What Causes Blossom End Rot?
Blossom end rot is the dark, sunken patch that develops opposite the stalk end of tomatoes, peppers and sometimes aubergines. It is linked to calcium not reaching developing fruit reliably, usually because roots experience uneven moisture. It is especially common in pots, grow bags and during hot, changeable weather.
Confirm that you are seeing blossom end rot before changing your feeding routine. The affected area begins as a pale or water-soaked patch at the flower end, then turns brown or black and leathery as fruit enlarges. It is not the same as blight, which usually affects foliage and can make irregular dark marks elsewhere on fruit. Remove damaged fruit promptly with clean secateurs or by twisting it off carefully. It will not recover, and taking it away lets the plant direct energy towards healthy fruit. Check other young fruit every few days, because early removal and steadier watering can prevent a small problem becoming a large crop loss.
The practical cause is usually inconsistent water reaching the roots. When compost dries out, roots cannot move calcium efficiently to fast-growing fruit; when it is then flooded, root stress continues. Water at the base in the morning and aim for evenly moist, not saturated, compost throughout the root zone. In warm weather, check pots and grow bags daily, and check again in late afternoon if leaves wilt. Water slowly until excess begins to drain, then stop. Never rely solely on a dry-looking surface, as peat-free compost can appear dry while remaining moist lower down. Conversely, a heavy, wet container may need time to drain before it is watered again.
Give every plant enough root space and drainage. A tomato or pepper in a small pot dries unpredictably and is much more vulnerable than one in a container at least 30 cm wide. Make sure holes are not blocked and raise pots slightly so water can escape. Grow bags can work well, but water must be shared evenly between plants and checked carefully during heat. Do not pack containers tightly with several crops competing for moisture. In greenhouse borders, incorporate garden compost before planting and mulch the surface once plants are established. For outdoor plants, a 3 to 5 cm mulch of leaf mould or garden compost helps keep soil moisture steadier after sun and rain.
Avoid the tempting but unhelpful response of adding extra calcium or heavy doses of fertiliser. Most garden soils and good peat-free compost contain enough calcium for tomatoes; the issue is commonly water movement inside the plant, not an empty soil reserve. Overfeeding can make roots less healthy and increase the risk of other problems. Feed fruiting tomatoes or peppers regularly only as directed on the label, after watering dry compost. Do not use fresh manure in containers or around stems. If you use your own garden compost, ensure it is well-rotted and mixed into a suitable growing medium rather than relying on it alone for a long season in pots.
Manage heat and root disturbance, both of which can trigger a flush of affected fruit. Ventilate greenhouses and polytunnels early on bright days, and use temporary fleece shade during exceptional heat while maintaining airflow. Avoid letting a hose blast and wash compost away from exposed roots. Do not transplant, deeply hoe or prune roots while fruit is forming. In cooler areas, protect young plants from cold spring nights with fleece or a cloche, but remove covers during mild days to prevent overheating. Plants commonly improve after a short spell of stress, so look for clean, healthy new fruit rather than expecting marked fruit to repair itself.
Make prevention part of your weekly routine. At the same time each day, inspect moisture, lift pots to judge weight, open greenhouse vents and remove damaged fruit. Add a mulch before the first prolonged dry spell, and use a water butt or stored water so you can water consistently during restrictions or hot weather. If blossom end rot returns every year, increase pot size, reduce the number of plants in each grow bag and improve the organic matter in beds. Record which varieties are most affected, as fruit size and local weather both matter. Once conditions settle, many plants produce a perfectly usable later crop.
Frequently asked questions
Is blossom end rot caused by lack of calcium?
Calcium is involved, but a lack of calcium in the soil is not usually the root cause. The problem commonly occurs because dry, waterlogged or damaged roots cannot supply calcium evenly to developing fruit. Stabilise watering and drainage before adding anything, as extra feed rarely solves the issue.
Can affected fruit be saved?
No. The damaged end will not become healthy again, and the fruit is more likely to rot. Remove it promptly and compost it only if it is free from broader disease symptoms. The important aim is protecting younger fruit by keeping the root zone evenly moist from now on.
Why is blossom end rot worse in grow bags?
Grow bags contain a limited volume of compost, so they can heat up and dry out quickly, then become waterlogged after heavy watering. Several plants may also compete for the same moisture. Check them daily in warm weather, water evenly and avoid allowing the compost to swing between extremes.
General guidance for UK home gardens and allotments. Conditions vary by region, soil and season; the RHS or a local garden centre can give site-specific advice. Page updated 19 Sep 2026.