Growing under the cover of a low tunnel is an easy way to get a jump start with your spring vegetable garden. These temporary structures are miniature versions of greenhouses. The domed plastic is great at trapping and holding heat in the soil. Quick, easy and inexpensive to build, low tunnels can be popped up anywhere, so they also work well with crop rotation. These extend the growing season in the spring to warm soil for earlier planting and in the fall to keep cold hearty crops in the garden longer.
You’ll also need to start with cold hardy crops. Some of the best choices are brassicas (aka cole crops) such as broccoli, cauliflower, cabbages and kale, plus beets, radishes and spring greens.
The listed plants will start germinating at temperatures around 40 to 45 degrees Fahrenheit. Another good list is here.
But we need to make as important distinction between air and soil temperatures. It’s soil temperature that you need to pay attention to for germinating the earliest plants. Luckily for seedlings and plant roots, soil temperatures are more consistent than air temperatures.
Having a dedicated thermometer to track soil temps will help you keep a handle on things. I actually have a few 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.
The most effective way to warm the soil is to cover it. Coverage on the surface will help a little but bringing that covering up off the ground so the sun’s rays get trapped inside helps even more.
Let’s Build a Low Tunnel
Gather the following needed materials. (Use what you have on hand, but this will get you going.)
- 18- to 24-inch sections of ¼-inch rebar (I use six total for my 4-by-8 beds.)
- 8- to 10-foot sticks of ½-inch PVC (I use 4 total for my 4-by-8 beds.)
- 2- to 8- millimeter plastic sheeting (I use roughly 10-by-16-foot sheets for my 4-by-8 beds.)
- clips or blocks to secure plastic snug to ground
Start by finding the dimensions of what you want to cover, and pound in ¼-inch rebar every 2 to 3 feet along the perimeter, so it sticks out a few inches above the ground.
Then slide ½-inch PVC over the rebar, arching from one side of the bed to the other.
Secure an extra piece of PVC along the length of the top to stabilize the low tunnel and help keep it from collapsing after heavy snow.
Cover with 2- to 8-millimeter thick plastic. Rolls of plastic typically come in 10-foot-wide rolls. How tall you want the tunnel, and the width of your bed will determine the width of the plastic you need. For our 4-foot wide beds, the 10-foot-wide works great as we make our low tunnels pretty tall.
You can use clips to keep plastic in place, dig it into soil or use anything handy such as landscape bricks to hold it down. This does bring plastic into your garden but is reusable for many seasons to come, as we’ve been using the same materials for six years now.
Cheers to growing more food in the same space, getting in an earlier succession and harvests just by growing under a low tunnel!