What is Blossom End Rot? (Plus The 2 Best Ways to Avoid It)


Are the bottoms of your tomatoes rotting?  The problem could be blossom end rot.  If you want to know what blossom end rot is, what causes it, and how to treat it, you’ve come to the right place.

So, what is blossom end rot?  Blossom end rot occurs when the bottom (blossom end) of a tomato rots and turns brown or black.  Blossom end rot is caused by a lack of calcium in one part of the tomato fruit.  Blossom end rot can also affect peppers, eggplants, and other fruits.

Of course, there are many things that can cause the lack of calcium that leads to blossom end rot.  We’ll look into these causes in detail below.

We’ll also talk about how to treat and prevent blossom end rot so you can avoid the problem and get a better harvest.

Let’s get started.

What is Blossom End Rot?

According to the University of New Hampshire Extension, blossom end rot is a localized calcium deficiency in the fruit.  This means that the plant cannot get enough calcium to all the places it needs to go.

This lack of calcium causes plant cells in one part of the fruit to die.  The affected part of the fruit becomes water-soaked and then turns brown or black.

tomato blossom end rot
Blossom end rot causes a sunken brown or black spot to form at the bottom of tomatoes when there is a calcium deficiency in that spot.

Blossom end rot is common in tomatoes, but it can also affect peppers, eggplants, and squash.  The term “blossom end rot” is fitting, because the disease affects the bottom (blossom end) of the fruit, where the flower falls off.

According to the Michigan State University Extension, blossom end rot is more likely to occur on the first tomatoes of the season.  So, there is a chance that the problem will resolve itself as the growing season goes on.

However, if the problem continues, there are ways to treat the problem now and prevent it in the future.  Blossom end rot comes down to a low concentration of calcium in one part of the fruit, but that can be caused by many things.

Let’s take a closer look at those causes.

What Causes Blossom End Rot?

Blossom end rot happens when there is a lack of calcium in one specific part of a tomato or other fruit.  According to the Michigan State University Extension, calcium is an immobile nutrient.

This means that calcium tends to get “locked in place” within a plant.  Basically, the plant cannot move the calcium throughout its tissues once it is in place, leading to deficiencies in some parts of the fruit.

Lack of Water

A lack of water is one of the most common causes of blossom end rot in tomatoes and other plants.  In order for a plant to uptake calcium, the calcium must be dissolved in water in the soil.

water
Calcium needs to be dissolved in water in soil so that plant roots can absorb it.

The plant absorbs this water through its roots and takes the calcium dissolved in the water.  Without enough water in the soil, the calcium in soil does not dissolve, and the plant cannot absorb it.

As a result, the plant will not be able to get calcium and deliver it to the fruit for proper growth.

A lack of water in your plants can be caused by two things:

  • Uneven watering – if you forget to water your plant and the soil dries out, the plant will not be able to absorb any water through its roots.
  • Transpiration – this is when a plant moves water through its tissues.  Some of the water evaporates from the plant’s leaves, stems, and flowers.  On a hot, dry day, a plant will lose lots of water to transpiration.

Nutrient Imbalance

A nutrient imbalance is another possible cause of blossom end rot.  With too much of one nutrient in the soil, a plant will be unable to absorb others.

For example, excessive magnesium in soil prevents the uptake of calcium by plants.  The reason is that calcium and magnesium have similar chemical properties, because they are in the same column of the periodic table (Mg and Ca), as seen on Wikipedia.

If you add too much Epsom salt to your soil, you can end up with excessive magnesium.  Epsom salt is magnesium sulfate, and you can more about what it is and how to use it in my article here.

magnesium sulfate
Epsom salt, or magnesium sulfate, can make calcium deficiency worse by preventing plants from absorbing calcium.

According to the Iowa State University Extension, too much nitrogen fertilizer can also cause blossom end rot.  If you suspect that this is a problem, then you might want to avoid fertilizers high in nitrogen and see if it makes a difference for your plants.

Soil pH Imbalance

A soil pH imbalance is another reason that your plants may suffer from blossom end rot.  According to the Old Farmer’s Almanac, the soil pH should be around 6.5 for tomatoes to ensure ideal growth.

If the soil pH is too low or too high, the availability of calcium will be reduced.  You can see a graph of the effects of extreme pH on nutrient availability in this chart from Research Gate.

If your soil is too acidic (low pH), you can add lime (calcium carbonate) to raise it.  If your soil is too basic (high pH), you can add sulfur to lower it.

However, you should get a soil test before adding anything to your soil.  You can learn more about how to do a soil test in my article here.

Calcium Deficiency in Soil

If there is not enough calcium in the soil, then there won’t be enough dissolved in water for the plant to absorb through its roots.  This is less common than the other causes mentioned above.

