If you’ve noticed dark, sunken spots on your tomatoes, you’re not alone. This common problem has a specific cause, and understanding it is the first step to a solution. Let’s look at why are my tomatoes black on the bottom and what you can do about it.
Why Are My Tomatoes Black On The Bottom
The black, leathery spot on the bottom of your tomato is called blossom end rot. It’s not a disease caused by a fungus or bacteria. Instead, it’s a physiological disorder linked to a calcium imbalance in the developing fruit.
Calcium is essential for building strong cell walls in the tomato. When the plant can’t move enough calcium to the blossom end (the bottom) of the fruit, the tissues break down. This creates that characteristic dark, sunken lesion. It often appears when fruits are about half their full size.
What Causes Blossom End Rot?
The root cause is always a calcium delivery problem. However, this isn’t usually because your soil lacks calcium. More often, it’s due to environmental factors that disrupt the plant’s ability to take up or transport calcium.
- Inconsistent Watering: This is the #1 culprit. Fluctuations between wet and dry soil prevent calcium from moving consistently from the roots to the fruit.
- Root Damage: Damaged roots from rough transplanting, deep hoeing, or pests can severely limit calcium uptake.
- Rapid Early Growth: A surge of growth from too much nitrogen fertilizer can cause the plant to outpace its calcium supply.
- Soil pH Issues: Very acidic or alkaline soil can lock up calcium, making it unavailable to the plant even if it’s present.
- Cold or Waterlogged Soil: Cool temperatures or soggy roots early in the season hinder root function and nutrient absorption.
How to Fix and Prevent Blossom End Rot
Once a tomato has blossom end rot, the affected fruit cannot be cured. You should remove it so the plant can redirect its energy. The good news is you can prevent it on future fruits with a few consistent practices.
1. Master Your Watering Routine
Consistent soil moisture is critical. Aim for evenly moist soil, like a wrung-out sponge. Avoid the cycle of drought and flood.
- Water deeply and less frequently to encourage deep roots.
- Use drip irrigation or soaker hoses to deliver water directly to the soil, keeping foliage dry.
- Mulch heavily with 2-3 inches of straw, wood chips, or leaves. Mulch regulates soil temperature and dramatically reduces moisture evaporation.
2. Test and Amend Your Soil
A simple soil test is your best friend. It will tell you your soil’s pH and calcium levels.
- For pH: Tomatoes prefer a slightly acidic pH of 6.5-6.8. If your soil is too acidic (low pH), add garden lime. If it’s too alkaline (high pH), add sulfur.
- For Calcium: If a test confirms a deficiency, add calcium at planting time. Use gypsum (calcium sulfate) or crushed eggshells worked deeply into the soil. Avoid quick fixes like spraying leaves; calcium doesn’t move well from leaves into fruit.
3. Fertilize Correctly
Go easy on high-nitrogen fertilizers. They promote lots of leafy growth at the expense of fruit development and can worsen calcium issues.
- Use a balanced fertilizer or one formulated for tomatoes.
- Consider adding a handful of bone meal (which contains calcium and phosphorus) to the planting hole.
- If you’ve over-fertilized with nitrogen, water deeply to help leach some of it from the root zone.
4. Practice Careful Planting and Root Care
Handle seedlings gently to avoid shocking the roots. Plant them deep, as tomatoes can develop roots all along their buried stems, creating a stronger root system.
Avoid cultivating or hoeing deeply near your plants. You can easily slice through shallow feeder roots, which are responsible for grabbing water and nutrients like calcium.
Other Tomato Problems You Might See
Not every dark spot is blossom end rot. Here’s how to tell the difference between other common issues.
Early Blight vs. Blossom End Rot
Early blight is a fungal disease. It shows up as dark, concentric rings on the lower leaves first, often with a yellow “halo.” It can spread to stems and fruit, causing dark, leathery spots on the shoulders or sides of the tomato, not just the bottom. Good air circulation and fungicides can help manage it.
Sunscald vs. Blossom End Rot
Sunscald happens when green or ripening fruit gets too much direct, hot sun. It looks like a white, papery patch that can become flat and grayish. It’s usually on the top or side of the fruit facing the sun. Providing adequate leaf cover through proper pruning prevents this.
Catfacing vs. Blossom End Rot
Catfacing causes gross deformities, scars, and holes at the blossom end. It’s linked to cool temperatures during flowering and is more common in large, heirloom varieties. The fruit is often still edible if you cut away the scarred parts.
Step-by-Step Action Plan for This Season
If you’re seeing blossom end rot right now, follow these steps immediately.
- Remove Affected Fruit: Pick off any tomatoes with significant black spots. This helps the plant.
- Check Soil Moisture: Stick your finger 2-3 inches into the soil. Is it consistently damp? Adjust your watering schedule to achieve this.
- Apply Mulch: If you haven’t already, add a thick layer of organic mulch around your plants right away.
- Consider a Calcium Supplement: While soil amendments are best for long-term health, a foliar spray of calcium chloride or calcium nitrate might offer some help for developing fruit in the current season. It’s not a guaranteed fix but can be part of a rescue effort.
- Avoid More Nitrogen: Do not apply any more high-nitrogen fertilizer.
Choosing Resistant Varieties for Next Year
Some tomato varieties are less prone to blossom end rot. While no variety is completely immune, choosing wisely can stack the odds in your favor.
- Look for varieties described as having good “calcium efficiency” or “stress tolerance.”
- Many plum and paste tomato varieties, like ‘Roma,’ tend to be less susceptible than large beefsteak types.
- Cherry tomatoes very rarely get it due to their smaller fruit size and faster development.
- When starting from seed, ensure your seedlings get adequate calcium and aren’t held in small pots for too long, which can stress them.
Frequently Asked Questions (FAQ)
Q: Can you eat a tomato with black on the bottom?
A: You can cut off the black, sunken portion. The rest of the tomato is usually fine to eat, though it may not have the best flavor. Discard any fruit that shows signs of secondary mold or rot.
Q: Does Epsom salt help with blossom end rot?
A: No. Epsom salt is magnesium sulfate. Adding it can actually make the problem worse by competing with calcium for uptake by the plant. It is not a cure.
Q: How do you add calcium to tomato plants quickly?
A: For a fast supplement, you can use a foliar spray of calcium chloride or calcium nitrate, following label instructions. However, correcting soil moisture is the fastest way to stop new cases from occuring.
Q: Does overwatering cause black bottoms on tomatoes?
A> Indirectly, yes. Overwatering leads to soggy soil and poor root function, which disrupts calcium uptake. It’s the inconsistency between wet and dry that’s the real issue, not just one extreme.
Q: Will blossom end rot spread to other tomatoes?
A: No, because it is not a contagious disease. However, if the environmental conditions causing it (like uneven watering) are not corrected, other fruits on the same plant and nearby plants will likely develop it too.
Blossom end rot can be frustrating, but it’s a solvable problem. Focus on steady watering, good mulching, and balanced nutrition. By creating a less stressful environment for your plants, you’ll set the stage for a healthy harvest full of perfect, red tomatoes all season long.