You’ve worked hard to tend thriving tomatoes all spring, but now they won’t ripen! If you have a bunch of green fruits hanging from your plants, it could be a sign that the temperature and soil conditions are not ideal for plants.
Tomatoes need the right environment to produce the ethylene (plant hormone) needed to ripen the fruits and turn them red (or orange or purple).
Temperature fluctuations are the most common cause of tomatoes that aren’t ripening. Under-watering and over-fertilizing are other likely culprits for green tomatoes hanging on the vine.
Tomato plants require moderate temperatures, continuous moisture, and proper nutrient levels to ripen their fruit. Once a tomato plant has produced flowers and cross-pollinated, the fruit begins to develop. Depending on the type of tomato plant you grow, the early stages of fruiting will vary. Plan on around 40-50 days for a slicer tomato and 20 to 40 days for cherry tomatoes.
Once the fruit has reached its full size (“mature green”), the plant shifts its attention toward ripening. If your fruits won’t ripen, it’s likely due to one of the factors I’ll discuss in more detail below.
Cold Temperatures
Tomatoes require temperatures between 60 and 80°F to ripen properly. According to Purdue University, the ideal range is 68-77°F. The further temperatures stray from this range, the less likely fruits are to turn red.
Cold nights are particularly problematic because they stress out the plant and halt the ripening process. When cold nights alternate with hot days, your plants may have lots of green fruit.
The Solution: Protect your plants with row fabric or low plastic tunnels. Fabric or plastic buffers tomatoes from temperature fluctuations and adds extra warmth during cold nights. The key is to remove the row fabric or open up the tunnels during the day to allow continued pollination of flowers.
Too Much Fertilizer
Tomatoes are heavy feeders, but too much nitrogen fertilizer at the time of fruiting can prevent tomatoes from ripening. The most obvious symptom of excess nitrogen will be lush, leafy plants with little to no fruit. Blossoms may fall, and any fruit that forms may remain green for a long period of time.
Most plants require their nitrogen during the vegetative growth phase while they establish roots, stems, and leaves. When the plant shifts to the reproductive phase, it needs more phosphorus and potassium to fuel flower, fruit, and seed production.
Too much fertility may also manifest as:
- Overgrowth of leaves
- Weak vines
- Delayed flowering
- Poor fruit set
- Oddly shaped fruits
- Hollow fruits
- Blossom end rot
The Solution: Once a tomato plant is established and large enough to fruit, stop fertilizing. Avoid feeding a tomato plant nitrogen right before it flowers. You should fertilize your tomatoes with all-purpose, slow-release fertilizer at the time of planting.
When plants start to flower, you can optionally provide a dose of organic fertilizer that is high in phosphorus and potassium for the fruiting phase. If you already have a lot of compost and organic matter in your soil, this may not be necessary.
To neutralize excess nitrogen, add a small amount of bone meal or colloidal phosphate to the soil. You can also flush the soil with water and integrate more compost into the upper layers.
Overwatering
We know that tomatoes are thirsty plants. However, too much water during the fruiting phase can inhibit ripening and reduce the flavor of your tomatoes.
When I was a professional vegetable farmer, we always cut back on water right when tomatoes began to ripen. This ensured maximum color change and sugar accumulation in the fruit. However, you shouldn’t let the soil dry out completely, or you may stress out the plant.
The Solution: Check the soil near your tomatoes before watering. If the soil is soggy, refrain from irrigating for a few days. The upper inch or two of soil can be moderately dry.
If you wait until the leaves begin to wilt slightly before you water again, it can push the plant to ripen its green fruit.
Warning: This technique is only to be used in the final stages of fruiting when a plant has lots of mature, green fruit hanging on the vines. Do not withhold water during the vegetative and fruit growth phase! Too much water stress may cause the tomato plant to stop producing entirely. Balance is key!
Excessive Heat
Periods of extreme heat can slow or halt tomato ripening. If heat is the problem, you’ll probably notice greenish-yellow or yellow-orange fruits. The fruits usually start to ripen until the scorching heat stalls their progress.
Tomatoes cannot produce carotene and lycopene—the pigments that give ripe tomatoes their color— in temperatures above 85°F. This heat stress is compounded when plants don’t have enough water.
The Solution: While you don’t have control over the weather, you can take steps to protect your tomatoes from excessive heat. If you live in the north, this likely isn’t necessary. However, southern growers may need to use one or more of these methods to buffer against extremely hot weather:
- Mulch the soil with shredded straw or leaves to preserve moisture and keep the roots cool.
- Use shade cloth to protect leaves and fruit from sun-scalding.
- Plant tomatoes in an area where they receive slightly dappled afternoon shade (too much shade can be harmful).
- Use drip irrigation, soaker hoses, or an olla to ensure consistent soil moisture.
- Avoid over-pruning. Removing too many leaves can leave fruits exposed and vulnerable to extreme heat.
Tomatoes obviously love warmth and sunshine, but southern summers may call for planting multiple successions of tomatoes that can harvest in the milder weather of late spring and early fall.
