinking away from the side of the pot. Gross water deficiency can also be detected by the lightness of the pot. Both these observations indicate extreme dryness of the compost which is not easily remedied by top watering. Much the best way of dealing with this situation is to immerse the pot in water to just below the level of the compost and leave it there till the surface of the compost begins to darken as it dampens.
Less extreme dryness is best checked by pushing a finger into the compost and checking to see whether or not your finger tip remains dry. If it does then the pot needs watering.
Light watering Failing to water the pots is one obvious cause of the drying out of the compost but a much more common factor is the use of frequent but very light watering. This only wets the top layer of the compost and does not penetrate deeper into the pot where plant roots are growing.
Too much water
Too frequent, heavy watering can lead to the compost becoming waterlogged and soggy. This can have a lethal effect on plants since it restricts the flow of vital oxygen to the plant roots. These stop growing and can no longer supply the aerial parts with sufficient water, causing them to wilt. Waterlogging also favours root-rotting diseases so, unless the situation is quickly remedied, the roots start to rot and the plant dies.
Extreme heaviness of the pot is a clear indication of waterlogging. This diagnosis can be confirmed by knocking the plant out of the pot and checking on the condition of the compost and the plant roots. The remedy is to re-pot in fresh compost, having first of all washed off the old material and removed any damaged or browned roots.
When to water
Watering should be done to a fixed schedule. Not only does each type of plant have its own requirements but these vary according to the relative sizes of the plant and the pot and also to its situation. The type of pot also affects the frequency of watering since clay pots lose water more rapidly than do plastic pots. As a general rule, however, all plants require less water in winter. The only effective way therefore of determining when to water is to make regular checks on the state of the compost. First look to see if the suitce has dried out and then carry out the finger test to check the state of the upper layers of the compost — if your finger stays dry when pushed into the compost, the plant needs water.
Watering regimes for houseplants
Although each type of houseplant has its own special requirements for water they can for practical purposes be split into three main groups:
1. Dry in winter Cacti and succulents need to be kept short of water during the winter and, in fact, the compost can be left to dry out almost completely. In spring and summer, however, they should be treated as moist/dry plants.
2. Moist/dry Most foliage plants belong to this group. Throughout the spring and summer they should be given a generous watering each time the top layer of compost has dried out.
Less frequent and lighter watering is needed in winter. Some drying out of the compost is desirable since it ensures that there is adequate aeration of the roots.
3. Moist at all times Only a few plants flourish if the compost is kept continuously wet. These include marsh plants such as Acorus and Cyperus which benefit from having the base of the pot standing in water. Azaleas, which are usually potted in very free-draining compost, also fall into this group.
How to Water
The easiest and most convenient method is to use a watering can fitted with a long fine spout to apply the water to the top of the compost whilst avoiding wetting the foliage. Filling the pot to its rim gives adequate watering in summer. In winter, however, the water needs to be applied more sparingly in order to avoid getting the compost waterlogged.
This method is unsuitable for watering Saintpaulias since the velvety leaves can be damaged by water drops and it is difficult to insert the watering can spout under the closely packed rosette of leaves. Tuberous plants such as gloxinias and cyclamen also need special treatment since their crowns may become diseased if they are wetted. The best way of watering these plants is to immerse the pots in water to just below the top of the compost, leaving them there till the surface of the compost darkens. The pots should then be allowed to drain before they are put back on show.
Vase plants (Bromeiads) also require special treatment since, in addition to normal watering of the compost, the ‘vase’ (formed by the central core of leaves) needs to be kept filled with water. it is also advisable to empty and refill the ‘vase’ every couple of months. If possible, use rainwater, as in some areas alkaline top water can leave an unsightly residue on the leaves.