Yes, hot weather is the culprit with your thick-skinned tomatoes. During very hot temperatures, tomatoes will thicken their skins in an effort to conserve moisture. This is a natural response to high temps. It helps, though, to keep your tomatoes mulched so the soil stays cool and slightly moist at all times. Radishes are very intolerant to hot conditions and will show all sorts of stress when the temps climb. They are a cool-weather crop and can't tolerate heat."