Understanding Blossom End Rot on Tomatoes

Tomatoes turning black on the bottom aren't really rotten

Blossom end rot on beefsteak tomatoes (Solanum lycopersicum) caused by irregular watering
John Beedle/Getty Images

As tomatoes in your garden reach the half-grown stage, you may notice that fruits that seem fine otherwise are beginning to develop hardened brownish areas along the bottom. These gradually grow in size and turn leathery and black by the time the fruit ripens into redness. This symptom, which can also occur on related members of the nightshade family, such as peppers and eggplants, is usually a condition known as blossom end rot (BER).

Despite the name, blossom end rot is not a bacterial rot, nor is it a disease. It is a condition caused by a calcium deficiency experienced by the plant when the fruit is forming. Calcium is the nutrient necessary for forming the skin of the fruit, and deformity results when the plant is unable to deliver the necessary calcium at the crucial phase of growth.

Causes of Blossom End Rot

Although calcium deficiency is what causes blossom end rot, it is fairly rare for this to be caused by soil that is lacking in calcium. Instead, it results when various environmental conditions prevent the plant from taking up and transporting the necessary calcium. Although it is sometimes recommended that you add calcium to the soil, in most cases the soil already has plenty of calcium in it.

In precise terms, there are two reasons for blossom end rot:

  • The plant grows so fast that it is unable to take up sufficient amounts of calcium to keep up with the fruit development.
  • Stress factors render the plant unable to process the calcium the plant does take up from the soil.

These two causes are most commonly due to due to insufficient watering, particularly with container gardening. If the soil gets too dry, the plant doesn't get the calcium it needs to produce healthy fruit. At the same time, if the plant gets too much moisture, the same thing can happen. This often occurs when overeager gardeners water their tomatoes too frequently.

Blossom end rot can also be the result of over-fertilization during early fruiting. This, too, is often the result of an overeager gardener making a mistake through good intentions.

How to Prevent Blossom End Rot

While adjusting feeding and water rates to levels that encourage steady, moderate growth of the plant can somewhat reduce the occurrence of blossom end rot, it is difficult if not impossible to prevent it entirely, since garden conditions are so variable. And even though the tomatoes are turning black on their bottoms, blossom end rot does not spoil the entire fruit. It is fine to cut away the affected portion of a tomato and eat the rest.

If you're growing indeterminate tomatoes (those that set fruit all season) and you have a few tomatoes with blossom end rot, it doesn't mean that all your tomatoes will be affected. Even without treatment, some of your later season tomatoes may be fine. There are several ways to prevent blossom end rot from happening in the first place.

  • Plant tomatoes in the right soil. Soil that is well-drained and adequately amended with organic material, such as compost or peat moss, makes it easier for the roots to take up the crucial calcium as the fruit is just developing.
  • Test soil pH. Most vegetables do well in soil with pH levels of 6.2 to 6.8, but vegetables prone to blossom end rot need a pH level of at least 6.5, which frees up more calcium in the soil. If a soil test indicates a pH level that is too low (too acidic), take measures to raise the pH level. Working fast-acting agricultural lime into the top 12 inches of soil is a good way to do this.
  • Plant at the right time. Gardeners that are too eager often put their tomatoes in the ground when the soil is still too cold for the roots to adequately develop. This again deprives the plant of calcium at the critical growth period. If you are growing seedlings, harden them off slowly with gradually increasing periods of outdoor exposure before subjecting them to soil conditions.
  • Don't over-fertilize, especially with a high-nitrogen fertilizer. This accelerates the development of leaves and diverts energy away from fruit development.
  • Don't under-fertilize. Tomatoes are heavy feeders, and potting soil, unless it is pre-fertilized, doesn't provide all the nutrients tomatoes need.
  • Use good watering practices. Don't let your tomato plants dry out; keep the soil moist, not wet.
  • Lay mulch on the soil around tomatoes. This can help retain moisture in the soil, allowing for a steadier growth rate that makes blossom end rot less likely to occur.
  • Use a calcium spray solution. Commercial calcium spray products applied directly to the plants two or three times a week when the blossoms first appear may supply tomatoes with the calcium they need. However, this effectiveness of this treatment is limited.

Adding Calcium to the Soil

Although calcium deficiency in soil is rare, it does sometimes occur. Adding crushed eggshells, limestone, or calcium carbonate to the soil is worth trying if blossom end rot persists after other methods have failed. Additional calcium won't hurt your plants and in some cases may reduce the occurrence of blossom end rot.

Blossom End Rot Problems in an Earth Box

Blossom end rot sometimes occurs even in tomatoes grown in the popular Earthboxes, which have very consistent supplies of water and just the right amount of fertilizer and dolomite. Normally, this should provide all the calcium the plants need, but in an Earthbox, the blossom end rot is probably caused by unusually rapid growth—the plant can't take in enough calcium quickly enough. If you experience blossom end rot in an Earthbox, mix 1/4 cup of lime with one gallon of water and pour it into the reservoir. Do this only once—it should fix the problem.