MT Mitchell, Castleton Farm

Using VPD (Vapour Pressure Deficit) for improved irrigation control

Originally dairy farmers, the Mitchell family bought Castleton farm in 1992, part of which was a soft fruit crop of 15 acres. In 1999, they decided to stop dairying and increased their soft fruit production.

Since then, the soft fruit area has grown to over 210 acres of strawberries, raspberries and blueberries.

Two years ago Ross Mitchell, son of founder Murray, was investigating better ways to initiate irrigation cycles for a blueberry crop grown on substrate. He discovered that there was no shortage of conflicting opinion, and many of the available solutions seemed to involve considerable investment in new hardware.

CMW suggested that he might think about using VPD, because Priva offer a VPD irrigation module originally developed for the American market and very popular there. Already a user of a Priva CIS fertiliser dosing system, Ross liked the idea that the Priva VPD module would enable him to initiate irrigation based on the plant's actual need - after all, VPD is a way of measuring the difference between the humidity at leaf level and in the environment around it.

A high VPD indicates that transpiration is unhindered, so by measuring the VPD and keeping a running total of 'VPD sum' the irrigation cycle starts automatically when the plant needs water to maintain active photosynthesis. CMW installed the additional VPD software, a Green Leaf sensor to mimic plant temperature and a wet and dry bulb sensor to measure the temperature and humidity in the tunnel, all necessary to establish the need for irrigation.

As you would expect, there was a short learning curve and some trial and error to establish the ideal VPD sum for the blueberry crop, but before long Ross came to the conclusion that his decision to try Priva's VPD module was the right way to go. 'I'm a grower' says Ross 'and like most growers I can walk into a tunnel and immediately 'feel' whether the crop is happy.

The VPD starts ensure that the crop gets water at just the right time - sometimes the irrigation even runs at night now because the prevailing environmental conditions have kept the crop active. That would never happen with a RADSUM start.' Other benefits that Ross has noticed is that the substrate is NEVER waterlogged now, and he has gradually gained enough confidence in the system to reduce run-off considerably. In fact, the VPD system has been so successful that Ross has now installed the system for his strawberries too.

The VPD module has been successful with every type of crop in the US and Canada, so Priva are hopeful that other UK growers will be inspired to try it out.

What is VPD?


In the strictest sense, VPD is defined as the difference (deficit) between the amount of water in the air and how much it can hold when saturated.

In the Priva module, VPD is expressed as the difference between the vapour pressure in the leaf canopy and the vapour pressure in the surrounding air.

Priva claim that their module provides a direct indication of the water requirement of the crop and that this makes their module unique. 

Systems reliant on a single 'VPD sensor' are not able to do this, say Priva.


The graph on the left was recorded recently over 24 hours at Castleton Farm.

In this example, the graph illustrates how the plant remains active (VPD sum increasing) overnight.

The VPD sum resets automatically after each irrigation start.

  • VPD sum (Red line)
  • Radiation (Green line)
  • Irrigation start (Blue line)

In this example, the graph illustrates how the plant remains active (VPD sum increasing) overnight.

The VPD sum resets automatically after each irrigation start.