How far apart should I space my tunnels?

The quick answer: 10–15 feet between tunnels is ideal. At minimum, allow at least one tunnel height, or 8–10 feet. They can be placed closer, but the variables below should factor into that decision.

Orientation and sunlight

Tunnel orientation gets more stress than it deserves. The general consensus — particularly for growers in the northern third of the US — is to run tunnels east-west, but the lay of your land and drainage should take priority over compass direction. If you're on a slope, run your tunnels with the slope rather than across it. My family's farm in Middle Tennessee, Bountiful Blessings Farm, has tunnels facing multiple directions and has never had a production problem attributable to orientation.

For spacing specifically, the concern is shadows cast by a neighboring tunnel. How much that matters depends on your growing season and crop selection — shading is mostly a winter issue, when the sun sits low, and it's less of a factor in the mid-latitudes and South. If you want to dig into this, see Which Direction Should Your Beds and Tunnels Face?

Rain

Tunnels create a watershed for neighboring tunnels and crops, which can cause flooding or seed erosion for direct-seeded crops on outer rows. Soil type, lay of the land, and typical rain intensity all play a role. With soils that absorb water better and areas with less intense rain, tunnels can be placed closer together. Planting a cover crop in between helps with water absorption and weed management.

For example, one customer positioned their tunnels with only a twelve-inch walkway between them and planted a cover crop. This suppressed weeds and increased water absorption, preventing muddy conditions. Shadows weren't a concern given their orientation and crop selection.

Snow

Winter growers need to account for snow. When snow slides off the side of a tunnel, it builds up against the walls. Tunnels placed too close together form a ditch-like pocket where snow collects. Any snow you remove also has to go somewhere. Depending on snowfall amounts and machinery size, count on at least 8–15 feet between tunnels so you can clear snow from the side. The greater the typical snowfall, the greater the distance needed.

You may also find this article on snow useful.

Wind

Wind passing over and around structures can create a wind tunnel effect, increasing wind speeds well in excess of the actual wind speed. Spacing tunnels out or creating windscreens can help.