Blossom End Rot in Tomatoes: It’s a Transpiration Problem

A tomato grower called with a familiar problem: blossom end rot, no obvious stress, irrigation looking fine, calcium levels good. The answer was not in his irrigation schedule. It was in what happened days earlier, on a cold, clear spring night.

Calcium moves through transpiration — and only through transpiration

The day before had been warm and sunny. Substrate was up to temperature. Roots were active. The plant was growing strong. Then the sun went down.

Outside temperature dropped fast. The greenhouse roof cooled rapidly through radiation to the cold night sky. The crop canopy cooled with it. Transpiration stopped.

And when transpiration stops, calcium transport stops with it. Calcium moves almost exclusively through the transpiration stream. No flow, no delivery. The developing fruit cells at the blossom end stopped receiving calcium. Cell walls were weak. The damage was done quickly.

Blossom end rot is not a nutrition problem. It is a transpiration problem.

The mechanism: outgoing radiation on clear spring nights

Outgoing radiation is driven by the temperature difference between the crop and the surface it perceives overhead. On a clear spring night without proper screening, that surface is your greenhouse roof — and it is cold. Canopy temperature drops. VPD drops. Transpiration drops. The roots stay warm, the plant keeps growing, but the calcium is not moving.

What often goes wrong in practice: screens are only partially closed, so the cold roof remains visible to the crop. Venting is stopped entirely to save energy, so humidity builds up and VPD drops further. The climate computer shows relative humidity within range — but the developing truss tells a different story.

What well-managed greenhouses actually do

Growers who manage this well, with a double-screen setup, follow a consistent approach. First, they vent at both wind and lee side — not to cool the greenhouse, but to remove humid air and maintain a VPD that keeps gentle transpiration going through the night. Then they close the upper screen fully and the lower screen to around 80–95%.

Warm greenhouse air rises through the gap in the lower screen and collects between the two layers, heating both screens from within. The crop perceives the underside of the lower screen rather than the cold roof above. Outgoing radiation drops. Calcium keeps moving.

The takeaway

Blossom end rot in spring is not always a fertilisation problem. It can be a climate management problem — one that started on a clear night, days before the damage became visible. By the time you see the symptoms, the cause is already in the past.

Monitoring canopy temperature, VPD, and screen position throughout the night gives you the data to act before the damage is done.

This post was originally published by Timon van Lemmen on LinkedIn. Read the original post →

Want to discuss climate management challenges in your greenhouse? Reach out to Timon or the rest of the Sigrow team at [email protected] (Mon–Fri, 9:00–18:00 CET) or [email protected] (Mon–Fri, 9:00–21:00 CET).

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