What Is Blossom End Rot and How Do I Prevent It?
Blossom end rot is a dark, sunken patch on the bottom of tomatoes (and peppers and squash). It's not a disease. It's a calcium uptake problem caused by inconsistent watering, and the good news is that it's preventable.
Your soil almost certainly has enough calcium. The issue is that calcium moves through the plant in water, so when watering is irregular (dry one day, soaked the next), calcium can't reach developing fruit fast enough. UC IPM confirms that blossom end rot is a physiological disorder, not a pathogen, and that consistent soil moisture is the primary prevention method. Adding calcium sprays or eggshells to the soil rarely helps because the problem is water delivery, not calcium availability.
In Santa Cruz, blossom end rot often shows up in early June when gardeners are still figuring out their summer watering schedule. It's especially common in containers and shallow raised beds that dry out quickly, and in areas with sandy soil like parts of Pajaro Valley. Coastal fog can also confuse watering routines: the air feels damp, but the soil may still be drying out underneath.
Prevention comes down to three things. First, water on a consistent schedule (same amount, same time, every day or every other day during fruiting). Second, mulch your tomato beds with 3 inches of straw or bark to stabilize soil moisture. Third, avoid heavy pruning of leaves, which reduces the plant's ability to regulate water uptake. If you see blossom end rot on a fruit, remove that fruit so the plant redirects energy to healthy ones.
This week: Check the soil moisture in your tomato bed by pushing your finger 3 inches deep. If it's dry, you're likely underwatering for the current conditions. Set up a consistent watering schedule and mulch if you haven't already.
Our free Troubleshooting Guide includes a visual identification section for common tomato problems. The Tomato Growing MasterKit covers watering schedules, mulching, and troubleshooting by Santa Cruz microclimate.
Keep Reading
- How Do I Set Up Drip Irrigation for Raised Beds?
- Common Tomato Problems and How to Fix Them
- When Can I Transplant Tomato Seedlings Outside in Santa Cruz?
- How Often Should I Water My Vegetable Garden?
Frequently Asked Questions
Will adding calcium or crushed eggshells fix blossom end rot?
Usually not. The problem is rarely a lack of calcium in the soil but rather inconsistent watering that keeps the plant from moving calcium to the fruit, so calcium sprays and eggshells seldom help.
How can I tell if my tomatoes are getting enough water?
Check the soil about three inches deep, and if it feels dry, water more often. Coastal fog can make the surface look damp while the root zone stays dry, so always test below the surface.
Why does blossom end rot show up so often in early June here?
Early summer is when watering routines are still settling into a rhythm, and the inconsistency stresses fruiting plants. Containers, shallow raised beds, and sandy soils like those in the Pajaro Valley dry out quickly and are especially prone to it.
