What Is Blossom End Rot and How Do I Prevent It?
A tomato can look promising from above, then reveal a dark, sunken patch when you turn it over. That leathery spot at the blossom end may be blossom-end rot: too little calcium reaching the developing fruit. It is a physiological disorder, not a contagious disease, and the first place to look is the root zone. Peppers and squash can develop it too.
UC IPM's blossom-end rot guide links the problem to calcium in the fruit and the plant's water balance. Dry soil, saturated roots and high soil salts can interfere with that balance. The symptom alone doesn't tell you whether the soil needs more calcium, so adding a supplement before checking the growing conditions can miss the cause.
Steady moisture, adjusted to your garden
In a Santa Cruz garden, a cool, foggy morning isn't a reliable watering gauge. Check the soil beneath the surface. A container or shallow bed may dry faster than a deep garden bed, and sandy soil holds less water than finer-textured soil. When the weather warms or the plants get larger, check again rather than keeping the same timer setting all summer.
The aim is an evenly moist root zone that can drain, not a fixed daily watering appointment. Feel the soil several inches down near the plant without cutting through its roots. After watering, check that moisture has reached beyond a small wet patch beside the emitter. If the soil stays soggy, investigate drainage before adding more water. Our guide to setting up drip irrigation for raised beds can help you work through the delivery side.
A loose layer of straw or other suitable organic mulch helps slow evaporation. Keep it away from the tomato stem and check the soil beneath it. Mulch supports an irrigation routine; it doesn't tell you when the plant needs water.
Keep roots healthy and feeding moderate
Avoid digging or hoeing close to established tomato roots. Be measured with fertilizer, too. University of Minnesota Extension's tomato disorder guide identifies root injury, excess nitrogen and moisture fluctuations as contributors to blossom-end rot. Use a soil test to guide amendments instead of assuming every damaged fruit calls for another dose.
Leave enough healthy foliage to shade the fruit. Heavy leaf removal can expose tomatoes to sunscald, a different problem that produces pale damaged areas on sun-exposed surfaces. Pruning isn't a cure for blossom-end rot.
Remove affected fruit; its damaged tissue won't recover. Later fruit on the same plant can develop normally, so a few spoiled tomatoes needn't mean losing the whole harvest. If the problem keeps recurring, review moisture, drainage, fertilizer use and soil-test results together. Varieties differ in susceptibility, which is worth noting when choosing next season's plants.
Frequently Asked Questions
Will calcium sprays or crushed eggshells fix it?
They won't repair damaged fruit, and they shouldn't replace a moisture and soil check. Crushed eggshells are not a quick source of available calcium. If testing identifies a calcium or pH problem, choose an amendment suited to that result. Foliar calcium is not a dependable substitute for correcting the conditions around the roots.
How can I tell whether I'm watering enough?
Look below the surface and check more than one spot. Soil that is drying through the active root zone needs a different response from soil that remains saturated. Adjust watering to the bed, weather and plant size, then check the result. A timer can deliver water regularly while still giving the wrong amount.
Does blossom-end rot happen at a particular time in Santa Cruz?
It often affects the first developing fruit, but there isn't a reliable local calendar date for it. Planting time, variety and growing conditions change when that first crop develops. If you're planning next season, our guide to transplanting tomato seedlings outside in Santa Cruz covers the earlier decision about when to plant.
This week: Check moisture around your tomato roots before and after an irrigation cycle. Look for a blocked emitter, a dry edge of the bed or soil that isn't draining, and correct what you find before buying a treatment.
For help distinguishing this from other garden problems, UC IPM provides plant-health and pest-management resources.
For more troubleshooting resources, visit the free Garden Toolkit. The Tomato Growing MasterKit offers a dedicated tomato-growing resource.
