Structures & Equipment
Inside View and benefits of screening
By Gary Jones
Growers weigh in on screening benefits
By Gary Jones
March 2016 — Climate here is a funny old thing, and less predictable than it used to be. But we still at least expect January through March to be colder than summer and plan accordingly. So it is that many greenhouse producers are routinely using screens to reduce energy costs.
Starting as a response to rapidly increasing energy costs, some producers already have a decade’s worth of experience. So, let’s see what growers have learned. Remember though, this definitely has a West Coast flavour!–
Do screens give the energy savings hoped for? Tomato grower Gord Yakel, of Delta View Farms, says that “numbers said we’d save 30 to 40 per cent energy costs. Of course we have to balance this with light being blocked by closed screens, and that as the screen gets dirty it blocks more of this light.”
Laura Bryce of Village Farms, Delta, says, “Over the last 15 years, our average gas usage is about 25 per cent less when using screens.” But “a bit of that is because we’re more conscious of not putting in pipe temperatures over 60 C ever, where 10 years ago people could be using pipe of 75 C. Village Farms typically runs the screen from planting (Week 50) to Week 20, and then again in Weeks 38-48, averaging about 1,500 screening hours per year.”
Do screens bring other benefits? Indeed they do. “Some growers use energy screens for shade in summer weather extremes to reduce crop stress and keep plant temperatures at achievable targets,” Bryce says.
At Delta View, Yakel says “getting the crop in the first week of January, young plants can get 600 joules. To utilize all this energy, I may need to run high 24-hour temperatures and the screen allows me to achieve this without raging hot pipes. I can get away with 50 to 55 C night pipes, whereas without a screen the pipes would be 70 C+ and those really hot pipes are a vegetative influence on the crop.”
Do screens bring any challenges? At Village, they add at least another weather-dependent decision to the day! Also, Bryce says growers have to decide on whether “to gap or not to gap? Some growers gap the screen to reduce air temperature, some prefer to vent above the screen. The point is, don’t gap if you’re creating cold strips in the greenhouse.”
Yakel agrees: “One really nice aspect of screens is being able to vent above it – especially when it’s cold outside. The screen prevents cold air dropping onto plants, but venting helps to create great climate/humidity levels that plants love. I’ve learned to use it cautiously. I mean there are days where it’s a no-brainer. On cold, rainy days when nothing is happening, keep it closed and save energy. But I’m quick to open the screen when outside temps are above 9 to 10 C. And I open it quickly as light intensity increases to utilize as much light as possible.”
He also adds that “when the vents are open in summer months, dust and dirt accumulates on the screen (even though it’s open and compressed against the truss), and figuring the best way to clean it is a challenge.”
What else have growers learned? Yakel advises that “in the beginning we were so focussed on saving energy that we actually over used it (keeping it closed for too many hours each day). We created some pretty humid climates, and saw a lot of botrytis that first year. We did come in under energy budget though,” he chuckles. “It’s crucial to be aware of how it’s affecting greenhouse humidity. If it’s too humid, you’re using it wrong: put a crack in it (maybe two per cent crack), open your windows, adjust pipes, or stop using it!”
Yakel also stresses the importance of good installation using quality materials. “Proper installation is really important in how well the screen performs. When it closes, it should be sealed tightly against the trusses and edges. Gaps create areas where cold air spills down onto plants, leading to unevenness.”
Would you do it again? “Over the years, it’s been a very useful tool for Delta View,” Yakel says. “When used properly, screens really can help create a nicely balanced crop.” So, I guess that’s a “yes!”
We could chat about screens and bees and fires and yield, and… but perhaps we’ll revisit those in another 10 years.
Gary Jones is co-chair of horticulture at Kwantlen Polytechnic University. He serves on several industry committees and welcomes comments at Gary.Jones@kpu.ca.