Building a Nursery GREENHOUSE Out of a Caterpillar Tunnel

When you are leasing land, or on a tighter budget, constructing an adequate nursery space can be a challenging prospect. While a beautiful high tunnel with cement floors, a drainage system, permanent plumbing, polycarbonate end walls, and automated heating and cooling systems would be IDEAL, it’s often not within reach–at least when starting out. 

When I was faced with this dilemma at the farm that I leased in Petaluma, California, I decided to purchase a portable caterpillar tunnel from Farmer’s Friend LLC., anticipating that whatever I built would likely need to be moved at some point in the future.

Unfortunately, the first 100’ caterpillar tunnel I constructed was lost to high winds in an extreme weather event. I had just finished constructing the tunnel, complete with wind bracing on both ends, and a fixed center purlin that ran the length of the tunnel. Wiggle wire fastened the plastic to both end hoops. All that was left were end walls.

An unexpected storm arrived in the fall before I had time to build my end walls. I had intended to install scissor doors within the next few days. When the storm arrived, I learned that my site was too level, not allowing for a high volume of rainwater to adequately drain away from the footers of the tunnel. An insanely high volume of water combined with freakishly high gusts of winds created the perfect storm. What seemed to be firmly packed footers became instantly flooded mush with inadequate drainage. Then the gusts of wind swept through the open ends of the tunnel and ripped it upwards. The plastic was destroyed, and the hoops were lifted up and out, with about 25% of them being bent and mangled. 

Using the hoops I was able to salvage from the 100’ tunnel that was lost, I was still able to put enough materials together for a 35’ nursery tunnel, and a 25’ wash/pack tunnel. I wanted to build a nursery tunnel that was extremely heavy-duty, but still fairly easy to take down if we needed to move. Consequently, I didn’t want to cement in the footers of the hoops. However, it still needed to be extremely sturdy and stormproof. I was determined that I would be ready for the next storm, come hell or high water.

Design: After researching, I found an instruction manual on  the Johnny’s Seeds website called the “7300 Quick Hoops Gothic High Tunnel Bender.” Eliot Coleman (one of my biggest influences) assisted in the design, so I figured it would be a wise choice. It was for a 14’ wide tunnel – the same width as my Farmer’s Friend tunnels.

This Coleman design had heavy-duty 2”x6” baseboards and hip-boards, and framed-out end walls. It would take an apocalyptic event to blow this thing away!

With a convenient downloadable expense calculator which allowed you to customize the dimensions of your tunnel, I was able to build the materials list and begin construction. 

I began by hiring a local guy with a small front-loader tractor to help me level the site and scrape it clean from any sod. The site was slightly uphill from the farm plot, so I knew there would be good drainage and no standing water in a storm.

After the footprint was cleared and leveled, I purchased landscape fabric, pinned it down, and left about a foot of fabric to extend beyond the perimeter of my tunnel, as a buffer for sod that would inevitably want to creep back in. 

I’m pretty handy, but not a full-on carpenter by any means, so I decided to hire a guy to help double-time the job. With 2×6 baseboards and hip boards, and lumber-framed end walls, we were shaping this thing up to be ROCK-SOILD! I wasn’t worried at all about it being blown away.

We used the bones of the design for general framing, and then made alterations for my context (making it a nursery rather than a production tunnel). The front wall had heavy-duty latching double doors (built according to the manual), but with a custom framed square above them to house the coming 24” ventilation fan, made by J&D Manufacturing. The opposite end wall had no doors. For that wall we framed in two openings (near the peak of the tunnel), to accommodate two side-by-side 30” ventilation shutters (aka “dampers”), along with the motorized openers.

For those unfamiliar with greenhouse ventilation kits, you typically have ventilation shutters on one end wall of the tunnel (where the air is drawn in), and then a ventilation fan on the opposite end wall (that sucks the air out). A thermostat is then wired into the system, which simultaneously triggers the shutters to open on one side, and the fan to turn on at the other side. This draws in fresh air from the outside and blows out the stagnant or hot air on the other side, creating cross ventilation. It’s amazing how fast the temperature will drop in a tunnel when the fans kick on, and the shutters open up!

HAF fans (which stands for horizontal airflow) can also be installed incrementally across the length of the house to help move the air through the tunnel from one end to the other. For my 35’ tunnel, I purchased two 12” HAF fans, and installed them overhead to the hoops using mounting brackets.

I ended up hiring an electrician to wire up the nursery for me. We needed to run power from our house to the nursery tunnel (about a 50’ run), and then GFI outlets in the tunnel, along with wiring up the ventilation fan, the thermostat, and the motorized shutter openers. There’s only so far you can go with Youtube tutorials. I had wired things up before, but this seemed a little out of my jurisdiction. It was an expense I couldn’t avoid. In retrospect, I’m really glad I outsourced. The time I would have had to invest in learning (and then executing) would have outweighed what I ended up paying the electrician. I dug the trench for the ground wire from the house to the nursery, and then let him do the rest. 

The water source for the nursery tunnel was supplied from a reservoir on the back of the landowner’s 40-acre property. The only issue with it was that it was full of algae. I had to install a heavy-duty 2” large capacity disc filter (made by Rain Bird). It did the job but needed to be rinsed out once a day so the pressure wouldn’t drop from the restricted flow.

After running the water line into the nursery, I decided to go with the Hi-Hose Watering System from Johnny’s. It’s sold as a kit that includes everything you need: a galvanized wire cable to be mounted overhead running the length of the tunnel, swivel pulleys,  S-hooks, and all other required hardware. 

The system allows you to have your water hose out from underfoot, and able to be conveniently pulled to any space in your nursery via an overhead pulley system. No kinks. No dragging. No tripping. It was worth every penny!

I also went with the “Wonder Waterer” watering wand from Johnny’s for an even gentle spray for the seedlings, followed by a heavy-duty brass in-line shutoff valve, made by Dramm.

I found an amazing deal on Craigslist for expanded metal 4’ wide greenhouse tables that ran the length of my tunnel on either side. With metal frames and legs, they would prevent the occasional rodent from climbing up onto the tables. I had heard many horror stories of rodents climbing up wooden legs onto greenhouse benches and destroying freshly seeded flats. 

I designed my tunnel with roll-up sides to allow for more ventilation when needed during the warm season. This lightens the load for your ventilation fan and also saves you on your electric bill. 

I went with the Sidewall Hand Cranks, also from Johnny’s, and was really happy with them. Mounted on a 3/4” metal conduit pipe, this crank and handle allow you to easily roll up or down your sidewalls when needed.

Finishing the nursery tunnel was a great feeling of accomplishment! It was a little on the big side for what I needed immediately, but that was by design. I started growing microgreens on the tables to take advantage of the space when it wasn’t being used for transplants. I added some shelving for storage, a second-hand germination chamber, and built a custom soil-filling station for preparing flats (more on that in a later post).

All in all, I felt it was the perfect nursery greenhouse for my context there at Winding Road Farms in Petaluma, California.  You can see images of the tunnel on my old account @windingroadfarms on Instagram.

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