Question 1

What is the dew point in a greenhouse?

Accepted Answer

It is the temperature at which the air can no longer hold its water as vapor, so condensation starts. It depends only on how much vapor is actually in the air, not on how warm that air is. Anything colder than the dew point gets wet: the cover, a screen, a gutter, a leaf.

Question 2

What is dew point depression?

Accepted Answer

Air temperature minus dew point. At six degrees of depression every surface has room to spare before it condenses. At one degree, the coldest leaf in the compartment is already on the edge of going wet.

Question 3

Why does condensation form on the crop at night?

Accepted Answer

Radiation warms the crop during the day and transpiration cools it, so crop temperature rarely matches air temperature. On a clear night without a screen the crop radiates away to a cold sky and drops below air temperature. If humidity is high the depression is small, so the coldest and most exposed parts of the canopy cross the dew point and go wet while the air sensor still reads a comfortable RH.

Question 4

How does dew point relate to Botrytis risk?

Accepted Answer

Fungi need free water on the leaf, and free water appears wherever a surface falls below the dew point. Botrytis does well when humidity is high and temperature is uneven across a compartment, because the cold spots condense while the rest of the house looks fine. In unmanaged greenhouse strawberry, Botrytis accounts for roughly 23.7% of post-harvest loss.

Question 5

Do anti-drip cover coatings prevent condensation?

Accepted Answer

No. They spread the condensate into a film rather than droplets, which stops the dripping and recovers some light, but the cover still condenses whenever it sits below the air dew point. Screens and a double cover keep the crop warmer and more even, which widens the margin.

Dew Point Calculator for Greenhouses

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