You go out to check on your tomatoes, feeling pretty good about the season, and there it is: a sunken, leathery, brownish-black splotch on the bottom of an otherwise perfect fruit. Your first thought is probably “blight” or “some horrible fungus is going to wipe out the whole plant.” Take a breath. It’s blossom end rot, and the good news is it’s not contagious, it’s not a disease, and it’s entirely something you can fix.
What’s Actually Happening
Blossom end rot shows up when the plant can’t get enough calcium to the fruit while it’s developing. That’s it — it’s a calcium delivery problem, not usually a calcium shortage in the soil itself. Most garden soils have plenty of calcium sitting around. The issue is that calcium moves through the plant with water, and it travels slowest to whatever part is furthest from the roots — which, conveniently, is the bottom of a growing tomato.
So anything that messes with steady water uptake messes with calcium delivery. Inconsistent watering is the number one culprit, especially the classic July pattern of bone-dry soil for a week followed by a torrential downpour or a guilt-driven soaking. The plant can’t regulate that kind of whiplash, and the fruit pays the price.
Other Things That Make It Worse
- Too much nitrogen fertilizer. Nitrogen pushes leafy growth, and a plant busy growing foliage prioritizes water and calcium there instead of sending it to the fruit.
- Root damage. Aggressive hoeing or tilling too close to the plant chops up the roots responsible for calcium uptake.
- Fast-growing early fruit. This is why blossom end rot often hits the first tomatoes of the season hardest — the plant’s root system hasn’t caught up yet to support rapid fruit growth.
- High salt levels in soil, often from over-fertilizing, which interferes with calcium absorption at the root.
The Fix (Spoiler: It’s Mostly Watering)
Despite what the garden center endcap of “Tomato Calcium Spray” wants you to believe, spraying calcium on the leaves does almost nothing. Calcium barely moves through leaf tissue to the fruit. Save your money.
What actually works is consistency. Aim for about 1 to 1.5 inches of water a week, delivered evenly rather than in one dramatic soaking session. A drip line or soaker hose on a timer will do more for blossom end rot than any product in a spray bottle. Mulch heavily around the base — two to three inches of straw or shredded leaves buys you a huge buffer against those sudden dry spells, and it keeps soil temperature more even too.
Ease off the nitrogen-heavy fertilizers once fruit starts setting. Switch to something balanced or slightly higher in phosphorus and potassium instead. And if you’re growing in containers, know that you’re fighting an uphill battle here — pots dry out fast, so check them daily during hot stretches, not just when you remember.
What To Do With Affected Fruit
Cut off the bad spot and eat the rest — it’s not toxic, just unattractive. The plant itself is fine and will keep producing. In most cases, once you dial in consistent watering, new fruit sets without the problem, usually within a couple of weeks. If you’re several tomatoes deep into the season and it keeps happening despite steady watering, get a soil test. Occasionally the issue really is low calcium or a pH that’s too acidic (below 6.0) for the plant to access what’s there, and in that case, working in some garden lime in the fall is the actual long-term fix.
Peppers, squash, and eggplant can get blossom end rot too, by the way, for the exact same reasons. So if your zucchini is looking rotten on one end, you already know what to do.
