What is blossom end rot, and what causes it?
Have you ever picked a beautiful tomato, turned it over, and discovered an ugly black spot on the bottom? If so, you've met one of the most frustrating problems tomato gardeners face: blossom end rot.
The good news is, your tomato doesn't have a disease or a fungus, and your crop isn't doomed.
What's wrong with my tomatoes?
Every summer I see photos online from discouraged gardeners asking the same question: "What's wrong with my tomatoes?"
The tomatoes look healthy until you turn one over. Then there's that discolored, sunken patch on the blossom end - the bottom of your tomato.
Most people assume it's a disease, or a fungus, or that insects have attacked the fruit.
Actually, it's none of those things.
Blossom end rot can affect other vegetables too
Interestingly, I've only dealt with blossom end rot once in my own tomato garden, the year I grew a few Roma tomatoes. I've actually had blossom end rot more often on my bell peppers than on tomatoes. It can affect peppers, eggplant, and occasionally squash, especially zucchini - as well as tomatoes.
In fact, one pepper was so funny-looking that I took a picture of it. The blossom end rot spots looked like giant eyes, and the shape of the pepper made it look like it had a huge grin on its face. It still makes me laugh.
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| Blossom end rot strikes more than tomatoes, like this bell pepper. |
Let's start by understanding what's really happening, and what causes blossom end rot.
What Is Blossom End Rot?
Despite the name, blossom end rot isn't actually a rot at all.
It's a physiological disorder. In other words, it's the plant's response to stress rather than something that's spreading from one tomato to another.
The affected fruit doesn't receive enough calcium while it's developing. Without enough calcium, the cells at the blossom end of the tomato - the bottom - begin to break down. That's why the spot starts out looking water-soaked or bruised before eventually becoming dark, leathery, and sunken.
So yes, calcium is involved.
But - and this is the part that surprises most people - that doesn't necessarily mean your soil is low in calcium.
The Calcium Myth
For years gardeners have dropped eggshells into planting holes, crushed up calcium tablets, sprayed calcium products, and tried all sorts of home remedies to prevent blossom end rot.
I used to do it myself, adding calcium tablets to the soil when I planted my tomato plants, after those Romas I was growing developed blossom end rot. I was afraid it would spread to other tomatoes, and I was also afraid it was something in my soil.
The problem is, if your soil already contains enough calcium - as many garden soils do - adding more calcium usually doesn’t solve the real problem.
Think of it this way: imagine your pantry is full of food, but the door to your pantry is broken and you can’t open it.
The problem isn't that you don't have food - the problem is getting the food to where it's needed.
That's often what's happening inside a tomato plant. The calcium may already be in your soil, but the challenge is getting that calcium into the developing fruit at the time it needs it.
Why Blossom End Rot Happens
Calcium travels through the plant along with water. Anything that interrupts that steady flow of water can interrupt the movement of calcium as well.
That's why blossom end rot often appears after periods of drought followed by heavy watering or rain.
It's also why extreme heat, damaged roots, compacted soil, waterlogged soil, or over-fertilizing can all contribute to the problem. Here in Oklahoma, we often go from soaking rains to weeks of intense heat with very little in between. Those weather swings can be hard on tomato plants.
So why were my roma tomatoes suffering from blossom end rot while the rest of my tomatoes were not affected?
Because paste tomatoes like the Roma variety are developing so much dense flesh, they need a steady supply of calcium while those fruits are growing. That rapid fruit development increases their demand for calcium, so any interruption in water movement is more likely to show up as blossom end rot.
The encouraging news is that blossom end rot often shows up in the first flush of tomatoes - the first round of ripe fruit - then disappears as the season continues and the plants develop stronger root systems.
There’s no need to pull your plants out and give up. Just keep caring for them as usual and wait it out.
What Actually Helps
So what should you do?
First, keep your soil moisture as consistent as possible. Tomatoes don't like feast-or-famine watering.
If your plants dry out completely and then receive lots of water all at once, calcium movement inside the plant can be disrupted. Too much water is just as hard on the plants as not enough water.
Second, mulch around your tomato plants.
A layer of mulch helps the soil hold moisture more evenly and keeps the roots cooler during hot weather. A couple of inches of organic mulch, like straw or shredded leaves, helps shade the soil and slows moisture loss.
Third, avoid over-fertilizing. Too much nitrogen encourages rapid leafy growth, and the plant may struggle to move enough calcium into developing fruit.
Fourth, protect the root system. Avoid cultivating deeply around tomato plants once they're established. Damaged roots can't absorb water as efficiently.
Should You Throw Away the Plant?
Absolutely not!
Many gardeners panic when they see blossom end rot on their first tomatoes and assume the whole crop is ruined. But usually, it isn't.
Instead of pulling up the entire plant and giving up, pick off the affected fruit, continue caring for the plant, and keep watering consistently. Very often, the tomatoes that develop later in the season will be perfectly healthy.
That's one of the most encouraging things about blossom end rot: the plant frequently grows out of the problem.
Can You Eat Tomatoes with Blossom End Rot?
Yes, as long as the rest of the tomato is healthy. Simply cut away the damaged portion and use the remaining fruit.
If the tomato has begun to spoil beyond the black spot, then it's best to discard it.
And yes, you can add that discarded portion of the tomato - or an entire tomato that has been affected by blossom end rot, into your compost pile. There is no fungus, rot or disease that will affect your compost.
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Look Beyond the Obvious
One of the biggest lessons gardening has taught me is that the obvious answer isn't always the right one.
A tomato with a rotten-looking bottom makes us think disease. The word “rot” makes us think that the tomato is disgusting and the plant needs to be pulled up and discarded. And the fact that it’s caused by calcium leads us to believe that we need to add calcium to our soil or our plants.
But sometimes the real problem isn't what we can see - it's what's happening beneath the surface.
Instead of reaching for another product to spray on your plants, or trying some of those “myths” like adding calcium tablets and eggshells to the planting hole when you transplant, or spraying your plant with skim milk, it's worth slowing down and asking what's really happening in the garden.
Instead, pay attention to the weather. Think about how you've been watering. Look at what has changed recently. And remember that sometimes the plant doesn't need us to do more; it needs us to understand what's keeping it from using what it already has.
And the next time you find blossom end rot on one of your tomatoes, you'll know what you're looking at, why it's happening, and what you can do about it.
Tomato Resources
Click here for more tomato resources from Oak Hill Homestead.
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Kathi Rodgers is the gardener and writer behind Oak Hill Homestead (est. 2006) and the host of HOMEGROWN: Your Backyard Garden Podcast. With over 30 years of gardening experience in a variety of climates and soils, she helps new and aspiring gardeners grow healthy, organic food right in their own backyards.
A passionate advocate for simple, self-reliant living, Kathi is the author of multiple ebooks, a published magazine contributor, and shares practical advice with readers who want real-life solutions they can trust.
Kathi lives in Oklahoma, where she grows more cherry tomatoes than she can count and keeps a watchful eye on tornado season. A proud grandma and great-grandma, she believes that wisdom - like a bountiful garden harvest - should be shared.
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