Now that the days are warming up and hot weather is here to stay, watering the vegetable garden can be a tricky business. The most common mistake a new vegetable gardener can make is over-watering. Outdoor garden plants generally need about one inch of water per week.
The best approach is to water deeply and infrequently (i.e. weekly). Deep watering encourages deep root growth–if watering is done often and sparsely, roots will will stay nearer the soil surface, eventually stunting plant growth. Rainfall should be taken in to account when assessing the water needs of garden plants. One strategy to ensure sufficient watering is to water at the base of a plant and count to 20. Come back in 30 minutes and check to see if the soil is moist down at least 6 inches.
The timing of when to water is also an important consideration. On hot days, it’s best to water very early in the morning before the sun is hot enough to evaporate the soil moisture of newly watered plants. Some home gardeners opt to water in the evening during hot weather, but if the leaves of garden plants are left wet overnight one risks the growth of powdery mildew.
The use of mulch (straw, wood chips, leaves etc.) can help retain soil moisture and suppress weed growth. In the case of tomatoes and potatoes, mulch serves an extra purpose as it keeps the soil from splashing on the leaves and infecting the plant with soil-borne fungi and disease.
Check out this article for more information: Illinois Extension: New Vegetable Garden Maintenance–Weed and Watering.