Blossom End Rot on Tomatoes: What Causes It and How to Stop It

You pull back a tomato leaf and find it: the bottom of the fruit is dark, sunken, leathery, like it rotted from the inside out before you even had a chance to eat it. That's blossom end rot, and it's one of the most disheartening sights in a summer vegetable garden.

The good news is that it's preventable. The frustrating part is that most of the popular "fixes" (eggshells, calcium sprays, milk) don't actually work. Once you understand what's really causing it, the real solution becomes obvious.

What Is Blossom End Rot?

Blossom end rot is a physiological disorder, not a disease. No fungus, bacterium, or pest is responsible. The affected area starts as a small, water-soaked patch at the bottom of the fruit (the end that had the flower) and expands into a dark brown or black, flattened, leathery spot. It can range from a small blemish to a patch covering the bottom third of the fruit.

The rest of the plant usually looks completely healthy. Other fruits on the same plant may look fine. The first flush of fruits in early summer is typically the worst affected. Subsequent tomatoes often develop normally, which gives you a clue about what's driving the problem.

Blossom end rot shows up most often on paste tomatoes, Roma types, and large slicers. Cherry tomatoes rarely get it, likely because their smaller fruit size places less demand on the plant's calcium supply at any one moment.

What Actually Causes Blossom End Rot

Here's the part most gardening advice gets wrong: blossom end rot is almost never caused by a lack of calcium in your soil. It's caused by a failure to deliver calcium to the fruit.

The calcium-water connection

Calcium moves through a plant almost exclusively through the xylem, the water-transport vessels that pull moisture and dissolved minerals up from the roots. When water flow is disrupted, calcium flow stops with it. The fruit doesn't get the calcium it needs, cell membranes break down, and the tissue at the blossom end dies. That collapse is what you're seeing as the dark, sunken patch.

This means anything that interrupts consistent water uptake triggers blossom end rot. Soil calcium levels are almost beside the point if the plant can't move water steadily from roots to fruit. According to the Royal Horticultural Society, the disruption to transpiration, not calcium deficiency in the soil, is the primary driver.

Why rapidly developing fruit is most vulnerable

Expanding fruit cells need calcium to build and maintain their membranes. When a tomato is sizing up fast, the demand for calcium at the fruit tip outpaces what's available if water flow is erratic. The plant doesn't have a reserve of calcium sitting in the fruit ready to deploy. It has to keep delivering it continuously through water transport.

Why the first fruits of the season are often the worst

Early in the season, root systems are still establishing and may not reach deep, stable moisture. Meanwhile, the first fruits are setting and sizing up under unpredictable spring weather. Shallow roots plus inconsistent moisture plus rapid fruit development equals peak BER conditions. Once roots deepen and watering stabilizes, later fruits often escape the problem entirely.

Why Consistent Watering Is the Real Fix

If BER is a water-transport problem, the fix is maintaining steady, consistent water flow through the plant from roots to fruit. Not more water overall. Consistent water.

How to water tomatoes properly

Deep, infrequent watering beats shallow, frequent watering every time for tomatoes. The goal is to wet the soil to a depth of 8 to 12 inches, which encourages roots to grow down into stable moisture rather than staying near the surface where they're subject to rapid drying. Mature tomato plants need roughly 2 to 3 gallons of water per week when rainfall isn't supplying it, according to University of Maryland Extension.

In practice: water deeply once or twice a week rather than a little every day. Let the soil dry slightly between waterings, but never let it get bone dry. Check soil moisture at the 3 to 4 inch depth before watering. If it's still moist, wait. If it's dry, water now.

Why mulch may be the single best thing you can do

Mulch keeps moisture in the soil longer, moderates soil temperature, and dramatically reduces the wet-dry swings that cause BER. A 3 to 4 inch layer of straw, shredded leaves, or wood chips around your tomato plants does more to prevent blossom end rot than any spray or amendment. If you're not mulching your tomatoes, start there.

The PNW Coastal Angle: Are You Actually at Risk?

If you garden along the Oregon or Washington coast, you might assume BER isn't your problem. Cooler temperatures, fog, and higher ambient humidity do make the violent wet-dry soil swings that trigger BER less common in coastal PNW gardens than in the Willamette Valley interior or California's Central Valley. In-ground tomatoes with consistent irrigation or reliable rain often sail through summer without a single affected fruit.

But that doesn't mean coastal PNW gardeners are off the hook.

In-ground vs. raised bed vs. container risk

In-ground tomatoes have access to a large reservoir of soil moisture and a deep, buffered root zone. Raised beds hold significantly less soil volume, drain faster, and heat up more quickly on warm days, all of which accelerate drying. Containers are the highest-risk scenario: limited soil, no connection to ground moisture, and rapid drainage mean the soil can go from saturated to dangerously dry within a day on a warm afternoon.

The container tomato problem

Many coastal PNW gardeners grow tomatoes in containers to maximize heat absorption. That's a reasonable strategy, but limited soil volume and fast drainage create exactly the conditions that cause blossom end rot.

If you're growing tomatoes in containers, consistent watering is non-negotiable. Self-watering containers with a built-in reservoir take much of the guesswork out of this. The plant can draw moisture steadily rather than relying on you to hit the schedule perfectly. Drip irrigation on a timer is another strong option for raised beds.

There's also a counterintuitive coastal risk: on foggy or overcast days, plants transpire less. When transpiration slows, less calcium reaches developing fruit even if soil is moist. Extended cloudy stretches rarely cause major BER in in-ground gardens, but container plants with inconsistent watering may tip over during a gray week.

