Blossom End Rot

Why do my tomatoes have a black spot on the bottom?

Why do my tomatoes have a black spot on the bottom?

After you put effort into planting and caring for a tomato plant, it can be disappointing when you don’t end up with beautiful, healthy tomatoes.

One of the most common problems with tomatoes is Blossom End Rot, which shows up as a black flattened area on the bottom of the tomato. There are many ways to prevent Blossom End Rot and, if you’re already noticing signs of this problem, it doesn’t mean that your whole crop is ruined

What causes Blossom End Rot?

Although you may have heard that Blossom End Rot is caused by a calcium deficiency, the problem is often with watering. Most soils in MN contain plenty of calcium, but it can be leached out in container grown tomatoes.

Tomatoes need to stay consistently moist, receiving about 1” of water per week. If you purchase large tomato plants at the garden center, the first tomatoes may have Blossom End Rot. This is because the size of the plants is controlled in the greenhouse by reducing the amount of water supplied to the plant. If this happens, just remove the first few tomatoes from the plant and, with consistent watering, any new tomatoes will not be affected by Blossom End Rot.


  • Mulch the soil around tomato plants in pots and in the garden to reduce soil fluctuations.
  • During dry periods, consider setting up a soaker hose to keep soil evenly moist.
  • Use fresh soil for container grown tomatoes every year.
  • Avoid using clay pots which dry out faster than plastic pots.
  • Fertilize plants with a slow-release fertilizer formulated for tomatoes (Tomato Maker, Jobes Tomato Fertilizer Spikes) when they start producing fruit.
  • If you already have Blossom End Rot Blossom End Rot isn’t passed on to later tomatoes. If the conditions of plant improve, later tomatoes will grow without problems.
  • Remove any tomatoes with signs of Blossom End Rot.
  • Use a slow-release tomato fertilizer or apply a product like Rot-Stop to the plant.
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