Tomato plants redden due to a variety of factors, including having enough sunlight for photosynthesis along with having the correct amount of nitrogen and spacing among plants. Other factors, such as extremes in temperatures or moisture stress, can also keep the tomatoes from turning red.
Due to the fact that they require plenty of sunlight and agreeable temperatures to ripen, many tomatoes will turn red fastest during the warmer summer months. Even in these warm months, however, factors, such as too much humidity or lack of rain, can cause a tomato to remain green longer than expected. Gardeners can remedy the situation in some ways, such as by watering the plants every few days during a dry spell.