A quick tip from the test garden
Should you prune tomatoes? In cool-summer climates, where tomatoes often don't get enough heat for the fruit to ripen, you can prune plants to let the sun in. Pinch out suckers that sprout from the crotches between the main stems and side branches. On plants trained to stakes, keep one vertical leader. For caged plants, train three or four vertical leaders along the sides of the cage and thin out congested growth to improve air circulation.
In warm-summer climates, tomatoes should be pruned only minimally to prevent sunburned fruit. Be sure to keep enough leaves to shade the fruit.