It can be frustrating to have a tomato plant full of green tomatoes with no sign that they will ever turn red. Some people think that a green tomato is much like a pot of water—if you watch it, nothing seems to happen. So the question becomes, “why do tomatoes turn red?”

As wearisome as waiting might be, you will be glad to know that there are a few factors that can either speed up or slow down how fast a tomato turns red.

The main determiner in how fast a tomato turns red is the variety—smaller fruited varieties will turn red faster than large fruited varieties. This means that a cherry tomato will not take nearly as long to turn red as a beefsteak tomato. Tomatoes cannot turn red, even forced by modern technology, unless it has reached the mature green stage, and the variety will determine how long this takes.

Another factor in how long it takes for a tomato to turn red is the outside temperature. Tomatoes will only produce lycopene and carotene—two substances that help a tomato turn red—between the temperatures of 50°F and 85°F. If it is any cooler than 50°F, those tomatoes will stay green. Any warmer than 85°F and the process that produces lycopene and carotene comes to a screeching halt.

Tomatoes are triggered to turn red by a chemical called ethylene. Ethylene is odorless, tasteless and invisible to the naked eye. When the tomato reaches the proper green mature stage, it starts to produce ethylene. The ethylene then interacts with the tomato fruit to start the ripening process. Consistent winds can carry the ethylene gas away from the fruit and slow the ripening process.

If you find that your tomatoes fall off the vine—either knocked off or due to frost—before they turn red, you can place the unripe tomatoes in a paper bag. Provided that the green tomatoes have reached the mature green stage, the paper bag will trap the ethylene and will help to ripen the tomatoes.

There are not many things that a gardener can do to hurry the ripening process up on tomatoes that are still on the plant. Mother Nature cannot be easily controlled and she plays a major role in how quickly tomatoes turn red, but heeding this advice should aid in producing the results you are looking for.