Tuesday, January 5, 2016

Winter high tunnel

In the hopes of continuing production past the fall shoulder season, I decided to fabricate a small, single layer poly (6 mil) unheated high tunnel. The intention is not only produce quality food, but to observe and note plant growth and development of low temperature tolerant varieties through the less than favorable environmental conditions. This includes, but not limited to:

  1. Large daily temperature fluctuations
      upwards of 60°F on a clear day
  2. Overall less than desirable temperatures
      cloudy, windy, and/or low outside temperatures leads to low ambient temperature in high tunnel
  3. Shorter day lengths
      lower photosynthetic activity, coupled with above low temperatures (ie. reduced metabolic activity) means further reduced growth potential
  4. Pest presence and pressures


Ideally it would have been in my best interest to start the plants in September or October (opposed to November when the high tunnel was constructed; unavoidable result of circumstance), thus they would have had longer/more time to establish before the environmental variable conditions declined, and remain so on some fronts. The direct seeded beets, kale, and spinach germinated rather quickly, however lettuce varieties were very poor.


The above limiting conditions are currently being combated here by:

1. Rolling up tunnel sides on sunny days to buffer temperature fluctuations, keep ambient temperatures no higher than 65-70°F.

2. Row cover, hoop and hoop-less, application before sundown and on cold days to help retain ground heat through the night. One must account for the permeability of light through the plastic covering, dew/frost which accumulates on it, as well as the row cover, as the loss is compounded at each barrier. My row cover alone has a conservative 10% light loss, thus if it’s going to be a cloudy day and ambient temperatures are already low (rendering row cover non effective), I prefer not to also reduce light levels from 85% to 76%, and keep row cover removed.

3. Supplemental lighting would resolve this; however it’s not an option.

4. Enclosed systems retain higher water levels (and almost everything else) than open ones, especially when there isn’t sufficient ventilation. This is a drawback of a high tunnel in winter, in my case anyway, cold and damp. This unfortunate conundrum significantly increases favorable environmental conditions that facilitate the proliferation of pest/pathogens. Currently fungus gnats seem to be an issue as they prefer the damp, organic rich soil, especially the plastic mulched rows which remain damper and warmer than the bare rows. Yellow sticky traps are presenting themselves to be reasonably effective at catching adults. Infrequent watering is also helping by minimizing soil moisture and organic matter decay which the larva feed upon. Between these two methods (reducing the adult egg laying population and starving the juvenile larvae population, thus not becoming an egg laying adult) there is a disturbance in the overall population life cycle. Precise quantifications haven’t been tabulated, however my casual yellow sticky card observations have seen an overall decrease in captured adults per square inch per week than when implemented a month prior. This is indicative that either one or both control implementations are effective, probably in addition to the temperature decline during the same time period.

Infrequent watering, although: A.) depriving the plants of needed electron extraction from water molecules for ATP production B.) reducing metabolite/element uptake and transport  C.) diminished vacuole filling, thus smaller plant size, does have a beneficial effect by increasing plant tolerance of low temperatures. Less water molecules in plant cells and tissues overall means higher solute concentrations which equates to less water freezing and thus less cellular damage.1

Overall, plant growth has picked back up as day length is increasing and plants are now reasonably established. Also contributing is the long, and still somewhat delayed bitter cold winter weather of Vermont.


1 Excellent, extremely informative, and a very source rich publication from the FAO on frost damage. eBook link - “Frost Protection: fundamentals, practice, and economics. Vol 1”



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