Correctly watering your tomatoes through their life can make a big difference to the overall crop. One thing to remember is the old adage “more plants are killed by overwatering than underwatering”.
So, what is “overwatering” and how do you prevent yourself doing it?
Watering Seeds & Seedlings
If we start at the beginning, tomatoes are like most seeds/seedlings and hate being unevenly watered or overwatered. Your growing medium should be damp but should not discharge water when squeezed (I know you won’t squeeze the compost when there are seeds in it, but you know what I mean). At the same time, the compost shouldn’t be allowed to dry out completely. If it does, it will be difficult to wet again. Keeping the compost at “just the right” level of moisture gets more difficult if you’re growing seeds in small plugs as they dry out much more quickly.
The best solution I have found is to sow your seeds, stand the pots/plug trays “knee deep” in water, let them stand for a while until the compost is wet through then take them out of the tray and let them dry out for a couple of hours. Then put the whole thing in a plastic bag in a warm place where they will germinate. You’ll need to keep a close eye on them because once they germinate you’ll need to get them out of the plastic bag to give them the necessary sunlight and air movement.
However, you’ll need to keep watering them and watering from below in the same way as above is (in my opinion) the best way to avoid overwatering. Stand the pots/plugs in a tray of water and when the top surface of the compost is damp, take them out and leave them to drain. Repeat this every day.
If you overwater at this stage, you’ll discourage the seedlings from developing a decent root system which is critical for long term growth.
Keep doing this until they have developed their first true leaves.
Watering Small Plants
At this point, you’ll need to pot up your seedlings into small (approx. 7cm) pots. If you’ve got multiple seedlings in a plug or pot, you’re going to need to separate them. This is simplest when the compost is relatively dry. First fill up your target pots with compost (the compost should be slightly damp, straight out of the bag is good enough). Empty the pot or plug and roll the compost gently between the palms of your hands and the compost should fall away from the roots, leaving you with individual seedlings that you can transfer to your target pots. Personally I find the simplest thing to do is to poke a finger into the compost, drop the seedling in as deep as it will go with the seed leaves above the surface and then gently firm the compost around the plant. Then, like the seeds, stand the pots knee-deep in water until they are wet though and then take them out to stand in the light.
Again, water from below but only when the compost begins to look dry. Roots need oxygen as well as water to develop so the idea is to make sure that they aren’t drowning in too much water. They will also develop more effectively if your plants are dryer because the roots are searching for water. If you give them too much water, they won’t “bother” to develop and you’ll suffer later on.
So again, water from below when the compost is dryish. Keep doing this until your plants are ready to go out into their final growing place after frosts have finished, potting them up into larger pots before they get potbound which will stop the roots growing.
Watering Growing Plants
This should be the simplest part of growing but is often when plants get overwatered. I grow my tomatoes in 10 inch pots which I stand in trays. I water from below when the tray is empty. Every day I put about 1 litre of water into those trays which are dry. If a tray still has water in it, skip it until its dry. The main reason why there would still be water is because the weather hasn’t been sunny enough for the plant to take up the water.
Watering from below also means that you won’t wet the leaves, reducing the chances of blight.
If your plant develops adventitious roots (that is roots up the stem) then you possibly have a problem that the root system underground is struggling and you should put more compost in the pot, covering up the roots and encouraging them to grow. If you don’t, the chances are the plant will die back and all your effort will have been wasted.
As your plants grow, they will usually need watering every day, water in the morning so that the plant can take up water during the day and grow up. On days when you feed your plants, they won’t need additional watering, just feed them from above (into the top of the compost) with a weak solution of fertiliser.
When your plants have a decent number of trusses set, stop the plants and reduce watering by about 30%. Too much water at this point will go into the fruit, slowing down ripening and reducing the taste of the fruit. The best tasting and ripest fruit come from a plant slightly under stress. Continue to water every day if the tray is empty but with less water. Regular watering will help to prevent blossom end rot and split fruit both of which are caused by inconsistent watering.
Split fruit is easiest to explain. Once the fruit starts to ripen, the skin “sets”. If the plant gets plenty of water after this has happened, the fruit absorbs more liquid but can’t swell the fruit so the skin splits. Its more common with thinner skinned fruit but is easy to cause and has no solution once its happened.
Blossom End Rot is caused by calcium deficiency. There is usually enough calcium in the soil or compost but, due to environmental conditions, the plant can’t process it as its needed. Stress can be caused by the plant being too dry but also by being too wet so inconsistent watering affects the plants at both ends. It can also be caused by over-fertilising so, the enthusiastic amateur gardener (like me) is likely to hit the plants from all directions. (My own experience is that certain cultivars are more susceptible to BER and I just don’t grow them).
Finally, of course, if you’re growing plants outdoors, you’re much less in control of the amount of water that your plants receive but (hopefully) your plants will have developed substantial root systems and be able to cope.
Right, so if you water your plants perfectly throughout their life, you will get perfect tomatoes, ripe and flavoursome. Well it does happen sometimes.
Adventitious Roots 
Plants in trays