Dealing with winter weather in your hoophouse

December lettuce in our hoophouse after snowfall. Photo Pam Dawling

Dealing with winter weather in your hoophouse

We’ve just had Winter Storm Frida bring us a foot of heavy wet snow and, along with most of Virginia, several days without electricity. Six days in our case. Much longer, colder and snowier than I expected. I know, climate chaos makes predictions and expectations chancy! We are in zone 7a, with an average annual minimum temperature of 0-5°F (-18°C to -15°C) until recently.

In November 2018, I wrote a post for Mother Earth News entitled Dealing with Snow on your Hoophouse.

When the power goes out, close the air intake

Hoophouse air intake with a very useful flap to reduce or close the opening. Photo Kathryn Simmons

The first thing an operator of a double layer hoophouse should do when the power goes out (especially in winter) is to rush out and close the air intake. Without electricity to run the little blower, the air intake hole becomes an air outflow. Closing the flap on the blower stops the air escaping. In summer you might not mind, but in winter, the air between the layers of plastic maintains about 8F/5C warmer at night inside than outside, and plants can survive 14F/8C colder than they can outside, without extra rowcover; at least 21F/12C colder than outside with thick rowcover.

The “air bubble” also gives strength to the structure in withstanding loads caused by high winds or heavy snow. Soon after the power cut out on Monday morning, I set out towards the hoophouse. Then I remembered that when we changed the end wall plastic in the fall, we did not fit a cover flap. Worse yet, the air tubing projected out of the wall, making a hasty retrofitting impossible. I briefly considered “putting a sock in it” literally, but then pictured what would happen when the power came back on and sucked the sock into the motor, overheating it and starting a blaze. So, learn from our mistake – always fit a flap over the air intake!

Rolls of rowcover in our hoophouse ready to pull over the beds on very cold nights. Photo Wren Vile

We keep rolls of thick Typar rowcover at the ready and cover the individual beds if we expect the outdoor air temperature to be 8°F (-13°C) or lower. Otherwise we don’t use any inner rowcovers in our hoophouse. Without the insulating effect of the air bubble during the power outage, we decided to use the rowcovers when the outdoor temperature was going to be less than 16°F (-9°C). Several nights like that!

Perhaps this will be your first hoophouse winter. Some others will be remembering last winter, and hoping to find a better way of dealing with snow. Snow can be heavy stuff, so removing it is worthwhile. This can be done from the outside, pulling the snow down to ground level, and from the inside, bouncing the snow off the plastic. It isn’t necessary to remove all the snow. Once you have removed what you can, the daytime temperature inside the hoophouse will rise and help melt the rest of the snow. We have never needed to get out of bed in the night to tackle snow, but you might.

The Snobrum tool we use on the outside of our hoophouse.
Photo Pam Dawling

First we tackle the outside, or if snow is still falling or it’s frozen onto the plastic, we start with the inside. Either way, start with the south side, to get as much advantage from solar gain as possible. The important rule of snow removal is “First, do no harm.” Don’t make holes in the plastic in your efforts to remove snow! A hoophouse is most stable when the snow is evenly distributed. Since houses are stronger at the ends where they have end walls for support, it makes sense to start at the weaker middle and work in both directions if removing heavy snow.

Hoophouse snow scraping tool on a telescoping painter’s pole. (No snow in sight!) Photo Pam Dawling

For outside we use a tool (SnoBrūm™) which is sold for scraping snow off cars. It’s a foam board about 6″ × 18″ (15 × 45 cm) with a threaded insert that takes a telescoping pole. It is sold with a short pole, but a painter’s pole will give a much longer reach. If you haven’t yet got one of these handy tools, carefully use the back of a rake (wrapped in thick cloth). Pull the snow down between the bows, avoiding pulling the tool over the framework as much as possible, as that can easily make holes. Alternate with shoveling the snow away from the base of the hoophouse if the snow is deep (piles of snow pressing against the lower walls can do damage). Be careful not to hit the plastic with any metal tools. Don’t attempt to use the relatively fragile foam-board tool for pushing snow along the ground; keep it free of grit and use it only on the plastic.

