1) WATER YOUR SOIL PROPERLY
Many people water the very surface of the soil which will then evaporate rapidly without ever reaching the plant roots. Check with a trowel that you are actually soaking the soil beneath. It is better to give the soil a good soaking every few days rather than just wetting the surface regularly.
Target the water at the soil rather than wetting the foliage. Water on the foliage will just evaporate or remain on the leaves encouraging fungal disease. Drip irrigation systems are by far the most effective way of delivering water to plants, as water is targeted to the plant roots rather than wetting the soil surface. They take a lot of setting up, but once in place, watering takes very little effort.
Mulching with compost or straw has a huge effect on reducing the amount of water that evaporates from the soil surface. It reduces the amount of watering needed and will also suppress weed growth. There are many different materials that are suitable for mulching from newspaper and cardboard, hay and straw to grass cuttings and leaf mould. All are excellent at retaining moisture in the soil and reasonably cheap. Gravel and grit on pots are also useful but may not be quarried in a sustainable way.
3) WATER THE PLANT WHEN IT MOST NEEDS IT
There are critical stages when it is most important to water plants. For directly sown plants the soil should be kept moist otherwise the seeds won't germinate. Likewise after transplanting, plants only have poorly developed roots so will frequent watering. After this critical period, the water requirement of plants differs.
As a very general rule, more leafy vegetables (eg spinach) will require more continuous water to allow leaf expansion. For plants that produce fruits (eg tomatoes, beans) watering is most critical from fruit or pod set onwards. If you want potatoes to be free from scab then you need to avoid letting the soil dry out for about six weeks after tuber initiation. This is approximately when the plants are 15 cm high, but you may wish to dig up a few to check.
4) IMPROVE THE MOISTURE HOLDING CAPACITY OF YOUR SOIL
If you have a very light sandy soil, water will drain out quickly. Improving the organic matter content by regular additions of compost, garden waste, manure and crop residues will gradually, over time improve the ability of the soil to retain moisture.
For more about harvesting and reusing water, and greenhouses, become a member of GardenOrganic, www.gardenorganic.org.uk/.