Blossom End Rot on Tomatoes: Why It Happens and How to Stop It

GardenDraft Team · June 11, 2026 · 7 min read

Part of: Plant Problems & Pest Guides

You've been watching a tomato ripen for weeks, you turn it over, and the bottom is a sunken brown-black leathery patch. That's blossom end rot — the most common tomato disorder there is, and one of the most misunderstood. The good news: it isn't a disease, it isn't contagious, and it's almost entirely preventable once you know what actually causes it.

Blossom end rot is a calcium problem — but not a soil problem

Blossom end rot is a calcium deficiency in the fruit itself. The intuitive fix, dumping calcium on the soil, is usually pointless, because most garden soil already has plenty of calcium. The real issue is that the plant can't move calcium to the fast-growing end of the fruit, and calcium travels with water. Anything that disrupts steady water uptake starves the fruit's blossom end of calcium even when the soil is full of it.

The usual triggers mostly come back to disrupted calcium uptake:

• Uneven watering, drought stress followed by a soaking, repeated.
• Letting plants wilt, even briefly, in heat.
• Heavy rain after a dry spell.
• Too much nitrogen fertilizer, which pushes fast leafy growth that outruns calcium delivery.
• Root stress or poor soil pH, which can make nutrients harder for the plant to move even when the soil contains enough calcium.

Why it hits the first fruits hardest

Blossom end rot is most common on the earliest fruit of the season and during hot, erratic-weather spells, exactly when watering is hardest to keep even and the plant is growing fastest. It also shows up in peppers, squash, and eggplant for the same reason. The patch always forms at the blossom end (the bottom), never the stem end, which is how you tell it apart from disease.

How to stop it

It comes down to consistent soil moisture, and three habits deliver it:

1. Water deeply and regularly rather than a little and erratically: aim for steady moisture, never a swing from bone-dry to flooded. (See how often to water.)
2. Mulch. A 2–3 inch layer of straw or shredded leaves is the single most effective fix — it buffers the soil against the wet-dry cycle that causes the problem.
3. Go easy on high-nitrogen fertilizer, especially early, so growth doesn't outpace the plant's ability to ferry calcium.

What to do with affected fruit

Pick and discard the affected tomatoes — they won't recover, and removing them lets the plant redirect energy to clean fruit. Then fix the watering and avoid further root stress. Once moisture steadies, later fruit typically comes in perfectly normal, often within a truss or two. Calcium sprays are rarely the first fix unless a soil test shows a true deficiency; the lever that usually works is steady water. Get the watering rhythm into your routine from the start by planning it alongside everything else (see how to plan a vegetable garden) and time your tomato planting for your location with the planting calendar.