solar heating of high tunnels
DESCRIPTION
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.TRANSCRIPT
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Ground is slow to warm
Cold soils inhibit plant growth and can cause nutrient deficiencies
Require supplemental heat during cold spells
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Traditional Outside
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).
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Heat the soil beneath the high tunnel
Air is heated in solar panels, pumped through tile two lines three feet below the soil surface.
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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
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Two lines of T-tape per bed
“Dick’s Super Soil”◦ Decomposed peat,◦ pH 7.4, high CEC,
low potassium
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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
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Sun warms soil,
Soil emits infrared radiation, which heats air
Soil continues to emit IR
Infrared transparent Infrared trapping
plastic
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Warm soil would emit a large amount of infrared radiation, altering the nighttime temperature of the high tunnel
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Unheated Tunnel – April 10-11
Heated tunnel – April 10-11
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Traditional Tunnel Outside
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Heated Tunnel Outside
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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
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Temperature in heated tunnelBlue line = airBrown line = soil
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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
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Heated Tunnel Traditional Tunnel
Soil temperature in raised bed: March-April
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Outside
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Planted tomatoes and cucumbers on March 15
Tomatoes grew well, but cukeswere stunted
Started cutting spinach and kale first week of March
Radishes in early May
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• Started picking tomatoes on June 8
• Started selling tomatoes and cucumbers at local farmer’s markets on June 16
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High yields both in tomatoes and cucumbers
Quality was excellent at first
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Roll sides manually Put shade cloth on
first week of July Thermostatically
controlled exhaust fan at the peak of tunnel
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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
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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
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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
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Green beans Radishes Kale Carrots Lettuce
Everything was good except onions
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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
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Overwintered KaleOverwintered Spinach
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Sales increased 35% in 09
Tomatoes and cucumbers were 8 weeks earlier in heated tunnel than in unheated tunnel
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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
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Special thanks toMinnesota Department of AgricultureSustainable Agriculture Grants
Questions