Leah Zeldes' column: Herbs in horto

From too-cold to too-hot, tomato woes still come in lots

Last time I was warning about the problems too-cool temperatures could cause tomatoes. Now it seems as if too-hot weather may be more troublesome.

Tomatoes, like people, prefer night temperatures between 55 and 75 degrees. They also don't like daytime temperatures above 90 degrees. Nor do they care for humidity above 70 percent. If the weather doesn't conform to their preferences, they're likely to sulk by dropping their blossoms instead of turning them into tomatoes.

Spritzing the plants with a strong jet of water a couple of times during the heat of the day can help to cool off flowering plants enough so they'll set fruit.

Other problems can cause poor fruit set, however. Tomatoes need even moisture. If the plants don't get enough moisture, they'll drop their blossoms.

If they don't get enough sun, they won't flower at all. Tomatoes must have six hours of sun a day. (If you don't have a location that gets that much sun, consider growing tomatoes in containers that you can move to follow the sun.) Also, too much high-nitrogen fertilizer will encourage bushy leaf growth at the expense of flowers and fruit.

Plants that have already set fruit are also subject to weather-related problems. Tomatoes won't color evenly if the weather's too hot -- their red pigment doesn't develop at temperatures higher than 86 degrees.

And growth cracks, circular or radial splits in the stem end (top) of the fruit, are common in hot weather. This is particularly a problem when dry conditions are suddenly followed by heavy rainfall or watering at a time when the fruit has reached its full size and begun to redden.

What happens is that the moisture and warm, sunny weather encourage rapid growth and the fruit literally bursts out of its skin. Cracking is primarily an aesthetic problem. The shallow cracks frequently heal over and the fruit is perfectly edible. However, fruit with deep cracks may rot, and rough handling can cause cracked tomatoes to rupture. Cherry tomatoes often split in half.

Again, even watering -- about an inch per week -- helps prevent this. Some varieties are less susceptible to cracking, among them: `Early Girl,' `Heinz 1350,' `Jet Star,' `Roma' and `Sweet Million.'

How water reaches the plant can have as much effect as how much water does. Tomatoes prefer to keep their leaves dry. In warm weather, heavy rains or overhead watering can cause fungal and bacterial problems.

Anthracnose is a fungus disease that rots ripe tomatoes. The fruit becomes spotted with dark, sunken, concentric rings. Infected fruit should not be eaten.

Dark, raised scabs with sunken centers that appear on green fruit are a sign of bacterial spot. This plant disease also spots leaves and makes them drop off; flowers also fall off. The infected fruit rarely ripen properly and frequently rots.

With both these diseases, pick and destroy infected tomatoes immediately. Fungicides can help to eliminate them.

While you can't do anything about hot, damp weather, you can keep from encouraging these problems by watering with soaker hoses, furrow irrigation or other means that don't wet the leaves. Don't work around wet plants.

If you've been troubled by fungal or bacterial problems, destroy -- don't compost -- all plant debris after harvest, and don't use saved seeds, which can harbor bacteria. Give plants ample space and stake them well, so they dry out quickly after rainfall. Don't be tempted to prune leaves to cut back on the amount of foliage there is to get wet, however.

While too much damp weather can cause disease and enough sun is essential, too much sun can create its own problems. Sunscald develops on tomatoes exposed to direct sunlight in hot weather.

Both green and ripening fruit can develop this fault, light patches on the side facing the sun. These blister and develop a grayish, paperlike texture, susceptible to black mold. If you catch them before they mold, you can cut the suncalded parts out and still eat the tomatoes, but moldy fruit aren't edible.

People who prune their tomato plants, attempting to ripen the fruit faster, are most apt to have troubles with sunscald. Let the leaves alone. If you grow tomato plants in cages, rather than staking them, you never have to prune and you get larger crops.

If leaf-spot diseases or other problems have thinned your tomato foliage, you can protect the fruit from the sun by shading with Reemay or thin screening.

* * *

²The 39th Annual Dearborn Garden Walk and Heritage Festival occurs from noon to 6 p.m. Sunday, July 20. This event, America's oldest garden walk, features almost 60 private gardens on North Dearborn, State, Astor and LaSalle streets, as well as Sandburg Terrace between Division Street and North Avenue in Chicago.

Among the gardens open to the public will be the grounds of Archbishop Francis George's residence; the Graham Foundation Sculpture Garden; the Edgar Miller Gardens with mosaic-tile-edged pools and reliefs; and many more.

The Ambassadoor West Hotel, 1300 N. State Parkway, hosts garden seminars from 1 to 4:30 p.m. Nineteen vendors will sell flowers, plants and garden supplies and accessories on the 1300 block of North Dearborn.

For a $5 donation, visitors will receive a program mapping the gardens for the self-guided tour, and a schedule of events. For more information, call (773) 472-6561.

* * *

²The Chicago Botanic Garden, 1000 Lake Cook Road, Glencoe, has opened its new Garden of Eatin' or Regenstein Fruit and Vegetable Garden, the Midwest's largest fruit and vegetable display garden. The 3.8-acre site showcases more than 500 varieties of edible plants and includes kitchen, tea and herb gardens, a grain field, a nut grove, examples of small fruit trees and other displays designed for urban and suburban gardeners.

Special programs will take place each weekend, including gardening and cooking classes and seminars. The garden is open daily from 8 a.m. to sunset. Admission is free; parking is $5 on weekedays, $6 on weekends. Call (847) 835-5440 for more details.

* * *