Blossom end rot is a calcium deficiency caused by uneven watering

Blossom end rot appears as a dark, sunken spot on the bottom of the tomato — the end opposite the stem. It starts as a small tan or brown patch and grows into a large, papery lesion that makes the fruit inedible. The cause is not a disease or pest: it is a calcium deficiency in the developing fruit, triggered almost always by inconsistent soil moisture.

Tomato plants need steady water to move calcium from the soil into the fruit. When the soil dries out between waterings, the plant cannot transport calcium to the growing tomato. When you water again, the plant floods with water but the calcium has already been locked out of that fruit. The damage is permanent — the spot will not heal — but you can prevent it on new fruit by fixing your watering routine.

Blossom end rot shows up most often on the first large tomatoes of the season, when plants are setting heavy fruit and the gardener is still learning the plant's water needs. It is more common in containers and raised beds, where soil dries faster than in ground beds.

Key Takeaways

  • Blossom end rot is caused by calcium not reaching the fruit during development, triggered by dry soil between waterings.
  • The dark, sunken spot on the bottom of the tomato cannot be fixed once it appears, but preventing it means watering consistently so soil stays evenly moist.
  • Containers and raised beds are more prone to blossom end rot because they dry out faster than in-ground soil.
  • Adding lime or gypsum to the soil before planting can help, but consistent watering is the most important step.
  • Affected fruit should be removed so the plant can direct energy to healthy tomatoes instead.

Why calcium stops moving to the fruit

Calcium travels through the plant in water. The roots pull calcium-rich water from the soil and move it up through the stem and into the leaves and fruit. When the soil dries out, this water flow stops. The fruit that was developing at that moment — usually the largest ones on the plant — does not get the calcium it needs to form a strong cell wall at the blossom end.

Once the soil dries, the damage to that fruit is done. Watering again will not reverse it. The plant will move calcium to new fruit that develops after you restore consistent moisture, but the spotted tomato stays spotted.

This is why blossom end rot often hits the first big tomatoes: they are developing while the plant is young and the gardener is still figuring out the watering schedule. Later fruit, once the routine is established, often stays healthy.

Soil moisture is the main control

The single most effective way to prevent blossom end rot is to water deeply and consistently so the soil stays evenly moist — not soggy, but not dry. For most tomato plants, this means watering once or twice a week depending on heat and rainfall, but the exact schedule depends on your soil, your container size, and your climate.

The best method is to check the soil with your finger before watering. Push your finger two inches into the soil near the base of the plant. If it feels dry at that depth, water. If it still feels moist, wait another day or two. This takes the guesswork out of the schedule.

Mulch helps keep soil moisture steady. A two-inch layer of straw, wood chips, or shredded leaves over the soil (but not touching the stem) slows evaporation and buffers the soil against rapid drying between waterings. In containers, mulch is especially useful because container soil dries so much faster than ground soil.

Adding calcium before planting

If your soil is naturally low in calcium, adding it before planting can reduce the risk. The two common forms are ground limestone (calcium carbonate) and gypsum (calcium sulfate). Limestone also raises soil pH, so use it only if a soil test shows your pH is below 6.0. Gypsum adds calcium without changing pH and works in any soil.

A soil test from your local extension office will tell you whether your soil needs calcium and how much to add. Without a test, a general rate is one to two pounds of gypsum per 100 square feet of bed, worked into the top six inches of soil before planting. For containers, mix gypsum into the potting soil at planting time — about one tablespoon per five-gallon pot.

Adding calcium before planting helps, but it is not a substitute for consistent watering. Even calcium-rich soil cannot move calcium to the fruit if the soil is dry.

Remove affected fruit so the plant focuses on healthy ones

Once a tomato shows blossom end rot, remove it from the plant. The fruit is not salvageable — the rot will spread and the taste will be off — and leaving it on the plant wastes the plant's energy. Removing it signals the plant to direct resources to the healthy fruit still developing.

Do not compost affected tomatoes if your compost does not reliably reach 140°F or higher, because the rot is caused by a nutrient imbalance, not a pathogen, and the fruit will just break down. You can throw it in the trash or bury it in the garden away from the tomato bed.

Container tomatoes need more frequent watering

Tomatoes in pots and grow bags are at higher risk for blossom end rot because container soil dries out much faster than ground soil. A five-gallon pot in hot weather may need water every day or even twice a day, while an in-ground plant might need water only twice a week.

Check container soil daily during the growing season by pushing your finger into the top inch. If it feels dry, water until water drains from the bottom. Use a pot at least five gallons in size — smaller pots dry out too quickly to maintain steady moisture. A saucer under the pot can help retain moisture, but empty it after watering so the roots do not sit in standing water.

Nitrogen and salt can make it worse

High nitrogen fertilizer can worsen blossom end rot by pushing the plant to grow leaves and stems at the expense of calcium uptake. Use a balanced fertilizer — one where the three numbers (nitrogen, phosphorus, potassium) are close to each other, like 5-5-5 or 10-10-10 — rather than a high-nitrogen formula like 20-5-5.

Excess salt in the soil, from over-fertilizing or from salty water, can also interfere with calcium uptake. If you fertilize, follow the package rate and do not exceed it. If you use well water or live in an area with salty tap water, flush containers with extra water once a month to leach out salt buildup.

Frequently Asked Questions

Can I eat a tomato with blossom end rot?

No. The affected area is soft and has an off taste. You can cut away the spot if it is small and the rest of the fruit is firm, but the rot usually spreads into the fruit and makes most of it unusable. It is better to remove the fruit and let the plant focus on healthy ones.

Will blossom end rot spread to other tomatoes?

No, blossom end rot is not contagious. It is a nutrient problem in individual fruits. However, if your watering is inconsistent, other tomatoes developing at the same time may also show the spot. Fixing the watering routine will prevent it on new fruit.

What if I water every day but still get blossom end rot?

Daily watering may be too much if your soil stays soggy, which can actually block calcium uptake. Check that water is draining freely from containers and that the soil dries slightly between waterings. Also check whether your soil or water is very high in salt, which can interfere with calcium movement. A soil test can confirm.

Is blossom end rot the same as late blight?

No. Late blight is a fungal disease that causes brown spots on leaves and stems and spreads quickly in wet weather. Blossom end rot appears only on the fruit, starts at the bottom, and is caused by a calcium shortage, not a disease. The treatments are completely different.

Do all tomato varieties get blossom end rot equally?

Some varieties are more prone to it than others, especially large-fruited types like beefsteaks. Cherry tomatoes are less susceptible. But any variety can develop blossom end rot if watering is inconsistent, so variety choice is less important than fixing your watering routine.