Can overwatering tomatoes cause blossom end rot ?

(Question)

I was wondering if overwatering tomatoes can cause blossom end rot? If it does cause BER , why?

(Answer)

Thank-you for contacting Toronto Master Gardeners with your question.

The direct cause of blossom end rot is a calcium deficiency, not over-watering. Generally, in fact, it occurs because of a lack of sufficient water rather than over-watering. I am referencing a previous TMG answer on this topic below:

Blossom end rot is due to adverse growing conditions and often occurs where fruit develops rapidly when it’s hot and dry – and we’ve had lots of that type of weather this summer .

The ultimate cause  is  calcium deficiency, which – in the fruit itself – disrupts the permeability of the cell membrane, resulting in cell swelling, leakage and ultimately breakdown of membrane. As well, fewer new cells grow. This results in black, sunken areas of the fruit, which is vulnerable to attack by bacteria, fungi and pests.

Even if calcium is present in the soil, the plant cannot take up the nutrient if the soil is too dry, so fluctuations in watering are usually the preventable cause….

Ensure the soil/potting mix is not allowed to dry out between watering  (thus causing the plant’s roots to  dry out inhibiting calcium uptake). Conserve moisture around plant roots by mulching. Apply any fertilizer to moist soil – not to dry soil, as the concentrated nutrients can restrict the plant’s water uptake…and not too much fertilizer .

Finally, please remove the affected tomatoes immediately, to give other fruits the opportunity to develop normally.

Other resources you might find interesting :

Missouri Botanical garden. Tomato diseases and disorders.

Toronto Master Gardeners. Calcium for tomato blossom end rot.

I hope this helps you in overcoming this common problem in growing tomatoes.