Though hoop houses, plastic tunnels and cold frames are a lot more common these days, there’s another way to ensure a harvest of fresh greens almost all winter long. It’s called below-grade trench planting, and it’s an excellent and inexpensive way to keep the lettuce, kale, tat soi and other cold-weather crops rolling into the kitchen even when the snow flies.
What Is A Below-Grade Trench Planting?
Where other season-extenders rely on building materials to insulate plants from cold temperatures, below-grade trench planting relies on the insulating properties of the soil to protect plants. Instead of a fall crop being planted at soil-level and then covered by a protective structure, it’s planted deep in the ground, where it’s protected by the still-warm soil instead.
During extreme winters in northern U.S. gardening zones, the ground can freeze down to several feet, especially if there’s little snow cover. But, in typical winters, even in the north, the ground will not be fully frozen for a month or two after the regular gardening season ends, if it freezes at all. Gardeners who employ below-grade trench planting for year-round growing make use of their still-warm soil by planting fall and winter crops in a deep trench dug into the soil.
How To Make a Below-Grade Trench Planting Bed
Step 1: Select A Site
To start, select a location that receives full sun for the entire day. Do not pick a low-lying area that could become waterlogged. Your trench should run east to west, so pick a site that can accommodate that.
Step 2: Dig The Trench
A large-scale farmers can dig a trench with a small backhoe or another piece of mechanical equipment, but backyard gardeners can dig their trench by hand. It can be as large or as small as you’d like, but make sure you can reach into the center of the trench easily. If you opt for a large trench, at planting and harvesting time you might find it easier to climb down into the trench to plant and pick, rather than doing it from the surface.
Typically, below-grade planting trenches should be two to three feet deep at minimum. I prefer three to four feet deep because here in Pennsylvania we seldom have winters where the ground freezes much below a depth of about three feet.
To help determine the frost line in your region, look at your local building codes to see how deep foundations and support columns must be set.
Step 3: Shore The Sides (If Needed)
You need to complete this step only if you’ve dug a trench wider than about six feet and deeper than four feet. For large trenches like this, shoring the sides with cross braces and plywood is a good idea. To further insulate deep, wide trenches, you could also line the sides with straw bales.
Step 4: Prepare The Base Soil For Planting
When you finish digging the site for your below-grade trench planting, you’ll find you’ve left the best soil in a pile to the side of your trench. The soil at the bottom of the trench will be less-than-ideal for planting. To prepare the base of the trench for planting, put a few inches of the top layer of soil that you removed back into the bottom of the trench and mix it with two to three inches of finished compost.
Step 5: Plant Your Crops
Any cold-weather-tolerant crop can be grown in a below-grade trench planting site, but some favorites include the greens mentioned above, as well as cole crops and certain root crops, such as beets, radish and turnips. Sow the seeds into the bottom of the trench in late summer or early fall to ensure they’re already growing well before very cold weather arrives.
Step 6: Cover The Trench When The Time Is Right
The trench should be left open and uncovered while the growing season continues. But, as soon as regular frosts arrive, it’s time to close in the area to retain the soil’s heat. Lay 2-by-4 boards across the trench every few feet and then cover the entire trench with a layer of thick, clear plastic film or rigid plastic sheeting. Do not use opaque plastic as it will block light from reaching the growing plants. Place rocks or bricks around the edges to secure the plastic in place.
Step 7: Mark Or Fence The Site Well
To keep kids, animals and anything else from accidentally falling into the trench, fence it in or post clear signs warning of the danger.
Step 8: Water As Needed
The plastic can be removed whenever you need to water, but otherwise, leave it in place. Once cold temperatures arrive, watering will have to occur less frequently, if at all.
Step 9: Pull Back The Plastic On Warm Days (If You Want)
The ease of using below-grade trench planting instead of cold frames or hoop houses also comes from the fact that you don’t have to remove the plastic on hot days because not only does the soil insulate the plants from extreme cold, it also insulates them from extreme heat.
Step 10: Keep Snow Cleared
If you covered your trench with plastic film, the 2-by-4 cross supports will help hold the plastic in place under a small amount of snow. But, if more than a few inches of snow falls, regularly brush it off the plastic with a soft broom. This is less of a concern if you use rigid plastic that can hold more weight.
Once dug, below-grade trench planting sites can be used for many years, especially if the sides have been shored properly. Simply add more compost to the bottom when planting time arrives and sow a new crop.
