Greenhouse climate control

Greenhouse climate control looks like temperature management and is really transpiration management. Almost every apparently strange behaviour in a well-run house - heating with the vents open, closing screens on a bright day, running fans when the temperature is already correct - makes sense once transpiration is the target.

Why transpiration is the target

Water evaporating from leaves does two jobs. It cools the leaf, and it drives the flow of water and dissolved minerals from root to shoot.

The second job is the one that causes trouble. Calcium moves almost exclusively in that transpiration stream, and it does not redistribute once deposited. A plant that stops transpiring stops delivering calcium to its newest tissue, and the newest tissue is precisely what needs it: growing points and developing fruit. That is the mechanism behind tip burn in lettuce and blossom end rot in tomatoes and peppers, both of which are usually climate faults rather than soil calcium deficiencies.

Too much transpiration is also a problem: the plant closes its stomata to avoid water loss, and closed stomata cannot admit carbon dioxide, so photosynthesis stops.

VPD, and why it replaced relative humidity

Relative humidity is a poor control variable because it changes with temperature even when the absolute moisture content does not. Air at 80 percent humidity means something quite different at 15 degrees than at 28.

Vapour pressure deficit expresses the same physics in a way the plant responds to: how much more moisture the air could hold at its current temperature. Controlling to a VPD target keeps transpiration steady as temperature moves through the day, which is what the crop needs.

Values around 0.8 to 1.2 kilopascals suit most crops in active growth. Propagation and rooting want lower; anything above roughly 1.5 begins to cost photosynthesis.

Screens are the boring recommendation that pays

In any climate with a real heating season, movable thermal screens are usually the fastest-returning greenhouse investment available. They cut radiative night-time heat loss substantially for a fraction of the cost of reglazing, and modern screens serve as daytime shading and as part of humidity strategy.

The complication is that a closed screen traps humidity beneath it, which is why screen control and dehumidification strategy have to be designed together rather than separately. A gap left in the screen, or a scheduled brief opening, is the usual answer.

Sequencing the limits

Plants are limited by whatever is scarcest. Adding more of something that is not limiting produces nothing, which is where a lot of greenhouse spending goes wrong.

The usual order is light, then temperature, then CO2, then water and nutrients. On a dull winter day at high latitude, light is limiting: enriching CO2 will not help, and neither will more heat beyond what prevents disease. On a bright day in a sealed house, CO2 becomes limiting quickly and enrichment pays well.

Knowing which limit is currently binding is more valuable than any individual piece of equipment, and it is why measurement - covered for the outdoor case in weather stations - matters as much under glass as outside it.

Related

Irrigation under cover is fertigation and shares its control logic with irrigation automation. The fully enclosed case, where light is supplied rather than admitted, is covered in vertical farming and grow lights.