VPD Calculator

The relationship between temperature and humidity is extremely important when maintaining water movement through the plant. This relationship is referred to as vapour pressure deficit (VPD; measured in kPa) which compares the amount of moisture in the ambient air, to the amount of moisture in the air surrounding the plant. It is a driving force for transpiration and determines how quickly or slowly the moisture moves out of the plant. Ambient air that is low in moisture is going to draw a lot more water through the plant compared to ambient air with high humidity. The rate that the water is pulled out of the plant's stomata is linked to the amount of water pulled through the stem and up from the roots. The faster you pull moisture through those leaves, the more readily water needs to be replenished in the root system.   

High VPD

•  low moisture content in the ambient air
•  big difference in moisture content between the air and the plant
• water is rapidly pulled from the plant leaves, putting pressure on the roots to transport more water

Low VPD

•       high moisture content in the ambient air
•       small difference in moisture content between the air and the plant
•       water is not pulled from the plant as quickly
•       more vegetative plant

How do I know where I stand?

Dr Greenhouse has formulated a cannabis-specific calculator to help determine where you stand when it comes to your vapour pressure deficit! By plugging in the known temperature and humidity for your grow space, the calculator will let you know if you are on track with where you should be!

The chart posted on the website can be used to help visualize the energy relationships of moist air. By maintaining a VPD in the target zone (shown in green in the chart below), you can rest assured that the air-water relations have been optimized within the plant and discourage vegetative crop growth. Once you start steering towards the danger zones on either side, an adjustment should be made to get back into the target zone. Fluctuation in temperature or humidity is okay as long as the appropriate adjustments are made to keep the system in balance!

Posted by: Talia Plaskett

Getting Into the Weeds: Farming in a Humid Year

Every year, Perennia's Horticulture Team sits down to discuss what major issues, questions and themes came up throughout the growing season. While there are differences between the crops grown here, we are typically able to identify pain points that are applicable to all producers. Last year, with the unexpected hot weather and lack of rainfall, we decided to tackle the basics surrounding irrigation. We wanted to make sure that everyone was on the same page on how to design and implement an irrigation system, for both soil and substrate based systems. For those of you who missed it, or are interested in a refresher...you can find all of these sessions here.

So what were the major challenges of 2021 you ask? 

Most producers can agree that there was significantly more rainfall and a higher humidity measured throughout the growing season than what we would typically experience. So, with that, enter our theme for the upcoming 'Getting Into the Weeds' Webinar series...Farming in a Humid Year!

November 23 - Understanding the Infection Cycle: Targeting Effective Disease Management

November 30 - Year in Review: A Disease Summary of 2021 - What Did We Learn?

January 12 - The Fundamentals of Fungicides

February 3 - Why Herbicides Fail: Targeting Effective Herbicide Application

If you are interested in learning more and/or registering for these free webinars, check out our list of upcoming online sessions. 

Posted by: Talia Plaskett

To Prune, or Not to Prune...

 

Pruning is an important cultural practice which helps to control the humidity in the canopy, contributing to lower overall pest and disease pressure. As with anything, however, there is more than meets they eye when it comes to the best ways to prune your plants.

With the unexpected onset of cool and wet conditions in the Valley, it seems to be an appropriate time to cover some best practices when it comes to pruning your crop. These will help manage the risk associated with Botrytis infection and keep your crop growing strong.

Figure 1. An image of a botrytis infestation on raspberry. Photo credit: Amber Dort, Perennia

·         Be sure to remove the entire leaf and leaf stem, cutting right up to the main growing stem. Allowing residual tissue to stay there makes you vulnerable to botrytis infection, which will travel up the stem stub and into your main stem.

·         Try to prune on sunny days. This will ensure that there is less moisture in the growing space, and will dry up the newly created pruning wounds before temperature drops again through the night

·         Prune your plants first thing in the morning – this gives the plant an entire day to mend the pruning wounds, before temperatures drop, and condensation collects inside the production space

·        Try not to prune too heavily at once. The plant will need some time to adjust to the new source/sink ratio (# of leaves to # of buds), and it is best not to shock the system. If you have some ambitious pruning plans, spread them out over time

·         Pruning efforts are typically concentrated on the bottom of the plant, but don’t forget about the rest of the canopy. If you find there are overly dense sections in the middle or top, do not hesitate to remove one or two leaves to create more air space. Look at the overall plant before pulling leaves off and assess where the plant needs it most.

Regular pruning schedules are highly recommended and will help create a more consistent crop production schedule. Consistency from crop to crop is key, and managing air flow through the canopy is going to drastically reduce the amount of botrytis in the crop. 

Posted by: Talia Plaskett