It is impossible to overstress the importance of watering tomatoes when discussing how to grow tomatoes. Dr. Philip Minges, Cornell University professor, said "Adequate soil moisture during the fruit-growing period is the major factor for obtaining normal size of fruit."
Tomatoes are 93.5 percent water so it's obvious that watering is important. You'll have the best growing results if you can maintain an even soil moisture by following an inch-per-week rule. This rule is a much better guide than watering only when the tomato leaves wilt. Once tomato plants wilt they've already been stressed.
The exception to the inch-per-week rule is when fruit is setting, that is, before the first fruit is golf ball size. During this fruit-setting stage, water every 10 days instead of 7. This puts a slight stress on the plant that causes it to develop tomatoes.
The general advice for watering tomatoes is one-inch of water per week. If you use an automatic watering system or sprinkler one way to gauge the amount of water being applied is to place a can on the ground and measure the amount of water collected.
Too much water can drown a tomato plant, especially if the roots are planted in a location with heavy clay soil and/or poor drainage (see the article soil for tomatoes for more information). On the other hand, too little water will halt fruit production.
The recommended frequency of watering tomatoes is about once per week but watering needs can vary widely depending on these variables:
The following scenarios address some of these variables:
Question: My area gets very little rainfall and
my soil is a deep clay loam with an average temperature of 80˚
F (27˚ C). How often should I water?
Answer: Water about twice per month. With clay soil, drainage can be poor and if the tomato plant roots remain completely submerged in water, the plant will suffer. Water moves into the roots only when combined with oxygen. So if your soil is very clayey, water only as frequently as necessary so the ground isn't always saturated. You should also try to amend your soil for tomatoes with organic matter so over time the quality will improve.
Question: It rains a couple times a week here.
How often should I water?
Answer: You should water only on an "as needed" basis. If the rain isn't enough to thoroughly soak the ground, the roots will grow toward the soil surface, which places them in danger of drying up when the soil dries out. So as long as the soil is being soaked to a depth of six-inches or more you won't need supplemental watering. If it isn't, then follow the one-inch-per-week rule for watering tomatoes.
Question: My soil is very sandy. How often
should I water?
Answer: If your soil is sandy or shallow, you may need to water twice per week.
Question: My soil keeps drying out and the
plants wilt between watering. Is that bad?
Answer: Try to water regularly. It's best to maintain even soil moisture. Mulching with three or four inches of dried grass clippings (not treated with weed killer) or other mulches can help maintain soil moisture and keep soil cool. Using red or black plastic as a mulch is even better and a good choice if you're in a cool zone and want to warm the soil. Uneven periods of dry and wet soil can stunt tomato plant growth and cause small fruits.
Question: Is there a connection between
watering frequency and fruit cracking?
Answer: Yes, if plants are watered heavily after a long drought fruits may crack.
Watering tomato plants any time of day is fine. However, don't water the leaves of the plant at dusk because they won't be dried by the sun. When tomato leaves are left wet for hours they are much more susceptible to leaf-spotting fungi and leaf-blighting. Obviously the rain will come when it comes but when it's under your control don't wet the leaves excessively near nightfall.
There are two easy ways to measure the moisture of your soil. The first is by using your hand. Take a handful of soil and squeeze it. If you can squeeze out water the soil is too wet. If you release your hand and the soil crumbles evenly into small clumps it's just about right. If you release and the clump just falls apart it's too dry.
The second way is to buy a moisture meter.
Studies have proven that watering tomato plants deeply on a regular schedule without overwatering can make a big difference in the yield of your tomato plants.