The document summarizes the results of heating the soil in a high tunnel using an underground heating system. Key points: 1) Heating the soil allowed the high tunnel to remain 10-20 degrees warmer than an unheated tunnel, allowing crops to be grown 8 weeks earlier. 2) Tomatoes and cucumbers grown in the heated tunnel were of higher quality and yielded more than those grown outside or in an unheated tunnel. 3) A potassium deficiency caused hard centers in tomatoes late in the season, but was addressed with foliar potassium applications.

1. Dallas Flynn
Mary Flynn
Thaddeus McCamant
Frazee, Minnesota

2.  Ground is slow to
warm
 Cold soils inhibit
plant growth and
can cause nutrient
deficiencies
 Require
supplemental heat
during cold spells

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.  Heat the soil
beneath the high
tunnel
 Air is heated in
solar panels,
pumped through
tile two lines three
feet below the soil
surface.

7.  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

8.  Two lines of T-tape
per bed
 “Dick’s Super Soil”
◦ Decomposed peat,
◦ pH 7.4, high CEC,
low potassium

9.  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

10.  Sun warms soil,  Soil continues to
emit IR
 Soil emits infrared  Infrared transparent
radiation, which  Infrared trapping
heats air plastic

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

12. 0
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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
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20
30
40
50
60
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90
0
100
7:01
10:16
13:31
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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

13.  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

14.  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
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1-Mar
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15. 0
100
10
20
30
40
50
60
70
80
90
3/31
3/31
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4/1
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Heated
Outside

16.  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

17. • Started picking
tomatoes on June 8
• Started selling
tomatoes and
cucumbers at local
farmer’s markets on
June 16

18.  High yields both in
tomatoes and
cucumbers
 Quality was
excellent at first

19.  Roll sides manually
 Put shade cloth on
first week of July
 Thermostatically
controlled exhaust
fan at the peak of
tunnel

20.  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

21.  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

23.  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

24.  Green beans
 Radishes
 Kale
 Carrots
 Lettuce
 Everything was
good except onions

25.  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

27. Overwintered Spinach
Overwintered Kale

30.  Sales increased 35%
in 09
 Tomatoes and
cucumbers were 8
weeks earlier in
heated tunnel than
in unheated tunnel

31.  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

32. Questions
Special thanks to
Minnesota Department of Agriculture
Sustainable Agriculture Grants