One cause of calcium deficiency in soil is a lack of crop rotation.  If you plant the same crop in the same spot every year, certain nutrients will be depleted over time.

This is especially true if you fail to replace nutrients with compost, fertilizer, and other soil amendments.  You can learn more about what crops to rotate with tomatoes in my article here.

Root Damage

Damaged roots are another potential cause of blossom end rot.  Root damage often occurs during transplant, when you move young plants from indoors to outdoors.

tomato plant roots
It is easy to damage the roots of a young tomato plants when you transplant it into the garden, so be careful!

Another way that root damage occurs is when young plants are grown too close together.  Their roots end up tangled, and when you pull them apart, the roots get ripped.

Another way that root damage occurs is when young plants are left out in the sun with their roots exposed.  Leave your transplants in their tray (or in a bucket with the roots submerged in water) until you are ready to plant them.

You can learn more about transplanting tomato plants (including when to do it) in my article here.

A plant with damaged roots will not be able to absorb as much water from the soil.  As a result, the plant will not be able to absorb enough calcium (which needs to be dissolved in water for uptake by plant roots).

You will probably notice other problems caused by root damage, including:

  • Stunted growth – plants with damaged roots will not grow as large as plants with healthy roots.
  • Decreased vigor – plants with damaged roots will not look healthy, and may succumb to diseases or insect pests.
  • Yellow leaves – this is one frequent symptom of nutrient deficiencies.

Does Blossom End Rot Spread?

Blossom end rot does not spread from plant to plant.  Blossom end rot is not caused by any bacteria, virus, or fungus, so it does not spread from plant to plant like blight or other plant diseases.

For example, I could give you a cold if I was sick, since a cold is caused by a virus that can spread.  However, I could not give you a calcium deficiency if I had one, since there is no virus to spread to you.

However, blossom end rot can affect any other plant in your garden that is subject to conditions that cause the problem (such as uneven watering, excessive nitrogen fertilizer, etc.).

Although blossom end rot does not spread, you should remove affected fruit right away.

For one thing, blossom end rot makes infected fruit more susceptible to fungus.  Fungus can then spread to other fruits on the same plant or to other plants.

Also, removing affected fruit prevents the plant from wasting energy on fruit that might not be edible by the time it matures.

Since blossom end rot is not caused by a virus, bacteria, or fungus, you can safely compost any discarded fruit.

If only certain plants in your garden are affected by blossom end rot, you will need to do some work to figure out the problem.  Think about the differences between the plants.  For example:

  • Variety – if you are growing multiple different types of tomatoes, does one variety in particular tend to get blossom end rot?  If so, you might want to grow more of the varieties that don’t have this problem.
  • Watering – are you watering some plants more than others?  Maybe you are getting impatient and giving less water to the plants at the end of the row.
  • Nutrients – are some of your plants getting a higher dose of calcium and other fertilizers than other plants?  This is more likely if you fertilize by hand instead of measuring.
  • Sunlight – do plants in full sunlight tend to get blossom end rot more often?  Maybe the strong sun is causing water to evaporate from the soil faster and causing faster transpiration.  Both of these things could combine to cause a lack of water and thus a calcium deficiency, leading to blossom end rot.

How to Fix and Prevent Blossom End Rot

If you want to fix blossom end rot, there are a few things you can try.  There are also some ways to prevent blossom end rot in the future.

Let’s start off with proper watering.

Water Management

Lack of water is one of the most common causes of blossom end rot.  So, it makes sense that proper watering and management is one of the best ways to prevent the problem.

First, give your plants a nice deep drink of water when you transplant them into the garden.  Then, add a layer of mulch to the soil around your plants.

watering can
Give your plants plenty of water, and don’t let the soil dry out too much between waterings.

Don’t mix the mulch into the soil – just leave it as a layer on top of the soil.  Mulch helps to retain moisture in the soil by preventing evaporation due to heat and sunlight.

If you aren’t sure how to get started with mulching, you can https://greenupside.com/can-you-mulch-around-tomato-plants-what-to-use/learn more about how to mulch around tomato plants in my article here.

Second, check your plants every day to see if they need water.  Use your fingers to feel the soil around them at a depth of a few inches.

If the soil feels a little dry, go ahead and water deeply.  This encourages your plants to form a large, strong, deep root system, which helps them to survive droughts.

Don’t let the soil dry out, or you invite blossom end rot into your garden.  If you can’t check your soil every day, consider using a drip irrigation or sprinkler system to keep the plants watered.

If the leaves on your tomato plant curl up, it could be that it is lacking water.  However, there could be other reasons.  To learn more about why tomato leaves curl up, read my article about it here.

