Watering Seedlings
If you are planting tomatoes as seeds, they may need as little as a quarter cup of water per day. However, this number will rise quickly as the plant grows. A good way to know whether or not your plant needs water is by waiting 24 hours after watering to see if the soil is dry. If your soil is no longer moist after 24 hours, then your tomato plant definitely needs more water.
Potted Tomatoes vs. Ground Tomatoes
Watering tomato plants in pots is very simple: You water the plant until the water flows freely through the bottom of the pot. Tomatoes grown in soil, however, can be much more difficult to judge. It is important to water the tops of the plant, but it is also vital to moisten deep into the soil to make sure that the roots are saturated (but not too much). Fortunately, tomato roots are fairly shallow, so you don't have to do much to get the roots watered. Try watering with small amounts of water two to three times a day. Another way to water sparingly is to let the hose run on a slow drip two inches away from the stem of the plant.
If you decide on the slow drip technique, make sure you don't forget about it! Even a slow drip can flood the roots if left running for too long.
Dealing with Rain
Rain provides natural watering, but you still need to be aware of how much water your tomatoes are getting. If your area gets heavy rainfall, you may not have to worry about watering your tomato plants at all, but definitely still keep an eye on them. If it looks like they are getting too much water, consider keeping the plants indoors (if potted) or come up with a form of water diversion for ground tomatoes. Placing a plastic tarp on the soil around the tomatoes or propped above the tomatoes can keep excessive water from being absorbed into the soil. Don't forget to remove the tarp once the rain stops, or the tomato plants and/or soil may overheat.
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