All plants require adequate supplies of water and there are always occasions when it is necessary to water artificially. However, watering is not simply a matter of pouring water on to the soil. This can, in fact, do more harm than good.
First: never add water in small amounts. It is essential to apply enough to get right down to the root zone where it is needed. Otherwise, roots will come to the surface in search of the water, and there they will be even more vulnerable to the effects of heat and lack of moisture.
Second: although large quantities of water are required, you must apply it carefully. Water applied in the form of large droplets or with great force will cause the soil "crumbs" to break down and form a hard surface crust. This prevents further water entering the soil and it also inhibits the free interchange of air and gases, with disastrous effects. On a seed bed, this crust can actually stop the tender young seedlings from pushing through the soil to the surface. To prevent this occurring, apply water through a sprinkler with a fine spray pattern of small droplets.
When watering seed trays, use a can fitted with a fine nozzle. Pour the water to one side of the tray or planter, then pass the can over the seedlings, keeping the angle of the nozzle constant throughout. When you have finished, do not raise the can until it is clear of the tray or planter. The size of droplets is not as important where the soil is covered with grass, so lawn sprinklers are not generally made with such attention to optimum droplet size. Consequently, if you want to buy one sprinkler only, buy one with a fine spray; it will be perfectly suitable for the lawn as well as the seed beds.
WHEN TO WATER: It is not necessary to keep the soil moist all the time - water when the soil is dry, but before the plants begin to suffer - and, provided you use a fine sprinkler, you can water at any time of day. However, timing is important. For example, watering when fruits or vegetables are swelling will greatly increase their overall weight. Once fruits, in particular, begin to colour, though, extra water could invite a fungus attack, especially from botrytis.
HOW MUCH WATER TO GIVE: It is, of course, not difficult to overwater, especially with plants in planters. Try to strike a balance between an aerated soil or medium and one with sufficient moisture. A cold, wet, airless medium will not do anything to encourage plant growth. If you are watering in the ornamental or vegetable garden, leave the sprinkler on for at least an hour each time.
WATERING NEW PLANTS: When you have just planted a plant, encourage it to search for water, thus increasing its root system. Water it thoroughly immediately after planting, then leave it to its own devices for awhile, almost allowing the soil to dry out, before watering the plant again. Vegetables, bedding plants, and perennials are usually small when planted and have shallow root systems. These plants may have to be watered more often to ensure a consistent water supply. Check the soil with a trowel or spade to the depth of the expected root zone. The entire root zone should be moistened before the plants show signs of wilting. If the plants are allowed to wilt a few times, growth will be retarded and harvest yields reduced.
Planters: need special attention. Both volume of soil and total water available for plant use are limited. These plants have to be watered more often than plants growing in the ground. Watering should begin when the soil surface feels dry to the touch, not before. Frequency and amount of water depend on media, location, amount of sun, temperature, type of plant, etc. Planters that have been allowed to completely dry out may need to be soaked in water to rewet the soil.
While there are exceptions, read your plant or seed instructions, most plants should be allowed to moderately dry out between waterings. The potting mixture should be lighter in weight and colour and should feel dry and crumbly. If your plant starts to wilt, you have waited too long and you will have to soak your planter, by this we mean, if you're top watering, pour enough water into the planter that a little runs out the bottom.