Soil Nutrition

To avoid problems with calcium availability in the soil, you will have to do 2 things:

  1. Make sure the soil pH is in the correct range.
  2. Make sure there is enough calcium in the soil.

There are lots of options that will help you to keep soil pH and calcium levels in the proper range.  However, before you add anything to your soil, do a soil test!

A soil test will tell you the soil pH, which will indicate whether you need to treat it or not.  Soil tests can also indicate calcium levels, so you will know if you need to add calcium or not.

You can learn all about what soil tests tell you in my article here.

Lime

If your soil is acidic (pH too low), you can add lime (calcium carbonate) to your soil to raise the pH.  One advantage of lime is that it contains calcium in a form that becomes available to plants when dissolved in water.

calcium carbonate
Lime, or calcium carbonate, will raise the pH of soil and also provide calcium

Another option for increasing soil pH is dolomitic limestone.  Dolomitic limestone contains calcium carbonate, but it also contains magnesium if your soil needs it.

If you want to add calcium on a smaller scale (or on a per-plant basis), you can just mix a small handful of lime into the soil where each plant will go.

Antacid Tablets

You can also use antacid tablets like Tums to provide extra calcium for your plants.  These tablets contain calcium carbonate, which is the same thing that lime is made of.

Just put one antacid tablet in the soil with each plant.  Then, water it in to give your plants a boost of calcium.

You can do this when you transplant, or later in the growing season if you see your fruit starting to show signs of blossom end rot.

Gypsum

If you want to add calcium to your soil without changing the pH, gypsum (calcium sulfate) is a good option.  Gypsum is used in plaster, chalk, and drywall (for construction).

gypsum
Gypsum, or calcium sulfate, adds calcium (and sulfur) to soil without changing the pH.

Gypsum also provides sulfur, which is another nutrient necessary for plant growth.  You can find gypsum at Planet Natural.

Bones and Shells

You can also use powdered bones and shells to add calcium to your soil.  The finer the bones or shells are ground up, the more easily they will dissolve and become available to plants as calcium.

eggshells
If you grind up eggshells into a fine powder, they can provide some calcium for your plants.

You can find ground oyster shells at Planet Natural.

You can find also find bone meal at Planet Natural.  Bone meal has lots of calcium, but it also has a good amount of phosphorus, which is another important nutrient for plant growth.

If you want to make your own calcium-rich mix, you can use eggshells.  Just put the eggshells in a blender with some water.

Blend the water and eggshells to make the shells finer.  Then, pour the water and ground eggshell mixture over the soil.

Note: if the shells are not ground down to a fine powder, it will take more time for the calcium to become available to plants.

You can also try dissolving any fine powder containing calcium into water and using a foliar spray.  A foliar spray is applied to the leaves of a plant using a spray bottle.

This allows the plant to absorb calcium dissolved in water through its leaves.

Other Calcium Sources

I have also heard that you can use powdered milk as one calcium-rich source for your plants.  I have not tried this personally.

You can also try wood ash, which contains a good amount of calcium, depending on the type of wood.

wood ash
Wood ash can provide nutrients to soil, including calcium. It can also be used to raise pH.

Gardener’s Supply Company has a tomato rot stop spray you can try.

You can learn more about fertilizers that are high in calcium in my article here.

What Not to Use for Blossom End Rot

Calcium nitrate is a rich source of calcium for plants.  However, I would not use it to treat blossom end rot.

The main reason is that calcium nitrate is also a rich source of nitrogen.  As we already discussed, excessive nitrogen can cause blossom end rot to occur.

Epsom salt is another treatment I would not use for blossom end rot.  The reason: Epsom salt does not contain any calcium!

Epsom salt is magnesium sulfate, which makes it a fine source of magnesium and sulfur for your plants.  However, it will not do anything to add calcium to your soil if it is deficient.

Epsom salt also will not solve the problem if a lack of water is the cause of blossom end rot.  In short, using Epsom salt is not the way to treat blossom end rot caused by calcium deficiency.

You can learn more about how to treat calcium deficiency in my article here.

What Does Blossom End Rot Look Like?

To help you to identify blossom end rot, here are some pictures on various fruits.

blossom end rot on red tomatoes
Here we can see small dark spots of blossom end rot on the bottom of small red tomatoes. The spots can get much larger than this!
blossom end rot on green tomatoes
Blossom end rot can affect tomatoes long before they ripen, as you can see here.
Image courtesy of user Nnoddy at Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Tomendrot.jpg
pepper blossom end rot
Blossom end rot can also affect peppers, as you can see from the dead brown part of the bottom of this pepper.

Remember that according to the University of Wisconsin Extension, sunscald on tomatoes may look similar to blossom end rot!

Conclusion

Now you know what blossom end rot is, what causes it, and how to treat it.  You also have some ideas on how to prevent it in the future.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

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jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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