Other Causes That Make BER Worse

Inconsistent watering is the primary cause, but several other factors can compound the problem:

• High-nitrogen fertilizers: Excess nitrogen pushes rapid vegetative growth. The plant prioritizes leaves over fruit, increasing competition for calcium at the root. Ammonium-based fertilizers are particularly problematic because ammonium ions compete directly with calcium for root uptake.
• Fertilizing dry soil: Applying fertilizer when soil is dry concentrates salts around the roots, which restricts water uptake. Always water before and after fertilizing.
• Low soil pH: Calcium availability drops below pH 6.0. Tomatoes prefer 6.3 to 6.8. Deficiency is rare in PNW soils, but if symptoms persist after fixing your watering, a basic soil test is worth doing.
• Root damage: Hoeing or cultivating too close to the plant base can shear off feeder roots and disrupt water uptake. Work shallowly within 12 inches of the stem.
• High humidity: As noted above, high ambient humidity reduces transpiration and slows calcium delivery to fruit. This is most relevant during extended cloudy or foggy periods, another reason container growers need to stay on top of consistent irrigation.

It's Not Just Tomatoes: Peppers, Squash, and More

If you're seeing that dark, sunken bottom on more than one type of vegetable, you're not dealing with a crop-specific disease. Blossom end rot affects peppers, eggplant, summer squash, zucchini, pumpkin, and watermelon by the same calcium-transport-via-water mechanism.

Bell peppers are particularly susceptible and show BER symptoms on the sides and bottom of the fruit. Zucchini often shows it at the tip. The same prevention applies across all these crops: consistent deep watering, mulch, and avoiding ammonium-heavy fertilizers.

If you're seeing BER across multiple beds or crop types simultaneously, the common thread is almost certainly watering consistency, not a soil-wide calcium shortage.

Common Treatments That Don't Actually Work (and Why)

The popular treatments for blossom end rot are largely ineffective. Understanding why saves you time and money.

Calcium sprays and foliar applications

Calcium chloride sprays are widely sold as a BER treatment. The logic seems sound: the fruit needs calcium, so spray calcium on it. But calcium doesn't move through plants that way. Calcium travels almost entirely through the xylem, pulled upward by transpiration. It does not move readily from leaves to fruit, or from the fruit surface inward. The Royal Horticultural Society explicitly states that foliar calcium sprays are ineffective for blossom end rot because of this transport limitation.

Any apparent improvement after applying calcium sprays is almost always due to the gardener also paying more attention to watering. The behavior change, not the spray.

Eggshells

Eggshells are calcium carbonate, and they do break down into plant-available calcium. Eventually. That means years, not weeks. They won't help tomatoes fruiting this season. They're a reasonable long-term soil amendment for acidic soils, but not a treatment for BER.

Milk spray

Milk spray shows up repeatedly in gardening forums as a BER remedy. Milk does contain calcium, but the same transport problem applies. Plants cannot absorb calcium through leaf tissue and distribute it to developing fruit. There is no credible evidence from extension horticulture programs that milk spray affects BER outcomes.

Removing the plant

This one costs gardeners an entire season unnecessarily. BER typically affects the first fruits under the specific conditions present early in the season. Once you correct your watering practices and the plant's roots deepen, later fruits almost always develop normally. Remove the affected fruits. Keep the plant.

What to Do Right Now If You're Seeing It

If you just found blossom end rot on your tomatoes, here's what to do in order of priority.

Can you still eat affected tomatoes?

Yes. The dark, leathery portion at the bottom is not toxic, just unpleasant in texture. Cut off the affected end and the rest of the fruit is edible. Don't let BER fruits go to waste if the top half looks fine.

Step-by-step recovery checklist

1. Remove affected fruits now. They won't recover, and leaving them on the plant wastes the plant's energy and calcium resources. Pull any fruit showing BER symptoms.
2. Mulch immediately if you haven't already. Add 3 to 4 inches of straw, wood chips, or shredded leaves around each plant. This is the single highest-impact step you can take today.
3. Adjust your watering to deep and consistent. Water thoroughly to 8 to 12 inch depth, then wait until the top 3 to 4 inches dry out before watering again. Stop watering lightly every day.
4. Check moisture before watering. Insert a finger 3 to 4 inches down. Water when the soil is dry at that depth, not on a fixed schedule.
5. Hold off on fertilizer. Wait until watering is stable and symptoms stop. Fertilizer on stressed plants with disrupted water uptake makes BER worse.
6. Avoid root disturbance. No hoeing near the stem base while the plant recovers.
7. Consider a soil test only if symptoms persist. Most PNW soils have adequate calcium. If watering fixes don't stop the problem, test pH. If it's below 6.0, ground limestone is a reasonable next step.

For container tomatoes, consider switching to a self-watering container or setting up a drip line on a timer for the rest of the season. Manual watering is simply too unreliable for containers in warm weather. A day or two missed is all it takes to trigger BER in a pot.

Blossom end rot feels like a crisis when you find it, but the plants usually recover well once watering is steady. Most gardeners who address it mid-season end up with a full second and third flush of healthy fruit. The fix isn't complicated or expensive. It's consistent moisture, and mulch to hold it there.

If you're also seeing yellowing or curling leaves on your tomatoes, that's a separate problem worth investigating. Tomato leaf curl and early blight are both common in PNW gardens and have their own causes that don't overlap with BER.