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Solar Heating of High Tunnels

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Presented by Northland Community & Technical College Specialty Crops Management Instructor Thaddeus McCamant and grower Dallas Flynn at the 2009 Minnesota Statewide High Tunnel Conference in Alexandria, MN on Dec. 2-3, 2009.

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Solar Heating of High Tunnels

  1. 1. Dallas Flynn Mary Flynn Thaddeus McCamant Frazee, Minnesota
  2. 2.  Ground is slow to warm  Cold soils inhibit plant growth and can cause nutrient deficiencies  Require supplemental heat during cold spells
  3. 3. Traditional Outside 100 90 80 70 60 50 40 30 20 10 0 3/31 4/1 4/2 4/3 4/4 4/5 4/6 4/7 4/8 4/9 4/10 4/11 4/12 4/13 4/14 4/15 4/16 4/17 4/18 The high tunnel warmed the air to 80F by 11:00 AM, but the air temperature dropped quickly at night. Nighttime lows in tunnel equaled outside (empty tunnel).
  4. 4.  Heat the soil beneath the high tunnel  Air is heated in solar panels, pumped through tile two lines three feet below the soil surface.
  5. 5.  Fans push air from the soil through the solar panels back to tile lines  Fans are controlled by thermostats:  Fan starts when air in solar panel reaches 125F, stops when temperature in soil panel drops to 85F
  6. 6.  Two lines of T-tape per bed  “Dick’s Super Soil” ◦ Decomposed peat, ◦ pH 7.4, high CEC, low potassium
  7. 7.  Cold soil can cause nutrient deficiencies, root diseases, reduced germination, poor growth  Heating the soil could cause the air temperature to be warmer at night
  8. 8.  Sun warms soil,  Soil continues to emit IR  Soil emits infrared  Infrared transparent radiation, which  Infrared trapping heats air plastic
  9. 9.  Warm soil would emit a large amount of infrared radiation, altering the nighttime temperature of the high tunnel
  10. 10. 0 100 120 20 40 60 80 7:01 10:01 13:01 10-11 16:01 19:01 22:01 1:01 4:01 Traditional Tunnel 7:01 10:01 13:01 16:01 19:01 Outside 22:01 1:01 Unheated Tunnel – April 4:01 7:01 10 20 30 40 50 60 70 80 90 0 100 7:01 10:16 13:31 16:46 20:01 10-11 23:16 2:31 5:46 9:01 12:16 15:31 Heated Tunnel 18:46 22:01 1:16 4:31 Outside Heated tunnel – April
  11. 11.  Solar panels could not keep tunnel above freezing during coldest part of the winter ◦ Cold winter -40 two times in January  Soil-heated tunnel was warmer than unheated tunnel ◦ 10 degrees warmer (mean temperature) ◦ 20 degrees warmer (overnight lows)  Soil temperature: 31.4  Overwintered spinach, kale in heated tunnel *F F ly n n H T I 8 0.0 7 0.0 6 0.0 Temperature in 5 0.0 heated tunnel 4 0.0 3 0.0 Blue line = air 2 0.0 Brown line = soil 1 0.0 0 .0 -1 0 . 0 Fe b 0 5 Fe b 1 2 Fe b 1 9 Fe b 2 6 TM P TM PA TM PB
  12. 12.  Soil rose above freezing on March 4 in heated tunnel  Soil rose above 45 on March 15  Nighttime lows stayed above 45 on March 15  10-15 degrees warmer than unheated tunnel between March 7 and April 15 Soil temperature in raised bed: March-April Heated Tunnel Traditional Tunnel 90 80 70 60 50 40 30 20 10 0 10-… 12-… 14-… 16-… 18-… 20-… 22-… 24-… 26-… 28-… 30-… 11-… 13-… 15-… 17-… 19-… 21-… 23-… 24-… 25-… 27-… 28-… 29-… 1-Mar 2-Mar 4-Mar 6-Mar 8-Mar 1-Apr 3-Apr 5-Apr 7-Apr 9-Apr
  13. 13. 0 100 10 20 30 40 50 60 70 80 90 3/31 3/31 4/1 4/1 4/2 4/2 4/3 4/3 4/4 4/4 4/5 4/6 4/6 4/7 4/7 4/8 4/8 4/9 4/9 4/10 4/11 4/11 4/12 4/12 4/13 4/13 4/14 4/14 4/15 4/16 4/16 4/17 4/17 Heated Outside
  14. 14.  Planted tomatoes and cucumbers on March 15  Tomatoes grew well, but cukes were stunted  Started cutting spinach and kale first week of March  Radishes in early May
  15. 15. • Started picking tomatoes on June 8 • Started selling tomatoes and cucumbers at local farmer’s markets on June 16
  16. 16.  High yields both in tomatoes and cucumbers  Quality was excellent at first
  17. 17.  Roll sides manually  Put shade cloth on first week of July  Thermostatically controlled exhaust fan at the peak of tunnel
  18. 18.  Core of tomatoes was hard  Yellow shoulders  Okay for processing, but not for fresh market  Gave extra tomatoes at the market  Assumed problem was weather related
  19. 19.  Gray color right inside the skin  Difficult to see without cutting tomato  Hard center core  Not in heirloom varieties  Most likely due to potassium deficiency
  20. 20.  Began in late July  Applied weekly foliar potassium (KDL) and continued to early fall  Problem disappeared three weeks after starting the KDL sprays  Amended soil with greensand
  21. 21.  Green beans  Radishes  Kale  Carrots  Lettuce  Everything was good except onions
  22. 22.  Aphids in peppers, eggplants in August  Controlled with ladybugs  Poor fruit set in eggplants and peppers in spite of enormous plants  Will buy bumblebees early next year
  23. 23. Overwintered Spinach Overwintered Kale
  24. 24.  Sales increased 35% in 09  Tomatoes and cucumbers were 8 weeks earlier in heated tunnel than in unheated tunnel
  25. 25.  Good year to have a heated high tunnel  All crops were better in the heated tunnel than in unheated tunnel or outside garden  Good quality  Few diseases  Picked tomatoes from June 8 to November 15
  26. 26. Questions Special thanks to Minnesota Department of Agriculture Sustainable Agriculture Grants

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