What Causes Blossom Drop in Tomatoes?

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Robert Pavlis

Why are my tomatoes not producing fruit? This is a common question on social media and in this post I’ll look at the real reasons for blossom drop and poor fruit production, as well as some of the myths people spread about the problem.

In a gardening Facebook Group someone asked, “What am I doing wrong??? This tomato plant is over 8 ft tall and hasn’t produced a single tomato. It has tons of flowers but they always fall off. I have 20 varieties of tomatoes growing and none of them are producing fruit.”

Here are some of the answers that people gave to help this poor gardener.

Tomato blossom - What Causes Blossom Drop in Tomatoes?
Tomato blossom – What Causes Blossom Drop in Tomatoes?

More Water – or Less Water

Both opinions were voiced. The plant is growing well (a picture was provided), but it may have been pruned a bit too much. If it was getting too much water, the roots would rot and the leaves would show wilting. If too little water was given, the leaves would also show wilting.

A lack of water can lead to blossom drop, but there is no information in this case that suggests watering is a problem.

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More Phosphorus to Make Blooms

This is a very common myth. People believe that increasing phosphorus will produce more blooms on a plant but this is not true. Adding more phosphorus to soil that has enough has no effect on flowering.

Low levels of phosphorus in soil stunts growth – that is not the problem here.

Too Much Nitrogen

Excess nitrogen produces a lot of green growth and few fruits. That statement seems to be true to some extent but does the excess nitrogen stop flowers from forming, or fruit from developing? It has more of an effect on flower production which is not a problem in this case.

Too little nitrogen produces spindly vines with small leaves. The plant then can’t produce the sugars it needs to produce flowers and fruit. A healthy looking plant does not have this problem.

Since we have lots of flowers, nitrogen is not the issue. Fertilizing with the right amount of fertilizer is still important.

Need Pollinators

Many plants do need pollinators to produce fruit, but tomatoes are self fertile and most fruit is set without pollinators. The pollen is produced, and generally falls onto the stigma. Wind does help this process and pollinators can increase yield.

A lack of pollinators is not the problem.

Hybrid Seed

This is a new one on me. “Seed saved from hybrids does not produce fruit as well.”

Heirlooms are essentially line bred hybrids and they produce fruit.

Not sure how this idea developed, but if you know the origin for this, let me know in the comments.

Low Levels of Molybdenum

” Low levels of molybdenum keep the stomata from working properly.” Molybdenum is an essential micronutrient but it is rarely deficient. It is required for converting different forms of nitrogen, and it is used by nitrogen fixing bacteria. I found no connection to stomata control.

Even if there is a connection, how is this connected to pollination?

Sprinkler Washing Pollen Away

“If you use a sprinkler system where the water is spraying up onto plants, it washes the pollen out of the flowers. It is better to use drip irrigation.”

Take note that no information about the irrigation method was provided in the original description, but that does not stop people from finding solutions.

Does the form of irrigation affect the pollen? This has been studied, and they found that with both sprinklers and drip irrigation, tomatoes had similar pollinator visits, and both had the same amount of fruit set.

Admittedly, this study used an overhead sprinkler and the comment was about a sprinkler facing up into the plant.

I doubt the sprinkler is the issue. It might knock some pollen off flowers right at the time you water, but most of the time you are not watering. Flowers that open when the water is off will not be affected. Flowers that are already pollinated by the time water is turned on will also not be affected. A sprinkler might account for low yield but not for zero fruit.

Top the Plant

Several comments discussed topping the plant, or removing part of the plant. The reason given for this is that you want to “force nutrients into the body and roots of the plant so it produces more fruit.”

I can’t see the logic here. Removing some of the plant does not move nutrients into the roots. In fact, some nutrients like calcium only move up the plant and not down into roots. A reduction of green growth means the plant produces less sugars, which is not going to improve pollination. Remember the plant is producing flowers.

One person commented that you should “pinch the top to form more laterals which produce more fruit.” I don’t think this is true, but even if it is, the problem is not a low yield, but no yield at all.

Excessive pruning results in weak plants and blossom drop.

Calcium and Eggshells

You can’t have a discussion about tomato problems without someone suggestion the plant needs calcium. Calcium is rarely deficient, and eggshells don’t decompose in soil even if this was a problem.

Better to use TUMS – just kidding.

Spray Flowers with Blossom Set

There are several commercial products with names similar to “Blossom Set” that contain hormones which can increase the success of pollination. They are only effective at low night temperatures and they can cause blossom end rot. Spraying the hormones can be helpful early in the season when temperatures are still too low, but are not very useful later in the year. Spray only on the flower cluster since its toxic to plants.

