If you’ve ever grown tomatoes, you’ve probably faced the frustrating sight of a dark, sunken spot on the bottom of your fruit. This is a common garden problem, and understanding why do tomatoes rot on the bottom is the first step to fixing it.
That ugly lesion is called blossom end rot. It’s not a disease caused by a fungus or bacteria, but rather a physiological disorder. This means it’s related to how the plant functions, not a pest you can spray. The good news is that it’s often preventable once you know the causes.
Why Do Tomatoes Rot On The Bottom
Blossom end rot happens when the tomato plant can’t get enough calcium to the developing fruit. Calcium is crucial for building strong cell walls. Without it, the tissues at the blossom end—the opposite side from the stem—break down and rot.
It’s important to note the calcium deficiency is usually in the fruit, not always in the soil. The plant might have plenty of calcium available at its roots, but something is preventing that calcium from reaching the tomatoes. This is a key distinction for finding the right solution.
Primary Causes of Blossom End Rot
Several interconnected factors can lead to this calcium delivery problem. Here are the main culprits:
- Inconsistent Watering: This is the #1 cause. Fluctuating between very dry and very wet soil disrupts the plant’s ability to take up calcium. Calcium moves with water, so when the soil dries out, the flow stops.
- Rapid Early Growth: When plants grow too quickly due to excess nitrogen fertilizer, the leaves and stems get priority. The calcium is diverted away from the fruit, leading to the rot.
- Damaged Roots: If the root system is injured by rough transplanting, deep hoeing, or pests, it can’t effectively absorb water and nutrients, including calcium.
- Soil pH Imbalance: Very acidic or very alkaline soil can lock up calcium, making it unavailable to the plant even if it’s present.
- Cold or Waterlogged Soil: Cool temperatures early in the season or soggy soil can stunt root growth and function, hindering nutrient uptake.
How to Identify Blossom End Rot
Correct identification is crucial. Blossom end rot has distinct signs:
- It appears only on the blossom end (bottom) of the fruit.
- It starts as a small, water-soaked spot that looks tan or light brown.
- The spot enlarges, turns dark brown or black, and becomes sunken and leathery.
- It usually shows up on the earliest, largest fruits of the season.
- Sometimes, secondary fungi will grow on the rotten area, but they are not the primary cause.
Is It Safe to Eat Tomatoes with Blossom End Rot?
You can cut off the affected portion and eat the rest of the tomato. The rot itself is not toxic. However, the damaged area can invite other organisms that cause spoilage, so use these fruits quickly and inspect them carefully.
Step-by-Step Fix and Prevention Plan
Once you see blossom end rot, you can’t reverse it on that particular fruit. But you can absolutely stop it from affecting new tomatoes. Follow these steps.
1. Adjust Your Watering Routine Immediately
Consistent soil moisture is your best tool. Your goal is soil that feels like a wrung-out sponge—moist but not soggy.
- Water deeply and less frequently, rather than giving light daily sprinkles.
- Provide about 1-1.5 inches of water per week, including rainfall.
- Use a soaker hose or drip irrigation to water the soil directly, keeping leaves dry.
- Mulch heavily with 2-3 inches of straw, wood chips, or shredded leaves. Mulch is a game-changer as it regulates soil temperature and locks in moisture.
2. Check and Amend Your Soil
A simple soil test from your local extension service is invaluable. It will tell you your soil’s pH and calcium levels.
- For Low pH (Acidic Soil): Add garden lime according to test recommendations. Lime adds calcium and raises pH.
- For Adequate pH but Low Calcium: Add gypsum (calcium sulfate). Gypsum adds calcium without altering soil pH, which is a great option if your pH is already balanced.
- Always follow package directions for application rates. More is not better.
3. Review Your Fertilizing Habits
Go easy on high-nitrogen fertilizers (the first number in the N-P-K ratio). They promote lots of leafy growth at the expense of fruit. Switch to a balanced fertilizer or one formulated for tomatoes, which often has added calcium. A good rule is to fertilize at planting and again when fruit first sets, then hold off.
4. Handle Roots with Care
When transplanting seedlings, be gentle. Avoid disturbing the root ball. If you must cultivate around plants to remove weeds, do it shallowly to avoid cutting feeder roots. Healthy roots are critical for nutrient uptake.
Common Mistakes That Make It Worse
Even with good intentions, gardeners sometimes do things that contribute to the problem. Avoid these pitfalls:
- Over-relying on Foliar Sprays: Spraying calcium directly on leaves is not very effective. Calcium moves poorly from leaves into the fruit. Focus on the soil and roots instead.
- Adding Eggshells as a Quick Fix: Crushed eggshells decompose far too slowly to help current-season plants. They need to be composted for a long time or ground into a powder to be of any use.
- Panic Overwatering: Seeing rot and then flooding the plant creates more stress. Stick to a consistent, deep watering schedule.
- Ignoring Early Fruit: Remove affected tomatoes as soon as you see them. This allows the plant to redirect its energy into developing new, healthy fruit.
FAQ: Your Blossom End Rot Questions Answered
Q: Does blossom end rot affect other vegetables?
A: Yes! Peppers, eggplant, squash, and watermelon can also suffer from the same condition for the same reasons—calcium deficiency in the developing fruit.
Q: Can I use milk or antacid tablets to add calcium?
A: While these contain calcium, they are not reliable or recommended methods. They can disrupt soil biology and are difficult to apply correctly. Soil amendments like lime or gypsum are the proper solutions.
Q: Will all my tomatoes get it once I see it on one?
A: Not necessarily. If you correct the watering and soil issues promptly, new fruits that set after your corrections should be fine. The earliest fruits are most vulnerable.
Q: Are some tomato varieties resistant to bottom rot?
A: No variety is truly resistant, as it’s a physiological issue. However, some gardeners find that plum or paste tomato varieties, with their smaller fruit, show it less often than large beefsteak types. Fast-maturing varieties may also escape it by fruiting before stressful hot, dry periods.
Q: Can hot weather cause it?
A: Indirectly, yes. High heat increases plant water demand and can lead to moisture stress if you don’t increase watering. It also can cause rapid growth spurts. Mulching is extra important in hot climates to keep roots cool and moist.
Long-Term Garden Health
Preventing blossom end rot is really about promoting overall plant health. Building good soil with plenty of organic matter through compost improves both water retention and drainage—a seeming contradiction that healthy soil achieves perfectly. Regular crop rotation helps prevent nutrient depletion and reduces disease pressure.
Remember, gardening is a learning process. Seeing a few tomatoes with blossom end rot doesn’t mean you’ve failed. It’s your plants signaling that something in their environment is out of balance. By responding with consistent care, you can enjoy a full, healthy harvest for the rest of the season. Paying attention to soil prep, watering, and mulching will solve most cases of this common problem.