CO-Horts Blog

Thursday, February 6, 2014

It is cold! Plant Hardiness

Posted by David Whiting, Department of Horticulture & LA, Colorado State University

It is cold today.  Let’s review winter hardiness.

Hardiness refers to a plant’s tolerance to cold winter conditions.  Low temperature is only one of many factors influencing plant hardiness.  Other factors include:

·       Photoperiod – The photoperiod (increasing length of the night) is the first signal that trees receive that winter is approaching.  This is of special concern when the parent stock of woody plants is selected from southern latitudes then planted in more northern areas.  The change in photoperiods may not properly trigger the beginning of the hardening for the tree.

·       Genetics – Plants have a genetic minimum that they can survive.  However, this will be trumped by temperature patterns over a multi-week period and short-term temperature swings.

·       Low temperatures – Temperatures this week may cause problems on some less hardy plants.

·       Recent temperature pattern – Hardiness is a factor the temperature the past few weeks.  Trees significantly increase hardiness when the temperatures decline slowly over a period of week.  When I lived in Minnesota, it was interesting how many plants were tolerant to the extreme cold (-20° to -30° F common) that are not hardy in Colorado.

·       Rapid temperature changes – Plant are rather intolerant of rapid temperature swings, so common of the Colorado winters with spring weather one week and a sudden return to winter.  This is the factor that most limited our plant pallet for Colorado.

·       Moisture – Trees loose about 20° hardiness when they go into winter with dry soils.

·       Sun and wind exposure – In our sunny climate, bark temperatures on trees can heat to 70° to over 80° in the winter sunshine.  Then freeze at night.  This leads to winter bark injury.  On evergreens, exposure to constant wind is a major hardiness issue, dehydrating the needles. 

·       Carbohydrate reserve – General plant health plays into hardiness.  Plants with a good supply of stored photosynthates are more hardy then plants that experienced growth limiting factors the pervious summer.

The USDA Hardiness Zone Map indicates the average annual minimum temperature expected for geographic areas.  Keep in mind the map is looking regionally, and does as does not take into account the microclimate changes in your elevation, drainage, exposure (aspect), or thermal heat loading.  So, in mountain communities, the zone for your area may not reflect the actual microclimate of your yard.  Actual microclimates maybe one or even two zone warmer or cooler.

Examples of Winter Injury
  •  Bud kill and dieback – From spring and fall frosts or from extreme low temperatures.
  •  Root temperature injury – Roots have limited tolerance to sub-freezing temperatures.  Roots receive limited protection from soil, mulch, and snow.  Under extreme cold, roots may be killed by the lack of snow cover or mulch.  Perennial flowers that are hardy under snow cover, may not survive the winters without snow cover and thus become an annual rather than a perennial.
  • Soil heaving – The freeze thaw cycle of the soil can push plants out of the soil, breaking roots.  Protect with snow cover or mulch.
  • Trunk injury – This is common on the southwest side of tree and is directly linked to drought.  This is more common on trees with hardscape over the rooting zone and soil compaction limiting the water infiltration into the rooting zone.
o   Sunscald – Caused by heating of bark on sunny winter days followed by a rapid temperature drop, rupturing cell membranes in the freeze thaw cycle.  
o   Frost shake – Separation of wood along one or more growth rings, typically between phloem (inner bark) and xylem (wood), caused by sudden rise in bark temperature.

o   Frost crack – Vertical split on tree trunk caused by rapid drop in bark temperature. 

Southwest bark injury is linked to drought stress.  It is common on trees with a restricted root spread or hardscape over the rooting area (restricting water infiltration into the root zone).

Winter injury on evergreens

  • Winter drought – Water transpires from needles and cannot be replaced from frozen soils.  It is more severe on growing tips and on the windy side of trees.
  • Sunscald – Winter sun warms needles, followed by rapid temperature drop rupturing cell membranes.  It occurs typically on southwest side, side of reflected heat, or with sudden shade.
  • Photo-oxidization of chlorophyll – Foliage bleaches during cold sunny days.  Needles may green-up again in spring. 
  • Tissue kill – Tissues killed when temperatures drop below hardiness levels.
Winter drought, sunscald, and photo-oxidization of chlorophyll are common on arborvitae.  This is a poor plant choice for this windy site with little winter moisture.

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