Harvesting late-season, frost-sweetened crops is such a benefit to growing into the fall in cold climates. It also makes the most of the same garden space as it allows a grower to add another succession of crops to the garden. Season extension increases total harvests from the same square footage.
So many of our favorite fall crops (kale, cabbages, Brussel sprouts, Swiss chard, parsnips, beets, turnips, carrots, celeriac, rutabagas and radishes) taste sweeter after a frost. This is because these veggies are able to turn starches in their cells into a botanical “anti-freeze” to keep themselves from dying when temps dip. This chemical change, their “anti-freeze,” turns into sucrose—aka sugar.
To keep harvesting deeper into fall and possibly even winter—plus benefit from the frost-induced increase in natural sugars—try covering these crops with a low tunnel. Or if you plan far enough ahead you can even grow them in a cold frame.
Choose Your Cover
The basic idea is that, by covering a plant, you keep the soil’s warmth intact and let that radiate up into the air around the plants. The sun also warms the air trapped inside the tunnel. You can gain critical degrees under cover depending on sunlight, soil and height of the tunnel.
The thickness of the covering also makes a difference in heat retention. Anything less than 2-millimeter-thick plastic isn’t worth the bother as it just won’t hold in the heat and tends to tear easily.
I suggest 4- to 6-millimeter plastic for covering low tunnels.
Low tunnels are so easy to pop up and move around to follow your crop rotations. Basically, you just pound in 18- to 24-inch lengths of rebar at the corners (and along the edges if the span is longer than 4 inches). Then simply slide some 1/2-inch PVC over the rebar. Repeat this on the other side to make an arch over the bed you want to cover.
Next, add a stick of PVC the length of the bed (perpendicular to the arches) to add strength to the structure. Securing with zip ties makes this step super easy. Cover in plastic, ensuring you have enough to make contact with the soil all the way around. You can use a longer piece of rebar (or rocks, lumber, etc.) to weigh down the edges.
Secure the plastic to the PVC with clips or clamps.
You’ll want to monitor the temperatures inside low tunnels often, especially on sunny days when air temperatures can rise quickly (even when outside temperatures stay in the 30s).
If you want to protect an individual plant, consider a simple “cloche.” This is a fancy word that describes anything that will hold in the heat.
I’ve used 5-gallon buckets and clear plastic bins to protect salad greens and cabbages when we got a quick dip into the low 20s. Being able to cover plants for a day or two lets me keep them alive in the ground, thus holding onto nutrients longer.
Cold frames are structures, typically somewhat permanent, that insulate a garden bed. They can be constructed of lumber, logs, concrete blocks or straw bales and covered with a piece of Plexiglass or tempered glass. You’ll just want to make sure the angle of your cold frame’s sash slants between 25 to 55 degrees and faces south.
Season Extension Tips
- Set up low tunnels before the first frost of the season.
- You can make the most of dwindling sunlight from the south with an east-to-west orientation.
- Adding black compost or soil to the bed helps increase the soil temperatures more than light-colored straw or leaves.
- Get both soil and air thermometers to help keep temperatures moderated.
Forks in the Dirt
You can find more information on season extension here: Season Extension: Garden + Harvest into the Fall + Winter.