solar heating of high tunnels

Dallas FlynnMary Flynn

Thaddeus McCamantFrazee, Minnesota

Ground is slow to warm

Cold soils inhibit plant growth and can cause nutrient deficiencies

Require supplemental heat during cold spells

0

10

20

30

40

50

60

70

80

90

100

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

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).

Heat the soil beneath the high tunnel

Air is heated in solar panels, pumped through tile two lines three feet below the soil surface.

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

Two lines of T-tape per bed

“Dick’s Super Soil”◦ Decomposed peat,◦ pH 7.4, high CEC,

low potassium

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

Sun warms soil,

Soil emits infrared radiation, which heats air

Soil continues to emit IR

Infrared transparent Infrared trapping

plastic

Warm soil would emit a large amount of infrared radiation, altering the nighttime temperature of the high tunnel

Unheated Tunnel – April 10-11

Heated tunnel – April 10-11

0

20

40

60

80

100

120

7:01

10:0

113

:01

16:0

119

:01

22:0

11:

014:

017:

0110

:01

13:0

116

:01

19:0

122

:01

1:01

4:01

7:01

Traditional Tunnel Outside

0

10

20

30

40

50

60

70

80

90

100

7:01

10:1

613

:31

16:4

620

:01

23:1

62:

315:

469:

0112

:16

15:3

118

:46

22:0

11:

164:

31

Heated Tunnel Outside

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

-1 0. 0

0 .0

1 0.0

2 0.0

3 0.0

4 0.0

5 0.0

6 0.0

7 0.0

8 0.0

Fe b 0 5 Fe b 1 2 Fe b 1 9 Fe b 2 6

*F F ly n n H T I

TM P TM P A TM P B

Temperature in heated tunnelBlue line = airBrown line = soil

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

0

10

20

30

40

50

60

70

80

90

1-M

ar2-

Mar

4-M

ar6-

Mar

8-M

ar10

-…12

-…14

-…16

-…18

-…20

-…22

-…24

-…26

-…28

-…30

-…1-

Apr

3-Ap

r5-

Apr

7-Ap

r9-

Apr

11-…

13-…

15-…

17-…

19-…

21-…

23-…

24-…

25-…

27-…

28-…

29-…

Heated Tunnel Traditional Tunnel

Soil temperature in raised bed: March-April

0

10

20

30

40

50

60

70

80

90

100

3/31

3/31 4/

14/

14/

24/

24/

34/

34/

44/

44/

54/

64/

64/

74/

74/

84/

84/

94/

94/

104/

114/

114/

124/

124/

134/

134/

144/

144/

154/

164/

164/

174/

17

Heated

Outside

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

• Started picking tomatoes on June 8

• Started selling tomatoes and cucumbers at local farmer’s markets on June 16

High yields both in tomatoes and cucumbers

Quality was excellent at first

Roll sides manually Put shade cloth on

first week of July Thermostatically

controlled exhaust fan at the peak of tunnel

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

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

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

Green beans Radishes Kale Carrots Lettuce

Everything was good except onions

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

Overwintered KaleOverwintered Spinach

Sales increased 35% in 09

Tomatoes and cucumbers were 8 weeks earlier in heated tunnel than in unheated tunnel

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

Special thanks toMinnesota Department of AgricultureSustainable Agriculture Grants

Questions