Answer: How often you need to water depends upon the type of soil in the tub and the size of your tomato plant. If you're using potting soil rather than garden soil, you may have to water everyday - potting soil drains quickly. If you're using garden soil, every 2-3 days will probably suffice. It's important to tomatoes that you keep the soil evenly moist. If the moisture level fluctuates greatly (dry, then soggy, then dry), your tomatoes can develop blossom end rot and other problems.
So, water throughly, then wait a day or two and stick your finger down into the soil. If it is still moist at the second knuckle, you won't need to water for a few days. If it is dry, it's time to water.
Hope this information is helpful!
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