Removing snow from a hoophouse at Dripping Springs Garden. Photo Mark Cain

Inside, don a visor (to keep blobs of icy water from landing on your face) and grab an old broom with its head covered in a cloth or bubble plastic, taped on. Make sure there are no sharp protrusions from the broom head. First walk along the paths nearest the walls banging the broom against the plastic. We find a double bouncing action works best, and we can get in a rhythm. This can be a long and tiring job! Start at waist height because if the lower snow falls down, sometimes the higher stuff will follow it down, saving you work. When you need to reach higher, tie or tape the broom to a longer stick. Or find something that screws onto your painter’s pole, such as a paint roller (!) and cover that with bubble plastic. Balance applying your energy with being gentle with the plastic. Do not use the foam-board scraper for bouncing snow — it will crack.

When you’re tired and no more snow is easy to move, wait and try again later. The warming effect of incoming daylight will help melt the snow next to the plastic, so that it is easier to start it sliding off next time you do the rounds.

Sometimes ice accumulates on the plastic. If you get freezing rain that sticks on the plastic, it creates a rough surface that keeps any snow that settles on top from sliding off until the ice thaws. The best way to remove ice is to melt it using warmth from inside. If you have double plastic and it is not windy, switch off the inflation blower — the heat from inside will reach the ice sooner. I recommend against breaking the ice free mechanically, as there is a big risk of the sharp edges of the ice cutting the plastic. You also risk abrasion every time you use a device on the outside of the plastic to pull snow or ice off the tunnel. This makes the plastic rough, and snow won’t slide off as well in the future.

Prevent Hoophouse Frame Collapse

Snow comes in various consistencies and weights. A foot (30 cm) of light fluffy snow may only contain as much water as 1″ (2.5 cm) of rain. But heavy wet snow can equal 1″ (2.5 cm) of rain in only 3″–4″ (8–10 cm) of snow. Each inch (2.5 cm) of rainwater-equivalent will load a structure with 5.2 lbs/ft2 (25 kg/m2). This is about 6.5 tons (5.9 metric tons) on a 25′ × 96′ (7.6 × 29.2 m) hoophouse! Uneven snow loads make a frame more likely to collapse because there are points of higher pressure. This happens if snow is thicker on one side of the hoophouse or if adjacent hoophouses are too close and snow piles up between them when it slides off the roof. The weight of snow can buckle the side of the frame.

Some growers have wood posts ready to wedge under the trusses for extra strength. It sounds like a good plan. Our hoophouse doesn’t have any crosswise trusses, just lengthwise purlins. It’s still standing after 18 years.

When heavy snow is predicted, turn a heater on a couple hours before the storm begins, with the thermostat at 70°F (21°C) or higher. The cost of the fuel is less than a new hoophouse. A portable propane heater (non-electric!) is a good thing to have on hand for unheated tunnels or if a furnace fails or if the power goes out. If you have a very heavy snowfall and it is not possible to remove the snow, then cut the plastic and let the snow fall into the hoophouse to relieve the pressure and save the frame.

“If this, do that”

We have an “If this, do that” card at the front of our hoophouse log book, to help with decisions. It divides snow events into three types.

  1. If the night temperature will be higher than 25°F (−4°C) and you expect less than 6″ (15 cm) of wet snow, leave the inflation blower on, go to bed and hope the snow will slide off.
  2. If the night temperature will be lower than 25°F (−4°C), or there will be more than 6″ (15 cm) of snow and no wind, turn off the blower until the morning, letting the interior heat do its best to melt the snow. If it’s going to be extremely cold, you’ll have to balance this with your need to keep your crops alive.
  3. If there is hard sleet or freezing rain, cycle the inflation off for three hours, on for three hours, with the goal of letting the interior heat melt some of the accumulation, and then using the inflation to push the slush off (give it some help in the morning). Repeat this cycle as needed.

Two great resources are Vern Grubinger, University of Vermont Extension, Prevent Greenhouse Collapse and the Rimol Greenhouses Blog How to Reduce Storm Damage to Your Greenhouse and High Tunnel.

In November 2020 I wrote Winter Preparations for Vegetable Gardens

Winter-Kill Temperatures

My annual blogpost of Winter-Kill Temperatures for Cold-Hardy Vegetables is always very popular. Usually searches for this info increase in October and peak in early November, when growers are deciding when to harvest each crop as the cold increases. In mid-winter you might consult the table to determine which crops to prioritize for extra rowcover if a bad thing happens.

I have learned that there is more damage when the weather switches suddenly from warm to cold. And that the forecaster in Raleigh, NC says it needs 3 hours at the critical temperature to do damage. Also note that repeated cold temperatures can kill off crops that can survive a single dip to a low temperature, and that cold winds, or cold wet weather can destroy plants quicker than simple cold.

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