Tag Archives: Cold Hardiness

Winter 0f ’18 -’19: Cloudy, Mild With a Chance of….On Weather, Zones & Plants

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The center green band of these NOAA charts delimits the 20 year average for each day, the bottom marking the average minimum temp and the top marking the average high temp. The red and blue portions of the chart mark the range of temps between that and their record high and low levels. The dark blue ‘bars’ show the actual recorded temps for those days. This chart shows distinctly the initial cold span with the slight warming to well within normal range, followed by a more irregular cooling. The relative steadiness of these temperatures helped plants maintain their slower, more dormant metabolisms for the month. Typically February has been notable for the stability of its daily temperatures, the relative narrowness of the ‘green, normal, band’ indicates this.
This chart for March again shows the slowly warming/rising light green normal band contrasting with the dark blue bars of our actual temps. The March temps appear colder because they shifted the chart downward. The more extreme minimum temperature swings of Nov., Dec., and Jan. have even out, the lt. blue portion narrowing and showing more consistency.

It’s 41ºF at 5:30am on Mar. 12 as I begin to write this.  We appear to have come out of the longest sustained ‘cold’ period of the winter of ’18-’19 which began on February 4 and continued through Mar. 11, a period of 36 days.  Over those days we had freezing minimum temps at PDX, the official NOAA reporting station for the Portland area, on 26 of them.  On two of those days, Feb. 6 and 7, PDX recorded the winter’s lowest temp, 23ºF, making it a zn 9a winter, mild for us historically and especially so for the temperate US as a whole, much of which was experiencing its own much colder temps.  It’s mid-March and our high temps have climbed well above what they were and our forecasts call for milder, more ‘normal’ highs and lows now locally.  It looks likely that not only are we going to be on the ‘warm side’ of normal, but that our lows have shifted into a pattern well out of the freezing range.  (State ODF meteorologist, Pete Parsons, calls for a pattern of slightly warmer and drier weather than normal over March, April and May with the highest chance of this during May.) 

While weather consists of moments, recorded data points, we attempt to make sense of it in its patterns over time…our experience of it.  In this we are much like our plants.  Plants too have their ‘expectations’ of the weather and those conditions that take them outside of them, outside their familiar patterns, the relatively quick changes and perturbations, as well as the longer sustained patterns, and extremes, are ‘noticed’ and make a difference.  How does this winter compare? Continue reading →

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Chilling, Freezing & Surviving: Understanding Hardiness & Preparing Your Plants for Winter

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Parrotia persica, Persian Ironwood, has long been among my favorite trees, for its leaf shape, substance, fall color, the overall plant form and for its exfoliating puzzled bark. Hardy to zn4 this plant is tough as nails here requiring no protective effort at all.

[A note to the reader. This is not a scholarly treatment of all of the peer reviewed material on this topic.  There are no footnotes or listed sources.  This is a product of my more than 35 years of horticultural field experience and gardening along with what I’ve gleaned from reading several technical peer reviewed articles on the subject.  Such material is difficult to read and can be off putting and intimidating to even the educated layperson.  This posting is my attempt at interpreting the research and reviews that I read in a way I think is understandable without overwhelming the reader with bi0-chemistry and the technical esoterica scientists must consider in their pursuit of understanding.  Any faults are mine.]

I’ve been thinking about plants and their response to cold having watched the deciduous trees drop their leaves, dug tender plants out of the garden and moved pots around to where they would be adequately protected from freezing temperatures.  We all know what happens to water when it freezes, going from a liquid state to a solid one, its molecules forming crystalline structures, expanded and rigid, responsible for burst water pipes and snowflakes.  Water possesses some amazing qualities, as a solid, kind of counter-intuitively, it becomes less dense floating rather than sinking even taking on some insulative qualities and stopping the convective flow of heat that is normal in liquid water.  At the instant of freezing water releases a small but measurable amount of heat. What happens inside plants when temperatures drop below freezing? How does the plant keep from bursting its own cell walls like the water in pipes within an unheated crawl space or wall cavity of a building?  You’ve seen what happens to plants like Coleus and the sodden black heaps they become upon thawing out. Continue reading →