High Tunnel Blog
High Tunnel Blog
During the winter months we do our best to button up the high tunnel nice and tight and cover the plants with row covers to give as much protection from the cold as possible. Funny enough the flip side to all this effort is that is can be equally detrimental to the crops we are trying to protect. Inside the sealed up tunnel and underneath those blankets resides the perfect conditions for an outbreak of disease- most notably fungi and mildew. The combination of high humidity and stagnant air coupled with spikes of warmth on sunny days is just what these less-than-beneficial microorganisms need to multiply. What a catch-22 eh?
Ironically, one of the best things we can do to pre-emptively to avoid disease issues is remove the very covers being used to protect the plants. In addition, opening the peak vents of the tunnel is the best way to relieve some of the humidity and allow for some cross ventilation, i.e. bring in some fresh air. The ideal kind of days we are looking for are when the sun is out and the inside temps are above 50 degrees. If sunny days are few and far between, take advantage of those warm spells (if there are any) where outside temps are above 32 degrees. The last few week have been ideal for ventilation with some sunny days and even temps in the upper 30's. I have removed the covers and three times thus far since Christmas. I will open and close the peak vents daily but leave the row covers off until the weather forecast shows daytime temps dropping below 32 degrees again. Some commercial growers in Northern climates are even going so far as to leave their peak vents cracked all winter. The jury is still out on how much or how little to ventilate during the heart of winter. The main thing is to at least make a point of doing so at least once or twice during the months of January and February.
A question came in recently from Greta over at Washburn school about what temps one might be recording inside the tunnels this time of year. Greta purchased a high-low digital thermometer to record the nightly lows and daily high temps. She has a lowest low of 0 degrees and a highest temp recording of around 70 degrees. The inflation blower wasn't always plugged in either, so max insulation value probably wasn't realized.
To compare, the temps I have been recording here have been relatively consistent with what Greta is seeing in their tunnel. I have three sensors- one outside, one inside the greenhouse, and one inside the greenhouse under the row cover.
Outside 38 -17
Inside 64 -2
Under cover 79 13
Keep in mind that both of my greenhouses are single layer plastic tunnels. Nevertheless, you can see the 15 degrees of added protection inside and the 30 degree difference under the row cover. Even just the 15 degrees in this case can be enough to determine whether an over-wintering crop will survive the winter or not. This depends heavily on the number of days the root base has to sustaine those sub-zero temps. Last winter we had over 40 days where it did not get above zero under the row cover. Everything died except for a small amount of spinach.
The point being here is that the more protection you can provide the plants during the dormancy period, a greater chance of survival and earlier jump on growth can be realized. Some commercial growers will add two if not three layers over the plants this time of year. This might also be a fun experiment for students- monitoring the temperature differences under multiple layers (or different layers) of protection.
Watch the moisture build up however! Wet, humid conditions are a perfect breeding ground for mildew and mold. It is good practice to pull back the covers and even open the peak vents on the greenhouse during warmer and preferably sunny days to let things "breathe". I'll speak to this in greater detail on another post.
Agripreneur Students (2016 posts)