Ask a Specialist Answer

Question

I need to create some fast, inexpensive greenhouse space. What do you suggest?

Answer

In 2015, we bought a farm with some infrastructure but no greenhouse. We grow flowers and vegetables. Because we were low on capital, but high in experience and knew how to “hack” together season extending systems, we chose to build two souped-up caterpillar tunnels for our greenhouse space. We didn’t want to build anything permanent until we had a couple of years under our belts farming our new land, in order to know more about the patterns of the wind and sun. Additionally, we have really felt the effect of climate change more than ever these last two years, and it seems like the pattern that’s emerging is extreme weather events. We want to build a resilient farm with good systems, and inexpensive, easy-to-fix structures.

The most protected space on our farm, which sits high on a windy ridge, is tucked in southeast of our big pole shed. We can fit two 10 x 60 structures in this protected space. To make our temporary greenhouse more secure in the high spring winds, we spaced the PVC bows closer together than is usual, and added more rope to hold down the plastic. We also added some Lexan polycarbonate sheeting remnants that we received from a generous friend. We used those below the hip board to add stability.

Another great, easy structure for this same purpose that’s even more secure can be made with galvanized steel chain link fence rails bent as bows that are more sturdy than PVC. (Learn more about this structure at www.youtube.com/watch?v=5yBHaNqrbtI .)

We rolled the sides up and down depending on the weather, and entered through one place on the wall. Doors are better and make a more comfortable entry, but we got in and out without too much trouble.

Inside this house, we built 2-foot high tables with wood frames and polycarbonate panels offset for drainage. The tables are balanced on cinder blocks. We tried tables covered in poultry wire, but these were too unbalanced; the wire would inevitably sag in one area. Instead of heating the air, we heat the tables that the plant flats sit on. It’s more efficient all around, and especially in this hack system that isn’t extremely airtight.

We’ve tried table-heating two ways: first with electric heat mats and, second with radiant roll-out heat mats that operate with an on-demand tankless water heater system that runs on propane. Water, mixed with glycol, runs through the tubes in the heating mats within this closed system that keeps the water temperature between 90 – 120 degrees F, a relatively small differential. Keeping the soil temp level and warm enough does far more for good germination and the early days of seedlings than air temp.

We have an exhaust system, and a few makeshift fans in the greenhouse, but the downside of this system is definitely controllable air-flow. For our early growing purposes it hasn’t mattered. Also, with climate change bringing earlier spring, by the time good air flow makes more of a difference, we’ve been able to open up the sides and allow the spring winds to blow through.

Our favorite low-cost system for starting plants early (February) is to build a mini tunnel inside the larger greenhouse. We attached electric conduit (available at home improvement stores for about $2.50 for 10 feet) to the tables, and put plastic over the conduit for a mini greenhouse inside the larger greenhouse. On super cold nights (-10 outside) we threw some sheets and blankets over the whole structure and kept the soil temps above 55.

System Costs

The average cost to build a caterpillar tunnel is about $700. The set up for the electric heat mats was about $600. We could fit 22 flats on the mats. The cost to build the radiant roll-out heat mats that operate with an on-demand tankless water heater is about $700, not including the mats. We got mats from a former farm for a really good deal. They’re also available online through BioTherm along with the hardware needed to set up the system. We have run about 700 square feet of tables on one water heater, fitting about 350 flats. The folks at BioTherm would be able to help you work out what you need.

I can also share a couple of watering hacks that have improved our farm life:

Bottom Watering

We set our newly seeded and just germinated trays in open flats without drainage, then fill the flats as needed for watering. It’s a little tricky to get the balance of how much water to add to the flats given cloud cover, but with a little time and practice you get the hang of it. This improves germination as we seed many flowers that sit on top of the soil and require light to germinate. It also prevents disease and fungus because we aren’t spraying water on the leaves and top of the soil.

Overhead Watering

Once the plants are large enough to handle a little more adversity, we hook up the PVC pipes with 1/16-inch holes drilled into for easy overhead watering. We only do this when we get to the point where we need to be in the field and are spending less time in the greenhouse. It’s not ideal because the plants aren’t getting the same amount of water, but it’s a functional hack.

There are many, many more greenhouse hacks especially in larger systems. These hacks have been great solutions to challenges on our new farm without requiring a lot of capital.

Posted: Jun 2017
Answer By: Jennifer Nelson