Why Are My Tomatoes Turning Black on the Bottom? (2024)

What Is Blossom End Rot?

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 in the plant when the fruit is forming and will often resolve itself as additional fruit begins to set. Calcium is the nutrient necessary for forming the skin of the fruit. Deformity results when the plant is unable to deliver the necessary calcium at the crucial phase of growth.

The first sign is a light brown discoloration near the bottom of the fruit. These spots grow and darken until they cover up to half of the tomato and dark sunken craters form. It will look like the tomatoes are rotting on the vine.

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, most of your later season tomatoes will likely be fine.

Causes of Blossom End Rot in Tomatoes

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, the lack of calcium is a result of various environmental conditions that prevent the plant from taking up and transporting the necessary calcium.

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

• The plant grows so fast due to over-fertilization during early fruiting 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. The stress is most commonly due to improper watering, particularly in container gardening. If the soil gets too dry, the plant doesn't get the calcium it needs to produce healthy fruit. If the plant gets too much moisture, the same thing can happen.
  

3 Ways to Fix and Prevent Blossom End Rot

1. Remove Affected Tomatoes

   If you have tomatoes with blossom end rot on your plants, remove the damaged tomatoes as soon as you detect the problem. They will keep growing and use the plant's energy, which is better put to use producing new rot-free fruit.

2. Improve Your Watering Routine

   • Your best bet for treating blossom end rot is to adjust your watering to make sure you are using good watering practices.
   • Don't let your tomato plants dry out; keep the soil moist, but not wet.
   • Water your plants at ground level and avoid overhead watering as this can invite soil-borne disease that weakens the overall plant and affects both growth and yield.

3. Add Mulch

   Mulching tomatoes helps to conserve moisture in the soil. Use organic mulches suchas straw free of weed seed, grass clippings, peat moss, or wood chips.

   

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   Tip

   Some tomato varieties, particularly Roma types, are more susceptible to blossom end rot due to rapid growth and high yields.

How to Help Prevent Blossom End Rot

While adjusting feeding and watering 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. 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.
• Fertilize plants properly. Don't overfertilize with a high-nitrogen fertilizer that accelerates the development of leaves and diverts energy away from fruit development. But 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 mulch around plants. This can help retainmoisture 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, the effectiveness of this treatment is limited.
• Add Epsom salts at planting time. Dig a fairly deep hole and remove the seed leaves and lowest leaves of the plant. Tomatoes need to be planted deep with about a third of the plant buried in the soil. Mix 1-2 tablespoons of Epsom salts into the soil used to fill the planting hole. This allows the plant to establish a good root system with early uptake of nutrients for consistent growth.