The best time for watering is in the morning before heat and evaporation take their toll. Grass clippings and mulch spread on the soil help to prevent the water from evaporating and let it soak into the soil.
Watering in the evening after 5 p.m. when the heat of the day so cooling will also work for most plants.
But if you plan to water in the evening, make sure you give the plants and soil enough time to dry before dew forms in the night. If you water too late at night you will have a tendency to attract slugs and other non-beneficial insects and disease into your garden.
Water the soil in your garden completely once a week. This means there must be enough water to soak down through the soil and get to the roots of the plants where it is needed.
Note: A few minutes of surface watering only encourages weak and shallow roots.
New plants need to be watered daily for several weeks to root growth. This is especially true for shrubs and trees. Once your plants are established, be careful not to over-water them. Too much water will take air out of the soil and the plants will suffer or die from lack of air.
photo credit: Gnosticgardener
How to Water your Plants
Generally, plants should be watered at the base of the plant. Water the area long enough for the water to be able to soak into the soil. You can check how deep the moisture is going into the ground with you finger. You want at least an inch of moisture; 3 inches would be better.
A light misting of the leaves occasionally can be beneficial, especially for houseplants. But too much water on the leaves can promote fungal blights. And during the hottest hour of the day leaves outdoor plants can actually suffer from phototoxicity (burning of the leaves).
I like to water with a dipper and water each plant individually with water from a rain barrel. The water is warm and not as cold as from a sprinkler. Cold water can shock plants and set back plant growth and production.
But all people don’t have time to hand water and a sprinkler or irrigation-type hose is a much easier and faster. Just make sure to water the area long enough to get the moisture down to the plants roots.
And last but not least, different plants take more or less water. Check in gardening books and online to learn plants water needs and group plants in certain areas according to their needs.