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High winds and high tunnels

September 22, 2008

Tunnel DamageThe remnants of Hurricane Ike lashed through our farm last weekend and tore the plastic off our high tunnel (right). I was surprised by the ferocity of the wind, given that the center of the storm didn’t even hit Kentucky. Much of our region was without power for most of the week, and many schools are just re-opening today, eight days after the storm.

The high winds didn’t do much damage to our Energy Farm study plots. The field corn and soybeans have been drying in the field (letting the sun do the work saves a lot of the energy that would be used by drying bins). The plants stayed standing, but the beans lost their leaves, which will actually make our harvest easier. The sweet sorghum stood up to the wind too. Sweet potatoes, with their soil-hugging vines and most of their biomass below ground, are pretty much wind-proof.

2006 Damage

In 2006 I had sweet sorghum growing right next to the high tunnel when heavy winds came through at about the same time of year. The high tunnel survived but the sorghum came down (left). What was the difference? I can only speculate:

In 2006 the winds came with rain, which softened the soil and left the tall plants more vulnerable to lodging. This year the winds came after a month of drought, and didn’t bring a drop of rain. The soil is hard, and the plants stood firm. Another possible difference is that the tunnel was protected by the sorghum in 2006; the crops growing next to the tunnel this year were much shorter. Finally, the tunnel walls were rolled down when the winds hit in 2006, but were rolled up for ventilation when the winds hit last week. That allowed the wind to get under the tunnel and start to work away at the sides.

The use of high tunnels has important energy implications. Using only passive solar heat, a high tunnel uses much less energy than a heated greenhouse (My figure below, with more here… Thanks to Dr. Yoshihiko Wada for his data on greenhouse energy consumption).

High tunnel vs. greenhouse energy comparison

Both high tunnels and heated greenhouses can allow year-round vegetable production in many regions, which reduces the need for winter trucking of produce. The difference is that only cool-season crops (e.g. lettuce, carrots, beets) will grow in the high tunnel through the depth of a Kentucky winter, but a heated greenhouse can keep warm season crops (e.g. tomatoes, cucumbers, peppers) going all year long. Although it’s nice to have locally-grown tomatoes in February, it would actually take far less energy to grow field tomatoes in Mexico and truck them to Kentucky than to heat the Kentucky greenhouse that would allow winter production of warm season crops. The high tunnel can give us fresh kale and carrots in February, and tomatoes as early as May. Outside the tunnel the kale dies in December and tomatoes don’t bear fruit until the beginning of July.

We use two layers of plastic on our high tunnel. Between them is an insulating air pocket, maintained by a 60 W blower fan that runs continuously. As the figure above shows, the blower fan adds a lot to the energy needed to maintain the high tunnel. Many others use a single layer of plastic, which doesn’t retain heat as well, but eliminates the need for a blower fan.

Like high tunnels, greenhouses are vulnerable to wind. When I worked in large glass greenhouses in British Columbia managers would keep a nervous eye on the data coming continuously from wind speed sensors (anemometers) on the roof. They knew they had to get their workers out in a hurry during heavy winds because the glass panes would start to shatter, with potentially fatal results.

I can’t imagine that heated greenhouses will have much place in an energy constrained future. Those who promote vertical farming in urban areas apparantly disagree. That could be fodder for another blog post.

Michael Bomford provides research and extension services related to organic agriculture and small-scale renewable energy production through Kentucky State University’s Land Grant Program. He thanks Tony Silvernail, Brian Geier and John Rodgers for their help with maintaining the organic land at the KSU Research Farm.

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