It's the scourge of tomato lovers and gardeners—blossom end rot. After waiting what seems like forever for our tomatoes to ripen, it's disheartening to see those brown splotches on the bottoms of the fruit. While blossom end rot can't be cured, it can be prevented or minimized, and it's fairly simple to fix the causes. 

Blossom end rot isn't limited to tomatoes. It can also appear on peppers, eggplants, and even squashes like zucchini, but it is less common. The first batch of tomatoes or other fruits to appear in summer often suffers the worst blossom end rot. Tomatoes with blossom end rot aren't very visually appealing, and they’re often internally damaged as well, so instead of eating them it’s best to throw them away. 

What causes blossom end rot?

Blossom end rot, or BER, is not caused by an insect, fungus, or disease. It's caused by a calcium deficiency in the tomato plant. But don’t go pouring fertilizer or sprinkling eggshells right away.

BER presents as brown, hard, leathery spots on the blossom side of the tomato, opposite the stem. You'll notice it as a small, fingernail-sized spot that grows to cover much of the fruit. On peppers it may form on the side of the fruit. The spot can also get covered with a black mold. 

Blossom end rot is caused by a localized deficiency of calcium in the fruit at the end opposite the stem, called the distal end. The tissue doesn’t have enough calcium and collapses. But that doesn't mean there isn't a sufficient amount of the nutrient in your soil. 

The calcium deficiency is most often caused by a lack of water. Without moisture or with uneven moisture, the tomato plant struggles to take up calcium and move it around the plant. It's the most common reason for blossom end rot, especially for the year's first tomatoes. Even if the plant roots, stems, and foliage have grown with enough calcium, dry conditions at or immediately prior to fruiting time can limit the plant's ability to move the mineral to the fruit.

Overfertilization can also cause calcium deficiencies, leading to blossom end rot. Excess nitrogen and potassium in the soil caused by high fertilization rates can limit the uptake of calcium. In addition, because calcium is more easily transported to the leaves of tomatoes than the fruits, and nitrogen causes high vegetative growth, excess fertilization can also inhibit calcium from reaching the fruits. 

How to Prevent Blossom End Rot on Tomatoes

While blossom end rot isn’t curable, it can be minimized with proper care of your plants. Young plants making their first crop of fruit are the most likely to be affected by BER. Even if your initial tomatoes had blossom end rot, don’t give up hope. As the tomato or pepper plant continues to mature and focuses less on growing new foliage, more calcium is taken up by the now expanded root system and available to the fruits. Following the tips below will help.

Ensure even watering and soil moisture (use mulch)

The most likely culprit and easiest fix for blossom end rot in tomatoes is water, or rather uneven soil moisture. Start by mulching your soil under the tomatoes, which will serve the triple benefit of gentling the sometimes harsh boom-and-bust summer soil moisture cycle, reducing weed problems, and preventing soil-borne pathogens from splashing up onto lower leaves. Even tomatoes grown in pots can benefit from a little shredded straw or grass clippings as mulch.

Every day or two, stick your finger down deep in the soil and feel it. If the soil is cool and damp, that's perfect. If it's starting to feel dry, dusty, and hot, it's time to water. Start off with 1–1.5 inches per week of water (a rain gauge is helpful to measure with), or if you use drip irrigation, run the system until the water has penetrated at least six inches down. Watch the weather and postpone watering if a storm is coming or you've received a nice rain. When tomatoes are fruiting, a sudden excess of water can cause them to split. 

Adjust soil pH

The soil’s pH affects plants' ability to absorb nutrients. For tomatoes, a pH of about 6.5 is perfect. Don't stress if it isn't exactly that, but if your soil test reveals a large difference, add lime or sulfur in the off-season to adjust for next year. In highly acidic soils, calcium is less available to the plant. Bagged soils made for containers are usually already neutral or close to neutral in pH. 

Be careful when weeding

Deep cultivation near tomato plants (or any other plants, for that matter) can damage the roots. Damaged root systems don't take up as much water—needed for calcium mobility in the plant—or as much calcium. If using a hand cultivator, don't allow the tines or forks to go more than an inch deep. The same goes for using a hoe or trowel. 

Pulling weeds by hand and using mulch are better options near tomato plants, especially as they mature. Don’t allow a powered cultivator or tiller anywhere near your tomatoes or peppers.

Fertilize if warranted

If pH is in the sweet spot and your soil moisture program is spot on, send a soil sample to the lab. Your county extension office can direct you on where to send a soil sample and how to take one. There is usually a small fee to cover the expenses, but it's well worth the time and small expense to avoid creating problems by randomly applying expensive fertilizer.If your soil test indicates a deficiency, it usually also recommends a dosage to remedy the problem.

Remove affected fruit

Tomatoes affected by blossom end rot won't recover, and they can continue to rot, rendering the entire fruit unusable. If you notice BER in your tomatoes, go ahead and remove those fruits. Let the energy in your plants go toward creating good crops you can use. 

Experiment with different varieties

Some tomato varieties are more susceptible to BER than others. If you've fixed other potential problems, like moisture and pH, consider growing a tomato variety resistant to blossom end rot.   

Unhelpful Actions

Foliar Quick-Fix sprays

Many commercial products are advertised and sold as a cure or prevention for blossom end rot. Most involve spraying a water and calcium solution on the leaves, and aren't very effective or are, at best, inconsistent. 

Calcium absorption by the fruits via spraying is almost nonexistent, and calcium absorbed by the leaves won’t transfer to the fruits; it stays in the leaves. More often, the problem goes away with adequate watering, not with foliar sprays. 

Excess fertilizer problems

Overfertilization can cause problems in the garden, just like a lack of nutrients. As mentioned above, an excess of one mineral can inhibit the uptake of another. Heavy doses of fertilizer containing ammonium nitrate (a common nitrogen source) creates competition with calcium ions for absorption by the roots. The ammonium ions win, providing lush vegetation with low calcium levels and likely blossom end rot issues down the road.

While it can seem like quickly getting some nutrients out to the plant will solve blossom end rot, it's rarely the case that our garden soils are deficient in any of the lesser-used (but still important) nutrients like calcium. A soil test can confirm any fertilizer needs accurately, reducing potential issues.

Home remedies for Blossom End Rot

Social media experts may tout sprinkling eggshells or antacid tablets (calcium carbonate) under tomato plants to boost calcium. However the calcium in eggshells is unavailable to plants until broken down by the soil microbial organisms, which can take months to years. 

The amount of calcium in an antacid tablet is insignificant to raise the calcium in the soil under a tomato, and wouldn’t help unless the soil was actually deficient in calcium, which is unlikely. Another popular idea is to dissolve the tablets in water and spray on the plants. As discussed above, spraying the foliage and fruit is ineffective in solving the problem of localized calcium deficiency in the tomato itself. 

Epsom salts are often touted as a cure-all for tomatoes—another garden myth. Excess magnesium is a cation that can inhibit calcium uptake in tomatoes, and Epsom salts are about 10% magnesium. While magnesium is required in small amounts by plants, it typically isn’t deficient in garden soil. Applying Epsom salts in large quantities can cause blossom end rot, not cure it.