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Overwintering Plants In The Nursery

Posted by Keith Dintelmann

Overwintering Plants In The Nursery

When the leaves begin to change and the beauty of autumn is upon us, it is a signal to us in the nursery trade that winter, too, is on its way and we must begin making preparations.  In the days when nursery production was all bb, and all the plants were in the ground, there was nothing to winter prep.  Plants simply went dormant naturally and their roots systems, being in the ground, rarely needed any additional winter protection.  But when nursery production moved toward growing plants in containers above ground it became necessary to come up with overwintering techniques that would protect root systems and insure plant survivability. 

Most overwintering techniques used in nurseries involve some sort of structure covered in a poly film.  There are many different types of structures and different types and grades of poly film as well.  The decision of what type of structure and what type of poly to use for any particular nursery is dependant upon the species of plant being covered, the amount of snowfall, and the average low temperature.  Regardless of these factors though, it is necessary to consolidate plants in their growing areas into a “pot tight” situation.  This is the first step and one of the most important steps in winter protection.  

Plant Spacing

During the growing season, plants are spaced apart to allow for proper light exposure and unimpeded growth.  But if plants were to be left spaced during the winter months the entire 360 degrees of the root zone would be exposed to cold air temperatures.  It is the root zone, not the plant tops, that are really the concern.  Bringing a group of plants together and placing the pots tight against one another limits the amount of cold air that can work into and around the root zone.  By virtue of the pots being tight the tops of the plants are also tight together creating a solid canopy of vegetation over the group of pots (this is not the case regarding perennials due to their tops dying back).  This acts as an insulation blanket trapping radiant warm air that works up from the ground.  The most vulnerable plants to winter damage become those that make up the perimeter of a grouping due to the outside 50% of the plant that is exposed to the coldest air. 

Maximum insulation can be achieved when plants are consolidated in triangular formation as opposed to a square formation. It is the smaller air space that more easily traps and holds the radiant heat from the ground. 

Circles graph

The Importance of Double Cover

You may wonder why all this is important.  Polyhouses aren’t designed to completely insulate and keep plants from freezing.  After all, plants in the landscape are susceptible to the ground freezing many inches thick and during those winters when that does occur we don’t see massive death in the landscape.  So, why does it matter if the root zones freeze?  Well, what we are really trying to achieve is limiting large temperature swings on the root zone as well as limiting the amount of time root zones are exposed to temperatures in the teens.  This is variety dependant and so, on our more fragile species of plants, we add a second layer of poly inside the house that can be pulled over the plants when low temperatures dictate the need.  We call this

Double Cover.  It is this second layer of added insulation that keeps nighttime temperatures from dropping as well as keeping daytime temperatures from rising more than if the double cover wasn’t there. 

A few years ago we ran a test to see just what the differences were in the air temperatures that existed under the double cover and inside the poly-house compared to the outside air temperature.  In this particular case the poly that covered the house was 3mm white poly with 50% light penetration.  The double cover layer was also 3 mm white poly with 50% light penetration.  During this particular weather event outside air temperatures never went above 30 degrees farenheit and the lows reached into the teens.  As a result the layer of double cover was left over the plants for more than 96 hours.  Temperature recording devices were used that captured the lowest temperature recorded overnight but also gave current readings.  The overnight reading was the captured low temperature.  All other readings were manually taken at the time of day indicated.   Below is the table and graph of the data collected. 

 

 

 

Temperature in degrees F

 

 

 

Date

Time

dbl cvr

house

outside

conditions

comments

Jan 29

4:00 PM

58.0

47.0

22.0

sunny,

 

Jan 30

overnight

27.0

14.0

13.0

 

 

"

7:00 AM

28.0

17.0

16.0

cloudy, windy

 

"

10:00 AM

36.0

47.8

17.0

partly sunny, windy

 

"

1:00 PM

43.5

57.4

19.9

partly sunny, windy

 

"

4:00 PM

37.9

41.2

19.7

partly sunny, windy

 

Jan 31

overnight

23.7

7.0

10.0

overnight, calm

 

"

7:00 AM

24.8

8.6

11.0

sunrise, wind still

 

"

10:00 AM

35.6

42.2

19.8

mostly sunny, calm

 

"

2:00 PM

37.6

47.1

26.4

partly sunny, calm

 

"

4:00 PM

36.5

34.0

19.7

partly sunny, calm

 

Feb 1

overnight

30.4

24.6

21.9

sunrise, wind still

 

"

7:00 AM

31.5

26.8

22.6

cloudy, wind still

 

"

10:00 AM

35.6

43.2

26.2

cloudy, wind still

doors open, dbl cvr pulled back, truck being unloaded and plants put into house

"

2:00 PM

36.5

40.6

28.6

cloudy, wind still

 

"

4:00 PM

33.8

34.2

27.0

cloudy, wind still

 

Feb 2

overnight

29.7

18.0

16.3

 

 

"

7:00 AM

31.1

32.2

18.4

partly cloudy, some wind

"

10:00 AM

37.8

47.1

21.2

partly cloudy, some wind

"

2:00 PM

49.8

61.2

20.5

clear, some wind

 

"

4:00 PM

44.4

50.2

20.2

clear, some wind

 

 Temperature graph 1

It is interesting to note that the temperatures inside the house fluctuated greatly given that the days were sunny and greenhouse effect warming took place yet at night those warm temperatures did not hold.  At best the effect of having plants in a covered poly-house during this event was that plants were limited to the amount of time they were exposed to temperatures in the teens.  The data does show however how the double cover did regulate the air temperature not letting the overnight temperature to drop into the teens on any given night and also not rising with daytime sunlight and greenhouse effect. 

Thus the double cover does for us what we want it to do; limiting temperature  fluctuations and prolonged exposure to teen and sub teen temperatures thereby insuring the winter survival of the plant and it’s ability to thrive over the coming season.

Tags: Plants, General, Winter