This roundtable was originally published December 4, 2014.

About the Upstart Farmers Roundtables

Upstart Farmers Roundtables are an informal, biweekly event. Using Google Hangouts video calling, a small group of Upstart Farmers comes together to share their experiences and learn from each other. Each week is guided by a discussion topic, with growers welcome to ask their own questions.

Roundtables are exclusively for Upstart Farmers, and recordings are only released to the Upstart Farmers Community.

Roundtable: Greenhouse Coverings

Today’s discussion is about greenhouse coverings. I apologize that the video of us all talking did not record, however you can still hear us and see the agenda. 

Types of Coverings

• Glass
• Acrylic (really high end, 50 year rating, premium for keeping heat in)
• Poly-carbonate (20-30 year rating)
• Polyethylene film (Our recommendation! Really cheap, really available, easy to patch, easy to fix, hail and wind resistant, better R-value than most others (unless talking about triple wall poly carbonate. We’re definitely a fan, its hard to beat.)
• Fiberglass (poor R-value) 

What are the 4 things to consider when choosing a greenhouse covering?

1.  Longevity – How long do you intend to use the facility? 
2.  Light transmission – How much light will it filter out?
3.  Labor costs associated with having material.
4.  R-value – insulation value of covering (i.e. the lower the R-value, the more heat loss that will occur through the material.)

What coatings are there?

• IRR coating – infrared spectrum coating helps prevent infrared heat loss (reflects infrared wave lengths back in) We’ve used SunMaster brand, but really just choose whatever’s cheapest with the coating; it’s not that fancy so no need to splurge on a specific brand. Just remember to have the coating facing inside.
• Anti-condensate coating – As the name implies this prevent condensation. Make sure this is also facing inward not out. This coating helps keep water as humidity instead of it turning into condensation. However, condensation will happen still with those no matter what.

Tips & Tricks

• We pump air in from the outside for our double layer inflation because it’s less humid in our region.
• If you’re dealing with condensation between layers, you can punch tiny weep holes down near where it fits into the wiggle wire. The blower will be constantly inflating and pushing air out through weep holes. This will also push out the condensation.
• Always go with a double-wall covering unless you’re in the tropics!
• When installing a covering with coatings, look for the name brand printed on plastic. Install it so that the name brand is not backward when you’re reading it from the outside.
• Recommended humidity levels are 50-70% in almost any growing environment.
•  Diffused light is better received by plants that direct light. New rigid or semi-ridig polyethylene covers will deliver diffused light to the plants. Plants like diffused light, especially in towers where shadow effects have larger effect on plant growth. Having a diffused covering is pretty awesome and gives much more consistent production from top to bottom of the tower than you get under direct light in a regular covering.

Cold Weather Climates/ Cold Weather Tactics

• Get a roll-over thermo-blanket (inside or outside) to keep heat in at night. It’s essentially polyethylene bubble-wrap with Mylar. These will help you achieve an R-value around 5 or 6.
• Bump your greenhouse up to a R-value around 4 by using polyethylene with 18 inches of inflation (as per Haydn’s technique).
• Bump water temperature up at night to deliver supplemental heat to the plants overnight, rather than blowing it in with a heater entirely. This is cheaper and will gives you more buffer time between a heating system failure and crop failure. Remember – don’t overheat the water! 70-75 degrees Fahrenheit is reasonable.
• Geothermal is an option that’s probably more effective in western climates that the east. If you have moist soils and high water tables, steer clear of this.

Tropical Climates

• Strong poly films get dirty really fast and are hard to clean in the tropics. High humidity causes plant matter to try to grow on top of the greenhouse, causes yellowing, etc.
• Other types of films are weaker and can get holes faster in these climates.
• Vent! Vent! Vent! Use insect screen around all walls of the greenhouse, and the roof.