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Healthy root zone will help ensure a healthy top zone

The plant is a totally integrated system in which the function of one part affects the performance of other parts.

September 14, 2016  By Dr. Mohyuddin Mirza

October 2016 – Talking about root zone is always fascinating to me because it is the foundation of all greenhouse crops and it is hidden from our eyes. The tops we can easily see but the tops depend on the roots, and the roots depend on the health of the tops!

I last wrote about root zone health in the January 2016 issue of Greenhouse Canada. I discussed various aspects of what happens when inputs in the root zone are ideal and noted that when something goes wrong in the root zone the top of the plant is affected.

In this article I would like to focus on interactions between the environment and root zone performance. It again must be emphasized that the plant is a totally integrated system in which the function of one part of the plant affects the performance of other parts.  


Hot weather was reported in almost all parts of Canada this summer. Many growers observed temperatures inside the greenhouse over 35 C and a relative humidity of less than 50 per cent.

Photos A and B were sent by growers. Photo A showed a collapse of young tissue in a tomato crop. Photo B is of a tomato crop that is being grown after bedding plants. There is no nighttime heat used. The sidewalls are opened during the daytime. One can see serious leaf roll and cat-facing on some of the fruit – even the fruit has bleached somewhat. The root zone consisted of coir in the first case (Photo A) and a soilless growing medium in the second case (Photo B).

Case # 1: The air temperature was around 35 C and there was very high ventilation. I am guessing the relative humidity was around 50 per cent. If you calculate Vapour Pressure Deficit (VPD), it will be 28.2 millibars. This is a very stressful situation for the plants. The VPD will be higher when leaf temperature is taken into consideration. Without any assistance from the grower to humidify the greenhouse air, the message is sent to the root zone to send more water to the leaves. The roots will try their best to send water to the leaves but the roots quickly found out that Electrical Conductivity (EC) was very high. It was around 4.0 mmhos. The roots did their best but could not supply enough water to leaves and those are the symptoms you see in Photo A.

So, here are some points to remember:

  • Understand VPD. This is a calculation based on temperature and relative humidity. It is better to use leaf temperature to get a better reading of VPD.

Most of the vegetable growers can see this reading on their computers but this is based on air temperature. So once in a while check the leaf temperature and relative humidity to calculate this number. VPD tables are available in millibars and also in grams/m3 of air. Know the ranges that are recommended. There are several good publications available on this subject of VPD. Your computer companies can help you as well in adding this calculation to your database. An understanding of VPD helped to diagnose this problem with tomato tops.

Photos C and D are screen shots showing VPD graph (yellow line). The time is plotted on the Y axis and VPD in millibars on the X axis. In Photo C, the VPD was out of the recommended range of between 8 and 10 millibars from 11 a.m. until 5 p.m. when plenty of light was available. This means the plants were not “working” or doing their business.

Basically, what happens is that photosynthesis is reduced, respiration increases and any food reserves in the leaves are utilized to keep the leaves cool. Growers must assist the plant under such conditions. I recommend introducing plain water when VPD goes over 10 millibars or over 8 grams/m3 of air.

The VPD graph in Photo D is relatively better. The plants were working for a longer period of time compared to the graph shown in Photo C.

Growers may have noticed that under very high VPD conditions, the EC in the root zone could jump by one to two units.

I have seen such sudden jumps in tomatoes from 2.5 to 4.5 millimhos and increasing the leach percentage with the feed solution doesn’t seem to help.

  • Understand the role of EC in water uptake.

Higher EC values mean less water is available in the root zone to be transported to where it is needed. Under such high VPD conditions, that is, above 10 millibars, plant roots will absorb more water and leave the nutrient ions alone. If more water is extracted from the root zone, then the EC will rise. I have seen more development of blossom end rot of tomatoes under such situations of high VPD.  

Case # 2: I want to refer back to the first set of pictures and Photo B. This crop is being grown under “bare minimum” conditions. No control on climate. There is no heat to maintain proper night temperature. There is no system or effort to control daytime temperature. Many bedding plants growers follow this practice after the space is empty.

One can see from the picture that plants become very generative. Leaves roll mostly inward and fruit is highly exposed. The fruit starts ripening before it reaches full maturity.

In such situations, the root zone has to work hard to keep up with the supply of water and nutrients. Many symptoms on the leaves, which look like phosphorus deficiency (purple spots) and magnesium deficiency, become common. The EC in this case was over 5.0 millimhos.

Growers just let it go, primarily due to the fact that they can harvest and sell fruit through their own channels. I think paying some attention to root zone EC, shading the greenhouse roof and using any other practices to improve the climate can produce better quality tomatoes.

The point we’re making is to understand the relationship of the root zone with the top zone. Roots depend on food from the top and tops depend on roots for water and nutrients. Keep that relationship healthy.

Dr. Mohyuddin Mirza is an industry consultant. He can be contacted at You can also talk to him next month at the Green Industry Show and Conference, being held Nov. 17-18 at the Edmonton EXPO Centre.

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