How to Prevent Blossom End Rot on Tomato Plants

It’s disheartening to nurture a tomato from tiny yellow flower to full-sized fruit only to have it develop a black, sunken spot just before harvest time. Believe me, I know. While I’ve been growing tomatoes for nearly 20 years, it still happens to me. That’s because blossom end rot is a common problem that can plague any tomato grower no matter their level of experience. If blossom end rot happens to you, don’t fret. Instead, learn what causes the problem, adjust your gardening practices and keep growing.

What is Blossom End Rot?

Blossom end rot is a disorder caused by physical conditions; it’s not a disease caused by bacteria, pests or pathogens. The actual problem is a calcium deficiency, but that nutrient deficiency is caused by fluctuations in the tomato plant’s water supply, either too much water or too little. As fruit develops, the lack of calcium leads cells to break down on the far end of the tomato fruit, creating the dark spot. End rot can happen at any point during the tomato growing season.

Symptoms of Blossom End Rot

Blossom end rot looks like a brown, sunken, leathery spot on the end of the fruit not attached to the plant stem, which is called the “blossom end.” It may start as a small spot or may develop quickly, such as after a hard rain, into a large dark spot. Blossom end rot on a tomato fruit won’t go away once it’s started.

If you see blossom end rot, it’s best to go ahead and remove the tomato from the plant. You can cut that part off and eat the remaining tomato, if you like, or add it to your compost; there’s no need to worry about spreading end rot to other plants since it’s caused by physical conditions and not a communicable disease.

Causes of Blossom End Rot

It may seem counterintuitive that both too much water and too little water can cause blossom end rot. But both situations will disturb the regular intake of calcium into a tomato plant, so you need to watch out for these extremes. In general, extremes in physical conditions aren’t great for plants, and while you can’t control the weather, you can create conditions in your garden to help balance it out.

Blossom end rot is more common—and most frustrating—on tomatoes, but it can also occur on other vegetables, including peppers, eggplant, and all the cucurbits including cucumbers, squash, pumpkins and melons. The cause and solutions for end rot on these plants are the same as for tomatoes.

Preventing Blossom End Rot

So how do you create conditions in your garden that will stave off blossom end rot? As with most of gardening, it starts with the soil. Follow this advice to help prevent end rot.

• Ideal soil is loose and fluffy, allowing the flow of water and nutrients, and rich with organic matter. Incorporate compost into soil before planting to help create good conditions for healthy growth.
• The ideal pH for most vegetables, including tomatoes, is between 6.0 and 6.5, which is slightly acidic. (Neutral pH is 7.0.) But highly acidic soil, below 5.5 pH, can contribute to blossom end rot. Conduct a soil test to know your soil’s pH, and if it’s on the acidic side, incorporate lime into soil. This should be done a few months ahead of planting time, so if you have the problem with end rot this season, make a note to amend soil before next year. Remember that building up a productive garden is a multi-year process.
• It's always a good practice to mulch the soil around your vegetable plants. Mulching helps keep soil moisture levels consistent by slowing both absorption and evaporation. But again, it’s about consistency and balance, so be sure not to mulch so thickly or tightly that water can’t evaporate at all. A few inches of a loose mulch like straw, shredded leaves or shredded newspaper is sufficient.
• Overfertilizing can also induce blossom end rot. More is not always better. Follow fertilizer application instructions, including amount and timing, on whatever fertilizer you’re using. Better yet, if you’ve done a soil test, follow the fertilizing instructions provided in your test results.
• While heirloom tomato varieties are beautiful and full of flavor, some may be more susceptible to blossom end rot than hybrids developed for performance. Keep track of your tomatoes season to season to see which are more likely to get end rot. I like to grow a mix of both heirlooms and hybrids in my garden, reaping the benefits of each.
• Avoid disturbing tomato plant roots while the plant is growing and especially while it’s fruiting. Damaging roots prevents them from taking in enough calcium from soil.

Controlling Blossom End Rot

As mentioned earlier, if you see blossom end rot on a tomato or other fruit, pick it off the plant and either trim off the rotted end or toss the fruit in your compost bin. Then start trying to alter conditions to stop it from happening to the rest of the fruit on your plant.

Blossom end rot will not technically spread across a plant, or from plant to plant, like some diseases, but if your plant has produced some fruit with end rot, it could produce others. Focus on keeping soil moisture levels consistent using the prevention methods above: mulch, regular watering, not letting soil dry out and not overfertilizing.

You can also try applying a calcium solution to the soil around the roots of your plants. According to Alabama Cooperative Extension (which is where I studied for my Master Gardener Certification), use a solution of 4 tablespoons of calcium nitrate or calcium chloride per gallon of water. This end rot treatment should be applied at the soil level; while you may hear about spraying the plant stems or fruit directly with calcium, studies have shown this to be ineffective.

Frequently Asked Questions

How do I fix blossom end rot on my tomatoes?

Once blossom end rot has occurred on a tomato fruit, it can’t be fixed. You can remove the fruit and either cut off the affected end and eat the rest or compost the fruit. But blossom end rot isn’t like a fungus or pathogen that will spread. It’s a disorder caused by physical conditions that can be prevented through making sure the plants are getting consistent moisture and enough calcium. And just because you have a few tomato fruits with blossom end rot on a plant doesn’t mean all the fruit from the plant will be that way. Adjust your gardening practices, like adding mulch around plants or paring back the fertilizer, or try adding calcium to the soil, using the notes and advice given above.

Can overwatering cause blossom end rot on tomatoes?

Yes, overwatering is a major factor in blossom end rot, but it’s not the primary cause. Blossom end rot is a calcium deficiency caused by inconsistent moisture levels in soil. Like drinking vitamin water or an electrolyte drink, plants take up nutrients from soil as they take in water. If there is too much water, the ratio of nutrients goes down and plants don’t get enough calcium, but drought can also cause the same problem. Consistency of water is key. Tomatoes need 1 to 1.5 inches of water per week. Using soaker hoses on a timer will help you get water directly to roots on a consistent schedule, but you should also be willing to adjust that schedule with the weather, especially if a lot of rain is in the forecast.

What are two causes of blossom end rot in tomatoes?

Blossom end rot is caused by a calcium deficiency, and that deficiency is usually caused by too much or too little water in the soil around roots of the tomato plant. End rot can also be caused by soil that is too acidic (pH below 5.5) and by overfertilizing. Conduct a soil test to know your soil pH and to get recommendations for how much fertilizer to use. Otherwise, follow timing and dosing instructions of fertilizer on the fertilizer label.