Tomatoes require a sufficiently long growing season and suitable temperatures in order to ripen their fruit, otherwise the fruit may stay green and fail to ripen.
When growing conditions are less than ideal, there are ways for gardeners to turn things in their favour, to ensure a successful tomato harvest at the end of the season.
Short Growing Seasons and Tomato Ripening
Tomato fruit reach full maturity in around 6-8 weeks after the flower is pollinated, though this may vary depending on the tomato variety and weather conditions. Sufficient time is required to ripen fruit after flowering.
Some areas normally have to contend with short growing seasons, while other areas that have long growing seasons may experience cold spring weather with late frosts and short summers due to fluctuations in climate.
The two largest limitations to tomato growing in short season areas are:
- Frost, which damages the plant and flowers.
- Cool nighttime temperatures that can interfere with pollen formation and fruit ripening. (For more information, see article Why Are My Tomatoes Flowering But Not Setting Fruit?)
Areas with short season may also experience very hot days during summer followed by cool nights, which causes additional stress to tomato plants, interfering with pollination and fruit ripening.
The Effect of Temperature on Tomato Ripening
The optimum temperature for ripening tomatoes is 21-24°C (70-75°F). When temperatures exceed 29.5-32°C (85-90°F), the ripening process will slow down significantly, or even stop.
At these high temperatures, the pigments lycopene and carotene, which give the fruit their typical orange to red colours cannot be produced. This causes the mature fruit to stay green in colour for quite some time, until temperatures return back to normal, and the ripening process resumes once again.
Do Tomato Fruit Need Sunlight to Ripen?
Some gardeners mistakenly remove tomato leaves shading fruit in an effort to help them ripen faster. Don’t do this, tomato fruit don’t need light shining on them to ripen.
In fact, green or ripening tomato fruit exposed direct prolonged sunlight, especially during sunny, hot weather, can be sunburnt, leading to sunscald.
The symptoms of sunscald are large, light-colored blistered areas that develop on the sides of fruits facing the sun, that later on become dry and sunken, with a papery tan to white texture. This condition is most prevalent on plants that have lost their leaves due to disease, or have had their leaves eaten by insects.
Tomato fruit exposed to excessive direct sunlight can also heat up high enough in temperature to inhibit synthesis of the red-orange pigments that form during ripening.
Uneven or Blotchy Ripening of Tomatoes
Uneven or blotchy ripening of tomato fruit is caused by high levels of magnesium and low levels of potassium, or conditions that limit the supply potassium, such as waterlogged and/or compacted soils, insufficient application of potassium when fertilising, or excessive application of nitrogen when fertilising. This is a different issue to the slowness of tomato fruit ripening, therefore adding additional fertiliser will not help to quicken ripening.
The Best Ways to Grow Tomatoes in Short Season Locations
There are several methods we can use to work around the problem of short growing seasons and maximise our chances of success when growing tomatoes.
Choose Tomato Varieties With a Short Time to Harvest
If we look at any seedling label or seed packet, it will list a time to harvest for that crop. The time may approximately vary from 8-17 weeks (56-119 days).
Select the quickest maturing tomato varieties in short season areas, such as those that ripens in 68-75 days. Many of the tomatoes with larger fruit will take around 80-100 days to mature. That doesn’t mean that larger tomatoes can’t be grown though. Some determinate varieties of tomatoes (those that have more of a bushy habit, grow to their full size first, then produce all their fruit at once) tend to mature early, and do have larger fruit.
Keep in mind that the ripening time is often measured in warmer areas, and may take 10-15 days longer in cooler areas.
There are hundreds of varieties of tomatoes, and it’s important to experiment with different varieties to find out which ones do best in your garden. Check with other local growers or community gardens to see which varieties work best in your area.
Select the Optimum Location
Tomatoes are subtropical plants that need plenty of sun, water and warmth to grow. Grow them in a location that receives a minimum of 6-8 hours of direct sunlight, so they can generate enough energy to produce fruit
Ideally, select a location that receives midday and afternoon sun. (In the northern hemisphere this south and west exposure, in the southern hemisphere this north and west exposure.)
In colder locations, grow tomato plants next to heat-absorbing objects such as brick or concrete walls, water tanks or even boulders to help increase the temperature around them during cold nights, which can extend their growing season, and improve fruit set and ripening. Avoid low lying locations such as the bottom of a slope where cold air will gather, or where structures exist that could block the flow of cold air away from the area, creating frost pockets.
Harvest Tomatoes Before It’s Too Cold and Ripen Indoors
Tomato fruit will ripen normally during warm weather, but towards the end of summer, the air and soil temperatures become cooler, and the ripening process slows down.
Rather than wait, tomatoes can be harvested when green and brought indoors to ripen before the nighttime temperatures get too cold. If green tomatoes are exposed to temperatures below about 4.5°C (40°F), chilling injury to the fruits may occur, and they may fail to ripen, even when brought indoors.
Some gardeners choose to harvest their tomatoes early when the fruit begin showing the first changes in colour. This ensures best flavour when ripened indoors, as it indicates that the fruit is mature (fully grown) and is not chill damaged, because it has just started ripening.
How to Store Harvested Green Tomatoes
If the intention is not to ripen them immediately, mature green tomatoes can be stored for 1 to 6 weeks by placing them in warm location with a temperature of around 12.5-15.5°C (55-60°F), that is relatively moist (80-85% relative humidity).
To maintain the humidity levels, place the green tomatoes in a paper or plastic bag, and store it in a location with the required temperature, such as a cool room, semi-heated room or outbuilding for example.
How to Ripen Green Tomatoes Indoors
To ripen mature green tomatoes, move them to a location which is at room temperature, 20-22°C (68-72°F). Placing them in an enclosed environment, such as a brown paper bag folded closed, is the best method, as the ethylene gas released from fruit as they ripen will stimulate other fruit in the bag to ripen also.
The ripened tomatoes will keep for 4 to 10 days in storage, and the ideal storage conditions are cool 7.0-12.5°C (45-55°F) and moist (80-90% relative humidity). At temperatures below 7°C (45°F), tomato fruit are susceptible to chilling injury. Don’t refrigerate them, as that will totally destroy their flavour!
If the green tomatoes are picked when they show the first signs of ripening and not earlier, and are kept at room temperature to ripen them, their flavour and colour should develop similar to Those ripened on the plant.
- Cornell Cooperative Extension, Cornell Vegetable Program, Why Aren’t My Tomatoes Ripening? By Steve Reiners, August 22, 2012, https://cvp.cce.cornell.edu/submission.php?id=91
- University of Idaho Extension, Short-Season, High-Altitude Gardening Bulletin 864, Growing tomatoes in cool, short-season locations, by Michael Bauer, Danny L. Barney, and Jo Ann Robbins, 2009
- University of Maryland Extension, Sunscald of Vegetables, September 2016, https://ag.umass.edu/vegetable/fact-sheets/tomato-physiological-ripening-disorders