Are you itching to get planting but the soil isn’t warming fast enough?

Growing under the cover of a low tunnel is the answer to jump start your spring vegetable garden. Most of the veggies shown here were started under a DIY low tunnel.

Big, leafy vegetable plants in the garden.

These temporary structures are basically ‘low to the ground’ greenhouses, hence the ‘low’ tunnel. The simple, arched structures are efficient at trapping passive solar heat and holding it in the soil. In Minnesota we often get snow into April, so the reinforced arches are key to keeping plants happy during spring storms.

Inexpensive and easy to build, low tunnels can be popped up anywhere, so they’re perfect for small space gardens. They also work well with crop rotation, as you can move where you’re growing your earliest crops each season.

Learn about Succession Planting – replanting more crops into the same place, something you can also accomplish when you start earlier.

Of course, you’ll also need to start with the right seeds. Many plants will start germinating at temperatures around 40–45F. For more information on my favorite crops to start early in the season, check out my Substack article on Starting Seeds Earlier.

Low tunnels extend the growing season both in the spring and fall. They can be put up again in the fall to keep cold hardy crops in the garden longer.

You can easily remove all the plastic on planting day once the soil has warmed up, giving you easy access to the soil for better planting.

Early spring planting after removing the plastic from a low tunnel.

Soil Temps

Let’s talk about the difference between air and soil temperatures. It is soil temperature that you need to pay attention to for germinating the earliest plants. Luckily for seedlings and plant roots, soil temperatures are way more consistent than air temperatures.

Having a dedicated soil thermometer to track soil temps will help you keep a handle on things. I actually have a few soil thermometers so I can keep them in the soil (which gives more accurate readings) in different places. This helps me track my soil temperatures in different garden beds.

Even after a spring snowfall, the sun can still warm the soil under the clear plastic!

Low tunnel almost buried in snow.

Growing Under Cover

Covering right on the surface of the soil will help hold in a little heat, but bringing that covering up off the ground so the sun’s rays get trapped inside helps even more. Angling the plastic towards the sun or having it arched will allow in the most heat. The thickness of the plastic (called poly in these applications) makes a difference as well. The thicker the poly, the better insulation value it has, but it might reduce the percentage of light coming through.

After pounding in the rebar, slid the PVC pipe over to create your arch.

Building a DIY Low Tunnel

First: Gather Materials

Remember to use what you have on hand and only buy what you need.

  • 18”-24” sections of ¼” Rebar or any solid ¼” stake (I use 6 in my 4X8 beds)
  • 8-10’ sticks of ½” PVC (I use 4 in my 4X8 beds) $3/piece
  • 3-6 millimeter thick plastic sheeting usually called Poly (I use roughly 10’ X 16’ sheets for my 4X8 beds) (10’x100’ 4 mil poly $31)
  • Large Binder clips or PVC Clamps
  • Scrap wood or logs to secure plastic snug to ground
  • Hammer
  • Twine or zip ties

Next: Instructions

  1. Start by finding the dimensions of what you want to cover, and pound in ¼” rebar every two to three feet along the perimeter, angling it into the bed just slightly. You’ll want it to stick out a few inches above the ground.
  2. Then slide the ½” PVC over the rebar, arching from one side of the bed to the other. If you want a little lower tunnel, you can usually bury the PVC into the ground a bit, or cut off a few feet to make extra low depending on what your needs are.
  3. Secure an extra piece of PVC (or a bamboo stake, etc.) along the length of the top to stabilize the low tunnel and help keep it from collapsing after heavy snow. Tie with twine or secure with zip ties.
  4. Cover with 3-6-ml poly; I usually use 4 mil. Rolls of plastic typically come in 8-10’ wide rolls. The width of your bed and how tall you want the tunnel will determine how wide your poly needs to be. For our 4’ wide beds, the 10’ wide works great as we make our low tunnels pretty tall.
  5. You can use binder clips or specialized PVC clamps to keep plastic in place along the top of the arched PVC. You can also dig the poly into the soil or use anything handy like scarp lumber or landscape bricks to hold it down along the bottom.

*This structure does bring plastic into your garden, but it can be reused for many seasons to come. We’ve been using the same materials for six years now. Also, it completely removes the plastic you would be using if you bought the produce in a store instead of growing it at home—not to mention the carbon cost of shipping the produce, etc…

Using a DIY Low Tunnel Throughout the Season

The basic structure can be used throughout the season to support insect netting (like below) and shade cloth as needed. I find myself popping these up in different formations all the time.

Using the low tunnel structure later in the season with insect netting

You will have to watch temperatures closely, especially on sunny days, as low tunnels are quite effective at collecting and trapping the sun’s heat. I’ve had air temps jump to over 100F by mid-morning even with outside temps in the 30s F. So, remember to vent these structures as needed. Opening one or both ends is very effective at quickly reducing temps.

**And if you don’t have time/interest in building a low tunnel, try turning a clear plastic tote upside down. They work wonders for warming a little space for planting early lettuces or peas!

Cheers to growing more food in the same space, and getting in an earlier succession and harvests just by growing under a DIY low tunnel!

Dig In,
Michelle