Get Your Garden Growing: Low Tunnels for Earlier Spring Harvests

Hoops around lettuce and onions that support a low tunnel garden.

Tetiana Strilchuk / Getty Images

We're a little over two months away from our last frost date, but I'm getting ready to sow a few seeds and plant cool season vegetables out in the garden. With a simple, inexpensive low tunnel, you can do the same, and be enjoying your first harvest by the time you'd normally just be able to start planting.Low tunnels are basically mini greenhouses. They warm the soil and provide a temperate micro-climate for growing spring vegetables. Even though there's snow on the ground, the temperature under the low tunnel in my garden is perfect for growing cool season vegetables such as mesclun, kale, kohlrabi, beets, and spinach.

Making a Low Tunnel

A square gardening box with three hoops for a low tunnel garden.

David Marcovitz / Flickr / CC BY 2.0

Low tunnels are key to organic farmer Eliot Coleman's four season harvest success. I have several in my garden, and prefer them over cold frames when I want to warm up an entire garden bed. Because you can make them any size you need them, they'll work well in any size garden. I have one bed at the back of my house that we cover with a low tunnel every winter, and we've been able to harvest spinach as late as January and as early as March (in the Detroit area, where our last frost date is in early May, this is a big deal!)

Low tunnels are also inexpensive. You can make the supports from copper plumbing pipe if you're concerned about using PVC, which is what most people use for the supports. However, bending the copper is a bit trickier than bending thin PVC pipes. If you've had a local election recently, and are able to get your hands on some of the wire frames from the hundreds of campaign signs that litter the landscape, those would also make excellent supports for your low tunnel.

Over the supports, sheets of thick plastic (I often use plastic drop cloths from the hardware store) are secured by weighing the ends down with bricks or sandbags. It's easy to assemble, and can be taken apart and stored when you no longer need it. I usually get a couple years' worth of use out of the plastic sheeting before I have to replace it. The PVC or copper frames have held up through several years of almost continuous use.

When to Plant

Covering wire hoops in a low tunnel garden that has fresh soil.

tanyss / Getty Images

The first thing to keep in mind is that nothing will grow until the soil is sufficiently warm. For this reason, it's best to construct your low tunnel a few weeks before you plan on using it (or put it up in the fall, so it's already in place when you want to grow early season crops.)

Most cool season vegetable seeds, such as lettuce and spinach, need the soil temperature to be around 40 degrees Fahrenheit for germination. You can purchase a soil thermometer, but if you have a meat thermometer in your kitchen, that will work well, too. Simply stick the thermometer about two inches into the soil.

If you don't have a thermometer, just take a guess. If the soil is no longer overly wet or frozen, it's most likely ready to plant. You can also plant transplants when the soil is at around 40 degrees, so if you find cool season veggie transplants at your local garden center, feel free to go ahead and plant them in your low tunnel.

Keeping It Warm

Low tunnel garden in the winter covered in snow and a blanket to keep vegetables warm.

tanyss / Getty Images

In most cases, the low tunnel itself will provide your plants with all of the protection they'll need from cold weather. However, there are a couple of things you can do to ensure that your plants stay nice and warm:

  • Use a floating row cover or lightweight sheet inside your plastic tunnel. Just lay it right over the plants on the very coldest nights. It will buy them a few degrees of frost protection during stretches of sub-freezing temperatures.
  • Place plastic milk jugs of water in the tunnel amongst the vegetables. During the day, the water in the jugs will warm up. And during the night, the heat from the water bottles will help keep the temperature inside the tunnel warmer.

Neither of these measures are necessary, but they can provide a bit of peace of mind if you're worried about it getting too cold after you plant.

Cooling It Off

Exposed lettuce on a hot day in a low tunnel garden.

DutchlightNetherlands / Getty Images

On very sunny days, temperatures inside the low tunnel can soar. I've had arugula bolting in February because I didn't ventilate during a few very sunny days. Luckily, low tunnels are easy to ventilate: simply remove the weights holding the plastic down on one or both of the ends, and pull the plastic up a bit to let air flow through. On days when it is both sunny and warm, pull the plastic off all together, then replace it in the evening.

What to Grow in a Late Winter Low Tunnel

Spinach growing in a low tunnel greenhouse in the winter.

Susy Morris / Flickr / CC BY-NC 2.0

If you normally aren't able to start your garden until late April at the earliest, this will give you a great jump on the season. Here are a few vegetables that do very well in a low tunnel in late winter:

  • Lettuce/mesclun
  • Beets
  • Kale
  • Chard
  • Onions
  • Broccoli
  • Brussels sprouts
  • Cabbage
  • Scallions
  • Kohlrabi
  • Spinach
  • Mache
  • Asian greens (Pak Choi, Tatsoi, Mizuna)

I hope this helps those of you who are itching to get out into the garden and start growing your own food.