Due to a shortage of calcium, the cell walls are not so strong. If the tomato plant has to transpire (evaporate) a lot of water, it will temporarily remove the moisture from the fruit. When the plant’sroots can absorb sufficient water from the soil, it stores this moisture in the fruit. This is normal in tomato plants. But cell walls of plants with calcium deficiency break apart when moisture is transpiredfrom the fruit. Then you get blossom end rot.

It matters a lot, which tomato variety you are growing. The larger and pointier the fruit, the more sensitive the tomato is to blossom end rot. For example, most cherry tomato varieties are hardly susceptible. Beef tomatoes, San Marzano types and plum tomatoes are more susceptible, but of course these varieties are just too good not to grow.

- Make sure there is sufficient calcium in the soil. Fertilize with extra calcium every spring.

 - Provide sufficient space between the plants. Do not plant the tomato plants too close together and prune the plants neatly.

 - Remove the old leaves at the bottom of the plants.

 - Make sure that the greenhouse air vents are always slightly open (except when it’s cold). Plants only absorb calcium when they are actively transpiring.

 - Provide a regular water supply.

Calcium is a nutrient that plants only absorb when there is a lot of transpiration. To ensure that the plants have sufficient calcium, they must breathe or transpire (evaporate moisture). Air between the leaves of the tomato plants is very important so do not plant the tomatoes too close together. Remove the side shoots and old leaves and ensure that you always keep the air vents open in a greenhouse.

Do not remove young fruit with blossom end rot, but leave it until it starts to color. If you remove all the rotten fruit, the plant will grow taller, because it has only a few tomatoes that require energy. Because of this extra growth, the calcium deficiency increases even more and the new fruit will also be calcium deficient and you will have an increased risk of blossom end rot.

Every year a number of my tomato varieties will produce some fruit with blossom end rot. For some of them, it’s just part and parcel of that variety. You can cut away the brown part of the tomato, and simply eat the rest. Usually fruit with blossom endrot are extra sweet.It is still good for something.