What Blossom End Rot Is and Why It Happens

Blossom end rot is a dark, sunken spot that appears on the bottom of a tomato — the end opposite the stem — and it is caused by a calcium deficiency in the fruit itself, not by disease or pests. The plant cannot move calcium to the developing tomato if the soil moisture is uneven: when the soil dries out and then gets soaked, the plant absorbs water and nutrients in surges, and calcium does not travel with that water the way other nutrients do. The fruit at the bottom of the plant, which develops first and fastest, runs short of calcium and the cells collapse, leaving a dark, papery lesion.

The problem is not that your soil lacks calcium — most garden soils have plenty. The problem is that your watering pattern is not steady enough for the plant to deliver what is already there. This is why blossom end rot shows up on the first tomatoes of the season, when plants are still small and soil dries faster, and why it often stops appearing on later fruit once the plant is larger and the soil stays more consistently moist.

Key Takeaways

  • Blossom end rot is caused by uneven soil moisture preventing calcium from reaching the fruit, not by a lack of calcium in the soil itself.
  • Watering deeply and on a regular schedule — aiming for soil that stays consistently moist but not waterlogged — stops the problem before it starts.
  • Mulch around the base of the plant to buffer soil moisture swings and keep the root zone from drying out between waterings.
  • Tomatoes already showing blossom end rot will not heal, but fixing your watering now will prevent it on new fruit developing higher on the plant.
  • Calcium amendments like lime or gypsum can help, but only if the real problem — uneven watering — is fixed first.

Set Up a Consistent Watering Schedule

Water your tomato plants deeply and on the same day each week, or every other day if temperatures are very hot. "Deeply" means watering until the soil is moist 6 to 8 inches down — not just the surface — so the roots have a steady supply and the plant does not have to search for water. Use a soaker hose or drip line if you have one, because they deliver water slowly to the soil rather than wetting the leaves, and they make it easier to water the same amount each time.

Check the soil with your finger before you water. If the top inch is dry but the soil 2 inches down is still moist, wait another day. If the soil is bone-dry several inches down, water when ready. The goal is to keep the soil consistently moist — like a wrung-out sponge — not soggy and not dust-dry. In hot weather, this might mean watering every other day. In cool weather or after rain, it might be once a week. Let the soil tell you, not the calendar.

Add Mulch to Stabilize Soil Moisture

Spread 2 to 3 inches of mulch — shredded leaves, straw, or wood chips — around the base of each plant, keeping it a few inches away from the stem so air can circulate and the stem stays dry. Mulch acts as a buffer: it slows evaporation so the soil dries out more slowly between waterings, and it keeps the soil temperature more even, which helps the plant absorb nutrients more steadily.

Avoid piling mulch directly against the stem, because that traps moisture and can invite rot or pest damage. Refresh the mulch once or twice during the growing season as it breaks down, but do not add so much that you create a thick, wet mat that stays soggy.

When to Add Calcium Amendments

If you want to add calcium to the soil, use gypsum (calcium sulfate) or lime (calcium carbonate), but understand that these work only if your watering is already consistent. Gypsum is faster-acting and does not change soil pH, so it is the safer choice if you are not sure about your soil chemistry. Lime takes longer to break down but also raises pH, which can help if your soil is acidic — though most garden soils already have enough calcium.

Follow the package directions for how much to explore, and water it in well. Do not assume that adding calcium will fix blossom end rot if you are still watering erratically — the plant still will not be able to move the calcium to the fruit. Fix the watering first, then add calcium if you want to, as insurance.

Remove Affected Fruit and Keep Growing

Tomatoes that already have blossom end rot will not heal or become edible. Remove them from the plant and discard them, or cut away the damaged bottom portion if the rest of the fruit is sound and you want to use it. Removing the affected fruit also lets the plant direct energy to new fruit developing higher on the stem.

The new tomatoes forming now will not have blossom end rot if your watering stays steady from this point forward. You may see the problem disappear within a week or two as new fruit develops under better conditions.

Adjust Watering After Rain

After a heavy rain, your soil will be saturated. Wait a few days before watering again, even if the top inch feels dry. Check 3 to 4 inches down: if it is still moist, hold off. Overwatering after rain can swing the soil from too-dry to too-wet, which creates the same calcium-delivery problem you are trying to avoid.

If you are in a region with frequent heavy rains and blossom end rot keeps coming back, consider planting tomatoes in raised beds or containers where you have more control over drainage and moisture. In-ground beds in clay soil can stay waterlogged for days after rain, making it hard to maintain the steady moisture that prevents the problem.

Container Tomatoes and Blossom End Rot

Tomatoes in pots dry out faster than in-ground plants, so they are more prone to blossom end rot if you forget to water. Water container tomatoes every day or every other day during the growing season, depending on heat and pot size. A 5-gallon pot needs water more often than a 10-gallon pot. Feel the soil before you water — if it is dry an inch down, water deeply until it drains from the bottom.

Use a potting mix that holds moisture well but still drains — not pure compost, which can stay too wet, and not pure sand, which dries too fast. A mix of compost, peat moss, and perlite in roughly equal parts works well. Add mulch to the top of the pot to slow evaporation, just as you would in the garden.

Frequently Asked Questions

Can I eat a tomato with blossom end rot?

Yes, if the damaged area is small. Cut away the dark, sunken spot and the tissue around it, and use the rest of the tomato normally. If the rot is deep or covers a large area, discard the fruit — it will taste bitter or off.

Will blossom end rot spread to other tomatoes?

No, it is not contagious. It is a physiological problem caused by how the plant is watered, not by a disease or pest. If other tomatoes are developing the same spot, it means the watering problem is still happening, not that the rot is spreading.

Do I need to add lime if my soil test shows enough calcium?

No. If a soil test shows adequate calcium, adding more will not stop blossom end rot — the problem is water delivery, not calcium availability. Fix your watering schedule first. If blossom end rot continues after two weeks of consistent watering, then consider a calcium amendment, but it is rarely necessary.

How often should I water tomatoes in very hot weather?

In temperatures above 85°F, check the soil every day and water whenever the top 2 inches feel dry. You may need to water every day or even twice a day in extreme heat. Mulch becomes even more important in hot weather because it slows evaporation and keeps the soil cooler.

Can I prevent blossom end rot by watering the leaves instead of the soil?

No. Watering the leaves does not deliver moisture to the roots, where it is needed to move calcium through the plant. Water the soil at the base of the plant, not the foliage. Wet leaves also invite fungal disease.