Worst case scenario, you can keep watering and caring for your plants while you wait for cooler weather. The fruits will begin to blush and produce lycopene and carotene pigments as soon as temperatures consistently fall below 85°F in the evenings.
Underwatering
Watering is always a balance. Too much moisture and your tomatoes may crack, but too little moisture and the plant won’t have the energy it needs to produce those juicy tomatoes at all.
Tomatoes need about 1-2 inches of water per week to achieve optimal growth. If you’re growing in containers or in extra hot weather, your plants may require more irrigation.
Tomatoes that don’t get enough water in the initial stages of fruiting are more likely to:
- Droop or wilt
- Have brown, shriveled leaves
- Drop their flowers
- Develop small, stunted fruit
- Produce fruit with Blossom End Rot
- Stop ripening their fruit
The Solution: Check your tomato beds every 2-3 days to ensure the soil feels moist and the leaves appear perky and hydrated. Your plant will clearly communicate if it’s thirsty by wilting, shriveling, or drooping. Fruit may appear stunted or dried out.
If you stick your finger 4-6” into the soil and your skin comes out dry, the roots are likely very dehydrated! Give them a deep watering and check back in a couple of days. For potted tomatoes, water deeply until water pours out of the bottom drainage hole.
Sunscald
Tomatoes are sun-lovers, but the fruits are not meant to be exposed to direct sun rays all day length. Tomato plants that are defoliated by pests, diseases, and over-pruning are vulnerable to sun-scalded fruit.
The harsh rays of sunlight on the green tomatoes may actually inhibit ripening. You can often see brown, whitish, or yellow blisters developing on the sides of the fruit that face the sun. The areas may become papery or moldy.
The Solution: Avoid removing leaves to ripen tomatoes. This is an urban myth that may cause more harm to your plant than good.
Keep your tomato plants protected against pests and diseases. Maintain as much lush foliage as possible. If the fruit is exposed, you can cover it with a lightweight material like shade cloth to shelter it from intense light.
Short Daylength
Gardeners typically find that their tomatoes have trouble ripening at the very end of the season. Tomatoes are proven to be a photosensitive species, which means they respond to the day’s length. When the sunny days of summer begin to fade, the reduced daylight hours tell the plant that it is entering the end of its lifecycle. This shift often coincides with the Autumn Equinox.
Day length is less problematic the closer you get to the equator near the tomato’s natural habitat. Northern growers tend to have more trouble ripening autumn tomatoes.
The Solution: Harvest any fruit remaining on the plants and bring them indoors to ripen. They may not be as tasty as summer vine-ripened tomatoes, but at least they won’t go to waste!
Place tomatoes on your countertop at room temperature near bananas or apples. These fruits produce lots of ethylene to promote tomato ripening. You can also put tomatoes in paper bags to concentrate the ripening hormone.
Do not place tomato fruits in the refrigerator. This will stall ripening and lead to a bland flavor.
High Magnesium
There is some evidence that excess magnesium harms tomato fruit ripening by altering the plant’s amino acid and metabolite ratios. Magnesium directly impacts how the plant uptakes calcium and potassium, which means a mineral imbalance can have a ripple effect on yields, growth, and flavor.
An excess of magnesium salts can lead to:
- Unripened fruit
- High incidence of blossom end rot
- Calcium deficiency
- Stunted growth
- Extra dark-colored vegetation
The Solution: Avoid applying magnesium to your soil unless you know there is a deficiency. In most residential soils, that would be “never” – because magnesium deficiencies typically materialize in heavily-overfarmed soils, and gardeners rarely experience such issues! If you ever add magnesium to the soil, ensure it also has calcium. These minerals interact together. Never use Epsom salt for your tomatoes; that’s a common garden myth, and it can often make blossom end rot worse.
In naturally high magnesium soils, you can add a small amount of gypsum (calcium sulfate) every year to slowly reduce the salt concentration.
Low Potassium
If you notice fruits ripening unevenly (blotches or large portions of fruit remain green while another side turns red), this could be a sign of low potassium. Tomatoes naturally ripen from the bottom up and from the inside out. When fruits display weird ripening patterns and the above issues have been eliminated, it’s time to address any mineral imbalances.
Signs of abnormal ripening due to potassium deficiencies include:
- Uneven ripening of fruit
- Yellow “shoulders” on fruit
- Blotchy ripening
- Margins of new leaves turn yellow
Potassium deficiencies are particularly problematic when there is too much phosphorus in the soil.
The Solution: Research shows that adding potassium sulfate can reduce incidences of ripening disorders. Kelp meal, hardwood ashes, and greensand are great organic options. Follow the fertilizer package instructions to add these materials around the time your tomatoes begin fruiting.
Better yet, feed plants with a fruiting-phase fertilizer at the time of flowering.
Final Thoughts
While there can be many reasons for tomatoes not ripening, the most common cause is excessively cold or hot temperatures. The best way to buffer against temperature fluctuations is to cover your tomatoes with a row cover at night and/or use shade cloth to protect them from excessive heat.
If you fix these factors and still can’t get your tomatoes to ripen, it’s best to harvest the green fruits and ripen them indoors on your countertop.
