The best time of day for watering established plants and lawns is carried out in the early morning or late afternoon, when plants will obtain maximum benefit from the water. With seedlings and seed beds however, it is better to water earlier in the day to enable the soil surface to dry a little by evening, as this considerably reduces the possibility of damage by damping off fungi. A dry surface also slows down the nocturnal movement of slugs and snails.
Note that seeds, seedlings and young transplants are very dependent on a frequent and adequate supply of water, which must be evenly distributed using as fine a spray as possible to avoid mechanical damage to the plant and reduce soil crusting.
Vegetable crops cannot tolerate periods of moisture stress. Vegetables use water best when the soil is at or near ‘field capcity’- that is, the soil contains as much water as it can hold after draining freely. Never wait until your crops are wilting before watering again. On very hot summer days, vegetables with large leaves and vine crops often wilt, but this is only a temporary water shortage, (they are losing water faster than they can take it in) and they will recover in the cool of the evening. Another watering next morning will replace water which has been lost.
Potted plants heat up a good deal more than the ambient air temperature (as much as 500 C) and therefore use up water reserves quite quickly. On very hot days, daily watering is required and in some cases, watering in the morning and again in the evening is required.
At other times, the exact quantity of water depends on many factors, such as pot size, the age and size of the plant and the type of soil mixture. For a start, try about a quarter of a cup of water for a 10 cm pot, or about half a cup for a 15 cm pot. Apply the water and then wait for five minutes or so. If excess water does not appear at the bottom of the pot, add more water. With a little practice and observation, you will soon work out how much water to give your plants.
A thorough watering, followed by a period of time to allow the soil to at least start drying out, allows air to be drawn in and about the root ball. Plant roots require oxygen for life and for growth, and if they are constantly saturated by frequent waterings they will soon die. But giving water little and often keeps the soil surface moist while the roots below soon die because they do not get any moisture.