Why Are My Tomatoes Rotting on the Bottom: Blossom End Rot Causes and Fixes

That sunken, black spot on your tomatoes isn’t rot—it’s a calcium deficiency screaming for help. I’ve lost entire crops to this before finally cracking the code on what’s really happening at the root level.

Blossom end rot hits when plants can’t pull enough calcium from the soil, usually because of uneven watering or pH imbalances. The fix isn’t complicated—just consistent moisture and a few simple amendments to get your soil back on track.

You’ll see the first healthy fruits within 2-3 weeks of making these adjustments. No more tossing half-rotten tomatoes in the compost pile while wondering what went wrong.

Works for container gardens, raised beds, and in-ground plants alike. Here’s exactly how to stop it for good.

Common Causes

Tomatoes rotting on the bottom is a classic sign of blossom end rot, a physiological disorder caused by several key factors. Here’s why it happens:

🌱 Calcium Deficiency

The most common culprit is a lack of calcium in the developing fruit. Calcium is essential for cell wall structure, and without it, the tissue at the blossom end (opposite the stem) breaks down. This isn’t always due to low soil calcium—uneven watering can prevent the plant from absorbing it properly, even if it’s present.

💧 Inconsistent Watering

Fluctuations in soil moisture—drought followed by overwatering—disrupt calcium uptake. When the soil dries out, roots can’t access calcium, and sudden rewatering doesn’t compensate fast enough. This stress triggers rot at the fruit’s weakest point: the blossom end.

🌡️ Extreme Temperature Swings

Hot, dry spells or cold snaps slow root activity, reducing calcium absorption. High temperatures also increase transpiration, leaving less water (and dissolved calcium) for the fruit. Even with adequate soil calcium, heat stress can mimic a deficiency.

🚜 Soil Imbalance

Over-fertilizing with high-nitrogen or high-potassium fertilizers can block calcium uptake. Excess salts from synthetic fertilizers may also damage roots, further limiting their ability to absorb nutrients. A soil test can reveal imbalances before they harm your crop.

Fix the cause, not just the symptom: Addressing these factors restores calcium flow to the fruit, stopping rot in its tracks.

How to Fix the Rot

Blossom end rot can be frustrating, but it’s often fixable with a few adjustments. Below are targeted solutions for each common cause, along with prevention tips to keep your tomatoes healthy.

🌱 Calcium Deficiency

Fix: Add calcium to the soil immediately. Use a calcium-rich foliar spray (like calcium chloride or calcium nitrate) and apply it directly to the leaves. For a quick soil boost, work crushed eggshells or gypsum into the top inch of soil around the plants.

Prevent: Test your soil before planting and amend it with lime or bone meal if calcium levels are low. Maintain consistent soil moisture to help plants absorb calcium effectively.

💧 Inconsistent Watering

Fix: Water deeply and regularly to keep soil moisture levels steady. If your plants are in containers, ensure the pots have drainage holes to prevent waterlogging. Mulch around the base of the plants to retain moisture.

Prevent: Use a drip irrigation system or soaker hose to deliver water directly to the roots and avoid fluctuations. Aim for 1–2 inches of water per week, adjusting for rainfall.

🌡️ Extreme Temperature Swings

Fix: Protect plants from extreme heat or cold. Use shade cloth during heatwaves and row covers or cloches if nights are unexpectedly chilly. Keep the soil temperature stable with a layer of organic mulch.

Prevent: Plant tomatoes in a location with consistent temperatures. Avoid early spring or late fall planting if frost or heatwaves are likely.

🌿 Over-Fertilization

Fix: Flush the soil with water to reduce excess salts from fertilizers. Switch to a balanced, slow-release fertilizer and avoid high-nitrogen options, which can hinder calcium uptake.

Prevent: Fertilize sparingly and follow package instructions. Opt for organic fertilizers like compost or worm castings, which release nutrients gradually.

💡 Pro Tip: Remove affected tomatoes to redirect the plant’s energy toward healthy fruit. With these fixes, new tomatoes should develop without rot!

Frequently Asked Questions