Thursday, January 2, 2014
Cold and Root Hardiness
I have been fascinated by the concept of horticultural zones and root hardiness for years. It may be because I have lived in the frozen tundra of central Wisconsin for a lot of my life, where it seems the artic dip from Canada visits us fairly often. It may be because I seem to live on the edges of zones fairly often and like to push them. I have gone so far as to track the daily temperatures and use the statistical formula to plot a particular winter's actual horticultural zone. (That year I was living in Elgin and it came up as zone 7a which at that time was zoned 5b.)
I don't know.
What I do know is with the extended bone-numbing cold we seem to be experiencing here in central Wisconsin I am starting to be a bit concerned about my garden which I left in fair condition but very abruptly because of my falling and fracturing my acetabulum. I directed a few final day plantings of potted plants, shuffled my potted annuals and houseplants plants off to a dear friend, and left.
I would not be concerned with a couple atypical days of bone-chilling cold, but the length and depth of this cold throughout December and now extending into January is starting to concern me. As gardeners many of us have not had experience gardening beyond our one zone; but as any gardener with a bit of zone envy and horticultural zone denial will tell you, it is not too difficult to have collected plants which push the zone in your garden.
Taking into account the number of days and the depth of cold, it is almost as though my garden has been magically transported from zone 5b into something like zone 3b. Up until 2012, my area was listed as zone 4a on the official chart. Gardeners being what they are, many in my area had been reacting to what they had been seeing, trowels in the ground.
We have been planting into zone 5b for a number of years.
On Tuesday, January 7, 2014, our predicted high air temperature (not wind chill) is -16 F.
Root hardiness is key to winter survival. Now the factors influencing overall plant hardiness in relationship to horticultural zone are many and they are discussed fairly in this document. I'm going to add my own observations to this information.
First, snow is a great insulator. In the absence of snow a good layer of mulch helps a lot. Snow exists at a temperature around 32 degrees Fahrenheit. Once water freezes, that's about as cold as it gets. Sometimes good snow cover in the swamps where I grew up, with the additional mulch layer of fallen autumn leaves could keep the ground beneath relatively unfrozen most of the winter.
This points up the second aspect of hardiness, the amount of organic material in our garden soils and its ability to retain moisture, and the hydration of our soil at the time it freezes, is also critical to how well our plants survive. The better the moisture content of our soil, the closer to 32 degrees our soil will freeze. This may surprise some. This is part of whether a plant will survive in a pot over winter. With less soil around a plant's roots, that soil is apt to be drier. Also with less soil separating the roots from the air temperature, the root are exposed to even colder temperatures than the temperature frozen soil achieves.
The last factor I would stress is how well prepared and how healthy a plant was going into dormancy. The growing season of 2012 and its winter was fairly hard on plants. Some were just coming out of that at the beginning of the 2013 growing season when a protracted late spring topped off by a suffocating late spring ice storm on April 12 of this last year caused significant damage and eve may have killed some plants.
There are some plants rated for my zone which with I have had poor results. They just do not grow well in central Wisconsin, and not just for me. These include hibiscus, some heuchera and tiarella, buddeleia, and ferns.
There are others which I have pushed the zone and which this might be their terminal year. These include my Japanese cypress 'Sagu suki, caryopteris 'Dark Knight', and the 'Bloodgood' seedling I transplanted in early October. Even plants like cotoneaster horizontalis, Korean boxwood, and viburnum may not be safe. Bold gardeners grow magnolia soulangiana and stellata here in their shrub forms rather than as trees for the opportunity for seasonal regrowth from roots when top growth in a tree form could kill or severely damage a specimen. Still, this may be a killing year for any newly planted or stressed magnolias.
And then there were the plants in trouble even before this winter; that bit of my privet hedge that had such a hard year in 2012-2013, my dappled willow which had suffered heat stress in 2012,and my smoke bush 'Nordine' that died to the ground in 2013.
Many years, I plant right up to within 4-6 weeks of the date I feel is average ground frozen day, Thanksgiving. This year I am thankful I did not have that opportunity. Our October was misleadingly warm and pleasant. Those of you that did plant late, I hope you mulched and watered well any transplants right up until Thanksgiving. It may well be your saving grace.