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Why Are My Tomatoes Flowering But Not Setting Fruit?

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It’s quite disappointing for gardeners when healthy and vigorous tomato plants,drop their flowers and fail to produce fruit after a good growing season.

Tomato plants have what are described as complete flowers, they contain both the male and female parts within the same flower. The tomato flowers grow in downward hanging clusters that are pollinated by movement and vibration, such as when they’re shaken by the wind. As such, tomato flowers don’t need insects to be pollinated. If a flower is not properly pollinated, blossom drop drop will occur, the flower will die and drop off.

Tomato blossom drop can be caused by unfavourable weather conditions, low humidity levels, water stress, excessive nitrogen fertiliser, nutrient stress, and plant stress caused by pests and diseases.

Unfavourable Weather Conditions

The main reason why tomato plants drop their flowers and fail to set fruit is exposure to unfavourable weather conditions.

The optimum daytime growing temperatures for tomatoes are between 21-30°C (70-85°F).

High daytime temperatures above 30°C (85°F) or high nighttime temperatures above 21°C (70°F) cause the pollen to becomes tacky and nonviable, which prevents proper pollination from taking place. When this happens, blossom drop occurs, the flowers dry up and fall off.

Low nighttime temperatures below 13°C (55°F) can also prevent pollination, and cause blossom drop in tomatoes. Low temperatures affect the viability of the pollen, as well as the growth rate of the pollen tube that forms shortly after a pollen grain lands on a mature stigma, the female part of the flower (see next section).

After the weather extremes have passed, and weather becomes more moderate, fruit set will increase in tomatoes.

When the weather is erratic, with extremes of hot and cold, it’s a good idea to plant cherry tomatoes because they set fruit over a wider range of temperatures compared to most large-fruiting tomato varieties.

Low Humidity Levels

For pollination to occur, pollen needs to be transferred from the male portion of the flower known as the stamen to the female portion of the flower known as the stigma.

In the process of pollination, pollen is transferred from the male stamens to the female stigma

The process of pollination is affected by the amount of moisture in the air, which is known as the relative humidity (RH). The ideal humidity range for pollination of tomatoes is 40% -70%.

If humidity is too low or too high, pollen can be either too dry or too sticky respectively for effective wind pollination.

When the relative humidity levels are lower than 40%, the pollen tends to become too dry and has trouble sticking to the stigma, the female part of the flower.

When the relative humidity levels are higher than 70%, the pollen becomes too sticky, does not shed very easily from the stamen, the male part of the flower, to reach the female part.

Water Stress

Dry soil conditions or excessive wind can cause water stress in plants, which is detrimental to good fruit set, and lead to blossom drop in tomato plants.

Plants have to expend a lot of energy and nutrients to support flowers and develop fruit, it’s very demanding on a plant’s resources. When plants experience adverse conditions that stress them, they drop their flowers to spare their remaining resources for their survival. 

Strong winds can cause flowers to desiccate (dry up) and drop.

The combination water stress and high temperatures creates very challenging environmental conditions for tomatoes to set fruit.

To avoid moisture stress when temperatures get too high, keep plants well-irrigated by deeply water tomato plants at least once a week, and preferably every morning before every hot, windy day. Use a layer of mulch around 5-10cm (2-4”) thick around the plant to preserve soil moisture and reduce water loss from the soil surface due to evaporation. During extreme winds or high temperatures, garden shade cloth (which provides 50% shade) can be used to reduce the intensity f the sun and wind.

Excessive Nitrogen Fertiliser

Excess nitrogen fertilizer increases leafy green vegetative growth vegetative growth and decrease the formation of flowers. It stimulates excessive growth of leaves and stems at the expense of flowers and fruit, causing flower drop and reducing production yields. Overfertilising by using too much nitrogen fertiliser can also cause potassium or magnesium deficiencies in tomato plants.

Use slow-release organic fertilisers, avoid synthetic fertilisers as these are high in nitrogen, and follow the directions for the application rate – how much fertiliser to apply, and how often.

Nutrient Stress

Harvest tomatoes that are ripe to encourage new flowering

Sometimes tomato plants that are fruiting very heavily will decrease production of additional flowers. This is because the plant is experiencing nutritional stress, as it doesn’t have sufficient nutrients in reserve to do both at the same time.

Each plant has a finite amount of nutrients which it can either direct into developing and ripening existing fruit, or producing new flowers to create even more fruit.

After a tomato plant has set a large quantity of fruit on its older and lower branches, there is competition for nutrients between these developing fruits which are growing in size, and new flowers developing on the younger middle and upper branches.

Most often, the older developing fruit wins the competition for nutrients, resulting in poor fruit set of the middle flower clusters of the tomato plant.

By harvesting the fruit on the bottom branches, the nutritional stress on the plants is reduced, allowing the upper flower clusters to set normally, and produce more fruit.

Plant Stress Caused by Pests and Diseases

Plants that are heavily affected by pests and diseases become stressed, and when their health an vigour declines, fruit set also reduces. Pest insects and diseases such as Botrytis (gray mold) and bacterial spot are rarely the primary cause of the problem of failure of fruit to set, but they can exacerbate the problem, making it worse.

References

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