Well-prepared growing beds are critical, because deep soil enriched with organic matter will give the roots the oxygen and even moisture and temperature that they need. Plants in compost-enriched soil also need less feeding. A two-inch-thick mulch of straw or chopped, half-rotted leaves will help retain the moisture and keep weeds back. Don’t use shredded hardwood mulch.
Incorporate one-third of a cup of garden limestone or bonemeal into the soil when planting, to provide the calcium the fruit needs. Some gardeners use crushed eggshells. If you haven’t done this, scratch in some limestone or bonemeal around the plant now.
Add a little balanced fertilizer when planting (such as 5-5-5) but use a low-nitrogen, higher-phosphate feed after flowering. Three tablespoons per vine every two weeks is fine, worked into the soil. If you use synthetic fertilizers, keep the granules away from the plant stem. Organic feeds pose a lower risk of “burning” the plant.
Some gardeners like to give a feed weekly of diluted liquid fish fertilizer emulsion.
Always try to water the roots, not the leaves, and don’t use an overhead sprinkler. Soak the soil at least once a week, and don’t rely on rain to do the job for you. Test the soil with your finger: It should be moist but not saturated. Container-grown tomato plants may need watering once a day in the height of summer. Containers must drain, so don’t place a saucer beneath the pot. Water until you see liquid flowing from the drain hole. Again, avoid watering the foliage.
The tomato hornworm is a large green caterpillar that can devour a lot of foliage, but it is really more of a problem for farmers than home gardeners, in my experience. If you see one, you can pick it off. (Don’t use a pesticide.) Then drop it in a bucket of bleach solution.
The bigger problem is with leaf diseases. Almost all tomato plants in Washington get early blight, which causes the leaves to develop yellow circles with brown centers. You can minimize this by laying a good mulch and snipping off the leaves as they discolor, from the ground upward. Discard them in the trash and avoid touching healthy leaves when doing this, particularly when it is wet.
Late blight is a far more serious disease. It can occur quite early in the season, in spite of its name, and is announced by the presence of dark lesions on leaves and stems. Eventually, the whole plant wilts and dies. Afflicted plants should be pulled before they spread the disease to others.
If the fruit develops a black rot at the base, it is suffering from blossom end rot caused by calcium deficiency and uneven watering. Remove those fruits before the plant invests any more energy in them.
By late summer, tomato plants can look pretty ugly, and by early fall the fruit flavor declines with cooling temperatures. Unless you are into green tomato dishes, I would pull vines by Labor Day and use the beds to sow lettuce, arugula and other fast-growing salad greens for the fall.