The Winter Landscape: Plant Hardiness

Plants are very specific about what soil, light and water conditions they need to thrive. Any gardener who has moved a sulking plant around 3 or 4 times before striking pay dirt understands this. It is simple to spot a plant that is unhappy.  Figuring out the cause of the trouble can be tough, as there are so many factors that come in to play. There are those plants, in defiance of every good intention and effort to find them a suitable home, that fail to prosper. I have killed outright plenty of plants. Given that every plant has a strong will to live and reproduce, those failures are frustrating and embarrassing. Figuring out what a plant needs to thrive is 2 parts science, 2 parts luck, and 6 parts good instincts. The hellebores in the above picture look unhappy, as they always do in February. They like the filtered sun, protection from wind, the friable soil and regular moisture that is available to them here in the growing season. They have what they need to thrive. What they do not like in February is the cold. Before any plant in a Michigan garden can thrive, it has to be able to survive our winters.

Plant hardiness zones indicate a worst case low temperature for different parts of the country. The USDA, in compiling data from weather stations, and factoring in mitigating circumstances which can influence low temperatures, has produced a plant hardiness map. Interested in that map making process?  mapping plant hardiness    It gives gardeners an idea of what the lowest average winter temperature can be in a given area. The hardiness map is a guide that can help gardeners select plants which are appropriate for their area. My area is at best a zone 6, and at worst, a zone 5b.  This means that plants that are able to withstand low temperatures in the -15 to -10 degrees should be able to survive in my garden. That includes these ratty looking  hellebores. Last year’s foliage may be burned from cold and wind, but they are very much alive, and waiting for some signs of spring.

The key to survival in very cold regions is dependent on that plant slowly going dormant over the metabolic slow down that we call fall, and a constant deep dormancy throughout the period of very cold weather. What the hardiness zone map does not provide for are weather events out of the norm. We have had temperatures in the 50’s and 60’s for a week. On February 18, the record high for that day of 62 degrees set in 1976 was eclipsed by a 65 degree day. One or two days of weather that warm might not affect any plants that are still dormant.  The frost is deep in the ground. But the days are staying warm. We have a low of 56 forecast for overnight tonight. Now I am starting to fret. It is too warm, way too early. Under that winter foliage, I see signs that the hellebore flower stalks have begun to emerge.

hellebore A plant breaks dormancy for a constellation of reasons.  Longer day length and warming temperatures are key. No plant reacts instantly to a brief change in conditions. But a change that persists is an invitation to wake up. Hellebores are greatly prized by gardeners everywhere. I do favor helleborus orientalis hybrids for my zone.  This group of hellebores, now known as helleborus hybridus, are bred from hellebores commonly known as Lenten roses. This group generally begins blooming in April in my zone 6 garden. Hellebore hybrids featuring the genes of Helleborus niger, commonly known as the Christmas rose, naturally bloom at a time which is winter in my garden. Do I have inter generic hellebores in my garden that would prosper better in zones further south?  Of course I do. I am always pushing my luck. I have had great success growing plants marginal to my zone. But nature is unpredictable. Nor does she care a whit for me and my love for my garden.

snowdropsOur bitter 2014 winter turned my zone 6 into a battle zone 4. None of my magnolias bloomed. My parrotias sustained extensive die back to their upper branches. My roses were killed back to within a foot of the ground plane. My clematis sputtered. My 20 year old boxwood had unbearable damage. It was a sobering experience, to say the least. Any plant marginal to my plant hardiness zone suffered damage of one sort or another. The winter of 2015 might have been worse. On February 20, 2015, the temperature in Roscommon Michigan was -39 degrees. The statewide average for that day was 18.5 degrees below zero. The damage done to evergreens these two back to back vicious winters was extensive. But those plants that were safely dormant managed to survive. The snow drops showing their faces yesterday worry me.  65 degrees in February is an anomaly.  The cold weather will return. How much cold can their flowers endure?

magnolia stellataOf course I am anxiously checking all of my early spring blooming plants.  This magnolia stellata was a 2 foot tall tree the day I bought my house twenty some years ago. It has prospered where it was planted.  It did not bloom in either 2014 or 2015. Its hardiness was sorely tested by a pair of bad winters.  The flowering in 2016 was a happening.  I am trying to decide if our warm weather is encouraging the buds to swell.  Not that I could intervene. Mother nature bats last. I am reading we have some night temperatures in early March in the teens.  Though I am ready to wash my hands of winter at the end of February, March is a winter month for us. Any early spring plant which is breaking dormancy right now may not fare so well in March. This is more a worry than a certainty.

