Watering tomato plants regularly and deliberately is very important so that they develop healthy fruit. Tomatoes are 95 percent water, so they need a lot of moisture to reach their full potential. If tomato plants are irregularly or insufficiently watered, they can develop problems such as blossom end rot. This disease eats away at the base of the fruit, rendering it rotten and inedible. There really isn't a way to cure blossom end rot once it starts; the best way to combat it is to avoid it from the start with proper watering.
Cover the garden bed with a several-inches-thick layer of natural wood mulch. The mulch reduces the effects of soil evaporation during hot summer months. Soil evaporation robs the plants of much-needed moisture.
Water tomato plants in the morning. This allows the plants to absorb as much water as possible before the hot sun appears.
Stick to a once-daily watering schedule, if possible. When watering tomato plants it's important to keep in mind that frequent, light applications of moisture can weaken the plants' root systems and keep them from absorbing necessary moisture and nutrients in the future. Tomato plants in containers may require twice-daily waterings because the soil drains into the pans beneath the pots.
Give plants that have not yet produced mature fruit about two quarts of water per day per plant. After the first harvest, each tomato plant will require two to four quarts of water. Use a two-quart watering can to measure the water so you can be sure the plants are getting the right amount.
Soak the soil but do not leave puddles of standing water on the surface of the dirt. If it rains, it's OK to skip a day or two of waterings. The soil should be damp but not saturated.