How to Use Low and Caterpillar Tunnels for Winter Crop Protection

With colder temps in the forecast and frost looming, it’s time to get the field covered up and tucked in for the winter.

At Hidden Springs, I plant the majority of my valuable spring crops in the fall, allowing them to develop strong roots over the winter. These strong roots help produce larger, more productive plants when spring arrives. To give them a slight edge over the elements here in Central Pennsylvania, I use tunnels to manipulate the climate just enough to pull them through the winter.

Tunnels: What’s the Deal?

While I don’t have access to permanent structures like greenhouses or high tunnels, I’ve found great success using two types of DIY tunnels: low tunnels and caterpillar tunnels. Both are set up in the fall and taken down in the spring.

Low Tunnels for Hardy Annuals

Low tunnels are made with frost cloth and lightweight metal hoops. I use them to protect hardy annuals like snapdragons, sweet william, and feverfew. While these plants could survive without cover, they benefit from the added protection. Low tunnels offer a few extra degrees of warmth and shield plants from harsh winter winds.

Since the frost cloth is water-permeable, I don’t need to worry about watering. However, snow can be an issue because the cloth and hoops can’t support much weight. Before a snowstorm, I remove the cloth, knowing that the snow will insulate the plants while the covers are off.

To create these tunnels, I place metal hoops every 5 feet along my 50-foot rows. With my 4-foot-wide beds, the tunnel arch stands about 23 inches above the ground. After securing the frost cloth with weights (like sandbags or cinder blocks), I’m ready for winter.

Low Tunnel Supplies:

• Frost cloth (I use AG-30 in Zone 6b)
• Metal hoops
• Sandbags, bricks, or cinder blocks

Caterpillar Tunnels for Corms

For plants that need a bit more protection, I rely on caterpillar tunnels. These are created using greenhouse plastic and heavier metal hoops made with electrical conduit. They’re not heated, but because they retain so much heat from the sun they keep the ground from freezing inside the tunnel. 

While anemone and ranunculus can handle some cold weather, the corms (sort of like their bulb/root) will rot if frozen so it’s imperative that the ground around them does not freeze. In Central Pennsylvania, we have long stretches of time where the ground is frozen during the winter, so without the caterpillar tunnels or a more expensive heated structure, I would not be able to overwinter these plants for early spring blooms. 

Building a caterpillar tunnel is a bit more involved than the low tunnels made with frost cloth. Over the years we’ve perfected the methods so that our tunnels remain securely in place all winter long, through harsh winter winds and snowfall. I am always in awe of the amount of heat they retain; even on frigid winter days it’s often steamy inside the tunnels. That said, the warmth of the caterpillar tunnel can quickly fry plants if left in place on warmer days so I am careful to vent them throughout the season as needed. It’s as easy as pushing the plastic up along the sides which is kept in place by the baling twine we use at each hoop. 

The greenhouse plastic is not water permeable and because of the freezing temps we can’t leave our irrigation hooked up through the winter. Instead, I make sure to fully vent the tunnels when we’re having a warmer, rainy stretch of weather. I find that doing this, combined with the moisture already in the soil, provides enough water for the plants over the winter. Once spring arrives I run drip irrigation through the tunnels and resume regular seasonal watering. 

You can create your own caterpillar tunnels using supplies mostly found at your local hardware store. We’ve created a video guide outlining exactly how we erect the tunnels and including a complete supply list. Find the video guide here. 

Caterpillar Tunnel Supplies:

• Greenhouse plastic
• Electrical conduit for hoops
• Baling twine
• Hoop bender

Which Tunnel is Right for You?

Consider your growing zone and the crops you're growing. Hardy annuals that just need a little extra protection will thrive under low tunnels. For crops that aren’t well-suited to your climate, caterpillar tunnels are the better choice. Either way, both options help extend your growing season and protect your plants during the harsh winter months.

At Hidden Springs, I've used both types of tunnels to give my crops a head start in the early spring. Whether you're protecting hardy annuals or tender corms, these methods can help you get the most out of your growing season.

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