Blossom End Rot in Tomatoes: Causes and Prevention Guide

Illustration of tomato plants in pots with bamboo supports

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Blossom end rot in tomatoes is a physiological disorder characterised by a dark, sunken patch appearing at the bottom of the fruit. Unlike blight or other fungal infections, it is not a disease caused by a pathogen, but rather a result of a localised calcium deficiency within the fruit. This deficiency typically occurs when the plant cannot transport calcium from the soil to the developing fruit, usually due to inconsistent watering or adverse environmental conditions.

Table of Contents

Identifying the symptoms of blossom end rot

The primary symptom is a circular, leathery patch located at the blossom end of the tomato (the end furthest from the stem). This patch can vary in colour from a greenish-brown to a deep black. As the condition progresses, the affected area becomes sunken, giving the bottom of the tomato a flattened or concave appearance.

The size of the affected area varies depending on the severity of the deficiency. In mild cases, the spot may only be 1cm in diameter, while in more severe instances, the blackened patch can exceed 2.5cm. Once these symptoms appear, the affected fruit cannot be cured or reversed, and removing damaged fruit lets you concentrate on protecting later fruit. Discard fruit with mould or spreading soft rot.

The role of calcium and water transport

While blossom end rot is caused by a lack of calcium in the fruit, it is rarely because the soil itself lacks calcium. Most garden soils, potting mixes, and growing bags contain sufficient calcium levels. The problem is one of transport rather than availability.

Calcium is moved through the plant via the transpiration stream—the flow of water from the roots up to the leaves. Because the fruits are distant from the root system and do not compete as effectively as the foliage for water, they are the first to suffer when the flow of water is interrupted. If the plant cannot move water efficiently, calcium cannot reach the blossom end of the fruit, leading to the collapse of cell membranes and the characteristic sunken rot.

Primary causes of calcium deficiency

Several environmental and management factors can disrupt the movement of calcium within a tomato plant:

Irregular watering: This is the most common cause. Allowing the soil to dry out completely and then saturating it creates a fluctuating water supply that prevents a steady flow of calcium.
Limited root space: Plants grown in pots or growing bags are at higher risk than those in border soil because containers dry out more rapidly and are more prone to moisture fluctuations.
High humidity: In greenhouses or polytunnels, very high humidity can reduce the rate of transpiration. When leaves lose less water to the air, the roots draw up less water, which in turn reduces the amount of calcium transported to the fruit.
Fertiliser imbalances: Applying liquid fertiliser to bone-dry soil can restrict water uptake. Furthermore, certain fertiliser ingredients, such as ammonium salts, can compete with calcium for absorption at the root level.

Prevention and management strategies

Prevention focuses on maintaining a consistent environment to ensure the uninterrupted flow of water and nutrients.

Consistent moisture management

The most effective prevention is to keep the soil or potting compost consistently moist. During peak summer heat, this may require watering two or more times per day. Water enough to moisten the root ball evenly and check how quickly it dries; repeated tiny surface splashes may leave the roots dry.

Improving water retention

To reduce the frequency of watering and stabilise moisture levels, gardeners can use the following methods:

Mulching: Applying a layer of organic matter, such as well-rotted garden compost or manure, at least 5cm thick over the soil surface helps conserve moisture and suppress weeds.
Larger containers: Using larger pots provides a greater volume of soil, which holds moisture more steadily than small containers.
Automatic systems: Drip irrigation or leaky pipes can provide a steady, slow supply of water, reducing routine watering work, provided flow, coverage and blockages are checked.

Environmental controls

For those growing in greenhouses, managing the internal climate is essential. Ensuring good daytime ventilation helps regulate humidity levels, which maintains the transpiration process and encourages the plant to draw up calcium-rich water from the roots.

Common misconceptions and ineffective treatments

Because the condition is linked to calcium, many gardeners attempt to apply calcium directly to the plant. However, certain methods are ineffective:

Foliar calcium sprays: Applying calcium nitrate or other calcium salts as a leaf spray is generally ineffective. Calcium is not readily absorbed by the fruit through the leaves, and the transport from the foliage to the fruit is poor. Additionally, the nitrates in these sprays can encourage excessive leafy growth, which may actually divert more calcium away from the fruit.
Adding lime to calcium-rich soil: Since most soils already contain enough calcium, adding lime to the soil rarely solves the problem if the underlying issue is inconsistent watering.

Next step for gardeners

Check root-zone moisture gently beside the plant without severing roots. Adjust watering to keep the compost evenly moist and ensure drainage remains clear; a suitable surface mulch can help where there is space.

Useful products for this job

The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.

Organic garden compost mulch: Used to cover the soil surface to conserve moisture and prevent the wet-dry cycles that cause blossom end rot. Ensure the compost is well-rotted to avoid burning plant roots. Search Amazon for Organic garden compost mulch.
Drip irrigation kit: Provides the consistent moisture levels required to maintain the calcium transport stream in tomatoes. Check if the kit includes a pressure regulator or timer for automated consistency. Search Amazon for Drip irrigation kit.

Related garden guides

Sources and further guidance

Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.