I am a huge fan of the autumn gardening season, so my garden is now stocked with young plantings of a dozen cool-season crops. All face a long list of threats, including hail, wind, insects and deer, but they are completely protected when grown beneath low tunnels. As the days get shorter and cooler in the coming weeks, the low tunnels will keep surface temperatures slightly warmer and delay freezing of the soil – if it freezes at all.
These are the same benefits you might get with an unheated greenhouse, at a fraction of the cost. And unlike a greenhouse, low tunnels can be moved to where they are most needed in a matter of minutes.
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One of the most common types of low tunnel consists of a series of hoops, stuck into the ground over the row, which are covered with fabric row cover. Inexpensive and effective, simple hoop-style tunnels are great in the fall, but they are prone to collapse under the weight of ice and snow. When creating a low tunnel for vegetables that will stay in the ground through winter, it is often better to use arches made of wire mesh fencing in place of hoops because they can hold more weight.
Best Vegetables for Low Tunnels
In the US, research from Massachusetts and Missouri on growing vegetables in low tunnels in autumn and winter have produced similar lists of the best vegetables to try. Overwintering onions, seeded in August or September and grown under cover through the winter have been quite successful in numerous climates, as have leeks and bunching onions.
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Onions protected by a low tunnel
Other good candidates include rocket, carrots, baby lettuce, spinach, and kale. Where winters are mild, you may have luck with chard, beetroot and radishes. Our Garden Planner includes low tunnel icons that automatically extend the sowing, planting and harvesting dates for plants grown under cover.
In addition to vegetables, research plots in Vermont and New York have produced bumper crops of fall strawberries from day-neutral varieties like ‘Albion’ and ‘Seascape’ planted in May and grown under protective low tunnels all season. The plants are strong producers because they are less threatened by diseases, and the warm temperatures under the covers in the fall enhances the flavour of the berries.
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Bricks can be used to hold down the ends of row cover fabric
Protecting Low Tunnels from Wind
Last winter when the poplar tree by my garage shed its leaves, a ready-made Halloween ghost appeared it its high limbs. The piece of row cover that had blown from the garden was cute at first, and then became an eyesore that was tricky to remove.
Hence the importance of respecting the power of wind when using low tunnels to extend the autumn season. Tunnels that are low and tight resist wind much better than loose ones, so it’s important to secure the edges. I like to secure the long edges with boards and use bricks for the ends, or you can use sand bags. Then use clothespins to fasten the cover to support hoops or wires.
Keeping covers tight and well anchored are key to avoiding blow-aways and tears, and you can add diagonal lash lines to further tether covers in place. Lash lines have kept my little Row Shelter Accelerators from shifting about in gusty winds, and they would boost the wind-resistance of kits like Haxnicks Easy Fleece Tunnels, too.
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Lash lines help keep low tunnels grounded
Planning for Ice and Snow
Low tunnels intended to protect overwintering crops need sturdy support, which is easily provided with arches of wire fencing. When protected from wind, ice and snow with a secure tunnel, spinach, overwintering onions and hardy little lamb’s lettuce seem to enjoy winter, and chard and kale can be kept in picking condition all the way to spring.
Should you need to further insulate a low tunnel from frigid cold, you can add a layer of clear plastic over the row cover in early winter. In cold winter climates, low tunnels equipped with double covers work like little igloos, retaining the earth’s heat while providing surface insulation.
I must add that working with low tunnels is fun. When you pull back the cover to weed your carrots, the soil will still feel summer-warm, with its surface loose and crumbly. You will never check a tunnel to find its contents consumed by rabbits or deer, but do watch out for bug-eating toads and salamanders. They like the toasty protection of low tunnels, too.
