How To Build a High-Tunnel Greenhouse

A high-tunnel greenhouse is a great way to extend your growing season and protect your plants from harsh weather. This simple, unheated structure can help you grow vegetables all winter long, making it valuable for gardeners in colder climates.

In the video above, Ask This Old House landscape contractor Roger Cook shares how he helped construct a basic high-tunnel greenhouse for a homeowner. Below is your step-by-step guide to building one yourself, along with tips for successful year-round gardening.

Building Your High-Tunnel Greenhouse

Creating your own high tunnel greenhouse is a relatively simple project that you can complete in a weekend. Follow these steps to get started, and be sure to gather all the materials and tools ahead of time to keep the process smooth.

Materials and Tools Needed

Here’s what you’ll need for the project:

• 4-mil plastic sheeting
• Auger
• Circular saw
• Cordless drill/driver
• Galvanized pipes
• Hinges and latches for doors
• PVC pipes (1-inch-diameter, 20 feet long)
• PVC pipe saw
• Pressure treated 2-by-4s and 2-by-2s
• Screws and washers
• Small sledgehammer
• Staple gun
• U-shaped metal pipe brackets
• Utility knife

Preparing the Site

Picking the right location and preparing it is key to your greenhouse’s success. Here’s what Cook suggests:

1. Choose a level area with good drainage and plenty of sunlight.
2. Clear the area of any debris or vegetation.
3. Mark out the dimensions of your high-tunnel (in this case, 10 feet by 12 feet).

Building the Base Frame

The base frame gives your greenhouse structure and stability. Follow this process:

1. Cut your 2-by-4s to size for the base frame.
2. Assemble the base frame using 3-inch screws at the corners.
3. Use a drill and auger to bore 1 1/2-inch-diameter holes into the ground inside each corner of the 2-by-4 base.
4. Bore two more holes, evenly spaced, along each long side of the 2-by-4 base.
5. Position the base frame on the prepared site and make adjustments as needed to make sure it’s level.

Installing the PVC Hoops

The curved PVC hoops form the main structure of your high-tunnel:

1. Attach reducing couplings and short sections of 1 1/4-inch PVC pipe to each end of the 1-inch-diameter PVC pipes.
2. Glue on the parts with PVC primer and cement.
3. Slip one end of the PVC pipe onto one of the galvanized pipes. Then bend the PVC and slip its other end onto the galvanized pipe protruding from the opposite corner.
4. Repeat the previous step to install the three remaining PVC pipes.
5. Secure the ends of each PVC pipe to the 2-by-4 base with U-shaped metal pipe brackets. Attach the brackets with 1 1/4-inch screws and washers.

Constructing End Walls and Doors

Strong end walls and functional doors are important for keeping your greenhouse secure:

1. Build two doorframes out of 2-by-4s, then screw one to each end of the 2×4 base.
2. Cut diagonal 2-by-4 braces and install one to each side of both doorframes.
3. Cut two long 2-by-4 beams to span the length of the greenhouse. Screw the ends of the beams to the doorframes.
4. Build two doors out of pressure-treated 2-by-2s.
5. Set the doors in the doorframes and attach each one with three 3-inch butt hinges.
6. Screw a short wood block to the doorframe to act as a latch.
7. Make sure the doors fit properly and open and close smoothly.

Covering the Structure

Finally, properly covering your high-tunnel greenhouse will protect your plants from harsh weather:

1. Drape the 4-mil plastic sheeting over the entire PVC pipe frame.
2. Secure the plastic to the 2-by-4s using 1/4-inch staples, spaced 3 inches apart.
3. Staple the plastic to the doorframes and doors.
4. Use a utility knife to carefully cut the plastic around each door opening.
5. Check that the plastic is taut to prevent sagging and wind damage.

Maintaining Your High-Tunnel Greenhouse

Make sure to maintain your high-tunnel greenhouse so that it lasts and remains effective. Here are some best practices.

Temperature Control

Maintaining an ideal temperature is key for plant health. We recommend doing the following:

• Install a thermometer to watch for temperature fluctuations.
• Monitor the temperature inside your high-tunnel greenhouse, especially on sunny days.
• Open doors or roll-up sides for ventilation when necessary to prevent overheating.

Structural Maintenance

Regularly check the frame, PVC pipes, and plastic covering for any signs of damage. Repair or replace any damaged parts as needed.

Pest and Disease Management

Look out for pests or rodents that could harm your greenhouse. To keep pests and diseases at bay, do the following:

• Use integrated pest management techniques to keep your plants healthy.
• Rotate crops to prevent soil-borne diseases and pests from becoming established.

Tips for Successful High-Tunnel Gardening

Make the most of your new high-tunnel greenhouse with these helpful tips:

• Add thermal mass: Add thermal mass, such as water barrels, to help regulate temperature fluctuations.
• Choose the right plants: Opt for hardy vegetables and herbs that thrive in greenhouse conditions. These include tomatoes, peppers, and basil.
• Use crop rotation: Rotate your crops annually to maintain soil health and reduce the risk of soil-borne diseases.
• Install irrigation systems: Set up a simple drip irrigation system to provide consistent watering and conserve water.
• Use raised beds or containers: Improve soil control and drainage by growing plants in raised beds or containers.