Late this past summer, I replanted my rose garden. Though I know there is a limit to their hardiness in my zone, it is better for me to start over, than to do without. That was a personal decision, not a decision driven by a plant hardiness map. As for the dogwoods, I hope they will be able to endure a freakish warm spell in the middle of winter. No small part of the winter landscape is an appreciation for endurance.

 

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Say It Ain’t Snow

more-snow.jpgIn Detroit, 49.3 inches of snow have fallen so far this winter, more than the full season average of 42.7 inches, and there are at least two months left in winter.  Detroit is seeing its snowiest January on record: Detroit has received 31.3 inches so far this month, breaking the old record of 29.6 inches set in January of 1978, according to the weather service’s White Lake Township station. And more snow is on the way.  There are those who might think this is a record of dubious merit.  But though I am truly tired of the endless snow, I am also much relieved.

DSC_7500The shop is about 30 miles north of downtown Detroit.  The USDA plant hardiness map puts us in Zone 6a.  This means that plant which are hardy from -10 degrees below zero to -5 degrees below zero should live and prosper here.  True plant hardiness is not all that formulaic.  There are invariably mitigating circumstances.  Some years those circumstances work for me.  Other years, marginally hardy plants slip away.  Some zone 7 plants will live for me, provided they are planted in very sheltered spots, and have good drainage.  But no matter my efforts, the proof is in the winter putting.

Jan 26 2014 (3)I have clients that have old Franklinia trees, 2 story tall American hollies, Brown Bracken magnolias- and other more southerly specific landscape plants that are healthy and robust, in spite of our plant hardiness zone designation.  I am sure that proper siting played a big part in their survival.  I have another client who lives in Detroit, and with much effort, mulching and wrapping, overwinters certain palms in the ground.  Palms-really?  Detroit is a big city with lots of buildings.  The mass of all of those buildings holds the heat, and the height of the buildings protects plants from wind.  Plants are very specific about what they want, and what they will tolerate.  The hardiness borders are not fixed.

Jan 26 2014 (8)There is no reason not to push the envelope, if there is a plant you feel you cannot do without.  The -10 degrees to -5 degrees is a worst case scenario in zone 6a.  So what gardener doesn’t like to plant dangerously? That is part of the fun of having a garden-stretching the limits.  But we have had temperatures that are coming close to that worst case scenario, and colder temperatures yet to come.  What does this mean for the garden?

Jan 26 2014 (22)First up, I need to refer back to the snow cover, as amply illustrated above.  Snow is water that freezes as it falls.  As this granular and crystalline form of water piles up, it forms a blanket which, despite its cold composition, has great insulating properties.  Think of all of this frozen water, with air trapped in between each crystal.   Years ago I was so surprised to learn that spring flowering bulbs never freeze solid.  The earth has a latent heat which keeps most bulbs just at or above freezing.  Bulbs that I plant in pots for the winter, that freeze solid through and through for lack of sufficient protection, will rot come spring.  Insulation is an important part of weathering the winter.

Jan 26 2014 (31)Where am I going with all of this?  The insulating role of soil, and in our present case, heavy snow, plays a major role in protecting landscape plants from cold injury.  My landscape plants began slowing down in August.  In an ideal season, the transition from the growing phase to the sleeping phase is very long, and imperceptibly gradual.  Sudden and extreme cold, like what we experienced several springs ago, will catch a plant off guard.  A plant which has begun leafing out has had a biological, final, and emphatic call to leaf out.  The new shoot that is out there is vulnerable to an abrupt change in conditions.   If the environment suddenly goes cold, the new leaves will suffer.  If the dramatic cold is cold enough, and long enough, branches of trees can die back.  In our cold a few springs ago, many Japanese maples died outright.

DSC_7503Tulips are well prepared for nature’s mood swings.  The leaves poke out of the ground-they test the early spring waters. Unforeseen cold weather can disfigure the leaves, but the buds stay tucked away underground until the weather reliably warms.  An unexpected freeze when the tulips are in bud-I shower them with water. The water will freeze before the blooms.  Gardeners, and spring blooming plants-they are endowed with incredible fortitude.   My trees and shrubs have been dormant a long time.  The roots of my magnolias are deep in the soil, and mulched with 40 inches of snow.  I am thinking – and hoping – that they will survive this brutal cold without incident.

Jan 26 2014 (27)If I have to have the cold, then I want as much snow as possible.  Middling cold winters with no snow cover worry me.  Though I know the will to live is incredibly strong, will is not everything.  Middling cold can damage or kill plants that have no insulation from snow.  As utterly bored and irritated as I am by the extreme cold and the relentless snow,  I am in the gardening gig for the long run.  That would mean a spring in which I see all of my plants leafing out, ready to grow.  So bring on that snow.

Jan 26 2014 (24)Underneath our 49 inches, a garden.