Troubleshooting
Blossom End Rot in Greenhouse Tomatoes: Why It Happens and How to Stop It
You see a perfect-looking truss of tomatoes, then turn one over and find a leathery black patch on the bottom. That is blossom end rot. It is not a disease and it does not spread from plant to plant. It is a disorder caused by too little calcium reaching the developing fruit, and in greenhouses it usually comes down to watering and climate, not a lack of calcium in the soil. Fix the conditions and new fruit usually comes out clean.
Quick answer
- Blossom end rot is a calcium delivery problem in the fruit, usually caused by uneven watering or stress, not usually by calcium-poor soil.
- Consistent moisture is the main fix. Avoid cycles of drying out then soaking, especially in containers and grow bags.
- Hot, very humid or very dry conditions can reduce calcium movement into fruit. Venting and shading help.
- Affected fruit will not recover. Remove it and focus on new trusses.
- Some varieties, especially certain plum and paste types, are often more prone to it.
What blossom end rot looks like
- A water-soaked spot at the blossom end (the bottom, opposite the stem) of green or ripening fruit.
- The spot grows, turns dark brown or black, and becomes sunken and leathery.
- Secondary molds may grow on the damaged area later.
- It often hits the first fruits of the season, or fruit that develops during a hot spell or a period of irregular watering.
The same disorder affects peppers, eggplants and squash, so if you grow those in the same greenhouse, watch them too.
Why calcium does not reach the fruit
Calcium moves through the plant mainly in the water stream that is pulled up as leaves transpire. Leaves transpire far more than fruit, so they win most of the calcium. When water supply is uneven, or transpiration patterns change sharply, developing fruit can come up short at the very time it is growing fastest.
Common greenhouse triggers:
| Trigger | Why it matters |
|---|---|
| Irregular watering | Dry spells interrupt calcium supply to growing fruit. |
| Small containers and grow bags | They dry out fast on hot days. |
| Heat spikes | Rapid leaf transpiration pulls calcium to leaves instead of fruit. |
| Very high humidity for long periods | Can reduce transpiration overall, slowing calcium movement. |
| Damaged or waterlogged roots | Roots cannot take up water and nutrients evenly. |
| Heavy nitrogen or high salts | Lush growth and high salt levels can compete with calcium uptake. |
Fix one: steady watering
This is the main lever for most hobby growers.
- Check moisture daily in hot weather. Grow bags and pots may need water more than once a day in summer.
- Water deeply and evenly. Aim for consistently moist, not alternately soaked and dry.
- Use bigger containers. A larger root zone buffers against drying out.
- Consider automatic watering. Drip lines on a timer, or self-watering systems, smooth out the peaks and troughs. Our drip irrigation guide covers options.
- Mulch the surface of beds or large containers to slow evaporation.
Fix two: control heat and humidity swings
- Shade in summer. Shade cloth or shade paint reduces heat spikes on bright days.
- Vent early. Open vents before the greenhouse overheats rather than after. Automatic vent openers help if you are out during the day.
- Keep air moving. A circulation fan helps leaves and fruit transpire more evenly.
- Avoid long, stuffy humid periods. Good ventilation helps keep calcium moving and reduces disease.
For more on balancing heat and moisture in the air, see VPD explained.
Fix three: sensible feeding
Many soils and potting mixes contain enough calcium. Adding more calcium will not help if the real problem is water delivery. That said, feeding still matters:
- Avoid excess nitrogen, especially early, which pushes leafy growth at the expense of fruit.
- Use a tomato feed as directed once fruit sets. Do not overdo strength, since high salt levels can make uptake harder.
- Get a soil test for in-ground beds if blossom end rot persists year after year despite steady watering. It will show whether calcium or pH is actually the issue.
- In hydroponics, follow your nutrient maker's guidance on calcium and monitor EC and pH.
About calcium sprays: foliar calcium sprays are sold for blossom end rot. Because calcium does not move readily from leaves into fruit, many extension sources regard them as of limited value. Fixing watering is the more reliable approach.
Grow bags and containers: the high-risk setup
Grow bags are popular in greenhouses because they keep tomatoes out of tired border soil. They also hold a limited volume of compost, and on a bright summer day a fruiting tomato can draw a lot of water from a small bag. That combination makes them the most common place to see blossom end rot.
Ways to make grow bags more forgiving:
- Plant fewer plants per bag. Two plants in a bag sized for three gives each more root room and water reserve.
- Use grow pots or rings. Bottomless rings set into the bag add depth, so you can water into the ring and let roots reach the bag below.
- Stand bags in trays. A shallow tray catches runoff that roots can draw back up, as long as bags do not sit waterlogged for long.
- Water little and often in heat. Two moderate waterings on a hot day are steadier than one big flood.
- Check by weight and feel. Push a finger into the compost. If it is dry a short way down, water.
Read the pattern
When blossom end rot appears is a clue to the cause. If it hits after a heatwave, look at shading and venting. If it hits plants on one bench but not another, compare their watering and container size. If it appears on every plant of one variety while others stay clean, the variety is the likely factor. Note the dates and conditions in a simple greenhouse diary and you will usually spot the trigger within a season.
Variety matters
Some tomato varieties seem more prone to blossom end rot than others. Elongated plum and paste types are often mentioned as being more susceptible. If one variety suffers every year while others are fine under the same care, try a different variety next season.
What to do with affected fruit
- Remove affected fruit so the plant puts energy into healthy fruit.
- Fruit with a small, dry patch can be trimmed and eaten. Discard fruit with mold or rot spreading inside.
- Do not panic about the plant. Once conditions improve, new fruit is usually fine.
Peppers and other crops
Peppers get blossom end rot too, often showing as a pale, papery or sunken patch on the side or bottom of the fruit rather than a dark leathery spot. The causes and the fixes are the same: even watering, adequate root room, and avoiding heat spikes. Because peppers are often grown in smaller pots than tomatoes, they can dry out faster than you expect.
Prevention checklist
- Use containers or grow bags that are large enough for the variety.
- Set up drip or self-watering if you are away during the day.
- Check moisture daily in summer, twice daily for small containers in heat.
- Shade and vent to prevent heat spikes.
- Run a circulation fan.
- Feed with a balanced tomato feed at the labeled rate.
- Test bed soil if the problem keeps recurring.
- Note which varieties suffer and adjust next year.
FAQ
Is blossom end rot contagious?
No. It is a physiological disorder, not a disease, so it does not spread between plants.
Will adding eggshells prevent blossom end rot?
Eggshells break down very slowly and are unlikely to supply calcium quickly. Consistent watering is far more effective.
Why are only the first tomatoes affected?
Early fruit often develops while roots are still establishing and watering habits are settling. Later fruit is often fine once conditions stabilize.
Can blossom end rot happen with plenty of calcium in the soil?
Yes. That is the most common situation. The problem is usually calcium getting to the fruit, not calcium in the soil.
Should I pick off affected tomatoes?
Yes. They will not recover, and removing them lets the plant put energy into healthy fruit.
The bottom line
Treat blossom end rot as a signal about water and climate. Even out your watering, give roots enough room, shade and vent to avoid heat spikes, and feed sensibly. Remove damaged fruit, and the next trusses usually come through clean.
Want to grow more consistent tomatoes overall? Our greenhouse tomato guide covers the full season.