Qualifying Questions

One of the biggest problems with solving such a problem on social media is that the original poster rarely provides a detailed description and people providing answers rarely ask any follow up questions. Few people criticize illogical ideas and this leads to new myths being distributed.

In this particular case, there are 29 comments before someone asked the first question, which was, “where do you live?”

The answer was Florida, zone 9b, and the date of the original post was August 4.

If you have not been to Florida in August, you might not know it is extremely hot, and this might have something to do with our problem.

Other questions that should have been asked before any responses were given, include things about fertilizer rates, watering frequency, past experience growing tomatoes and history of the soil.

Temperature and Tomato Blossom Set

Tomatoes are a bit fussy about the temperature. If it is not right the flowers fail to be pollinated and you get blossom drop.

Tomato plants can tolerate extreme temperatures for short periods, but several days or nights with temperatures above 86 F (30 C) [daytime] or below 70 F (21 C) [nighttime] will cause the plant to abort flowers”

Pollen is viable for about 2 days and will not set until nighttime temperatures are above 55 F (13 C) for at least 2 nights in a row. “When daytime temperatures exceed 85 F (30 C) or nighttime temperatures exceed 70 F (21C), pollination suffers due to pollen becoming “tacky” and non-viable.”

Once the qualifying question about location was answered it became clear that temperature was the problem. Several other people living in Florida then commented, that they get blossom drop in July and August and that tomatoes start producing again in September. Problem solved.

What Causes Blossom Drop in Tomatoes

What can cause blossom drop in tomatoes? Blossoms drop when the plant is stressed and can be caused by any of the following.

  • Not enough water
  • Missing nutrients, especially nitrogen
  • Too cold or too hot
  • Poor genetics
  • High winds
  • Humidity outside the range 40-70% 
  • Heavy fruit set – fewer fruits later in season
  • Insects and disease

There is some evidence that shading can keep plants cool enough to set fruits in hot climates.

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

12 thoughts on “What Causes Blossom Drop in Tomatoes?”

  1. Hi, I really appreciate your information coming from a science background. I particularly appreciate that you specifically clarified what temperature you were talking about after saying too hot,too cool. I am in zone 3a so what I consider hot…..some people would probably consider warm, lol but plants don’t know zones. So when gardeners simply say too hot/too cold it may or may not apply to my area.

    Reply
  2. Hand pollinate your tomato plants with a soft artists brush. I have also done this with fruit tree blossom with great success.

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  3. re: hybrid seed. I’ve never saved seed from a hybrid, but I’ve always been told (and I think it’s scientifically accurate) it might not be true to the parent plant, but rather revert to one of the original strains or be otherwise genetically unstable. Expanding on that, I’ve seen comments on social media essentially saying that hybrids aren’t likely to start with a cross of fantastic strains, but strains with certain good traits that make combine well. Essentially, the result is better than either original strain, or else they wouldn’t bother hybridizing. i.e. if seeds revert, then you are much more likely to get low yields, high disease susceptibility, poor heat tolerance, etc. etc. The original thinking boils down to hybrid seed is unstable, so any undesirable trait is possible. I think (though not certain) that this idea crystallized into a specific negative traits attributed to hybrid seed.

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    • Seed collected from hybrids is usually inferior – but it can also be superior – depends on the genes the seed gets.

      In reality all heirlooms are hybrids – they have just been line bred so they are more stable.

      Reply
  4. what do you know about rooting hormone formulas – commercial varieties and home made ones using things like honey, cinnamon, aloe vera or aspirin powder?

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    • Commercial products include a rooting hormone – so they work.

      Honey, cinnamon, and aspirin don’t contain the hormone – so they don’t work.

      Reply
  5. It is obvious that blossoms drop when the tomato plants are stressed. In my opinion, the main cause is too much heat

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  6. I’m not sure where you got the idea that tomatoes don’t need pollinators. I’ve heard it at advanced Mater Gardener trainings a couple of times from UC Davis professors that bumblebees are needed for successful pollination of most plants in the nightshade family, but that honeybees do not.

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      • The explanation is that transferring pollen directly isn’t the only way pollinators aid in fruiting. In this case, it’s the vibrating action caused by bees (“sonification”) that moves the pollen from stamens to pistils mechanically, similar to the wind’s action. Of course, that’s assuming the pollen is viable, which in high-heat situations it’s not.

        Reply

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