Will your spring-flowering shrubs survive the frigid winter?


Wisteria is gloriously, delicately mauve and hypnotically fragrant in June, as long as the buds haven’t been frozen over winter or in a late spring frost.


“Everyone talks about this winter’s storm damage to tree branches, but no one mentions the effect the cold may have had on spring-flowering shrubs and trees,” commented my friend Douglas Markoff as we chatted about the weather (what else?) last week. 

A light went on: What about the rhododendrons in our front garden? And the creamy white magnolia, my husband’s favourite spring shrub? Or the wisteria that drips mauve blossoms in June and smells divine on the arbour at the side of our house? They were all encased in ice for a week around Christmas and survived with no broken branches, so I wasn’t especially worried.

But spring-flowering shrubs set buds in autumn and the question now is: will these buds flower, or have they been frozen to death in the Arctic-like temperatures we’ve been experiencing?  I worry most about the magnolia and rhodo buds, which are are plumply visible at the ends of the branches, but forsythia, viburnum and serviceberry could be susceptible too.

Douglas is a serious gardener, a guy with a degree in botany who’s also executive director of  The Riverwood Conservancy, a municipal garden and wildlife area in Mississauga, where I live.  In other words he has credentials, but he admits he has no definitive answers.

“Plants have a remarkable ability to bounce back from weather challenges, but this has been an exceptional year,” he says.  Ever the optimist, he’s confident most hardy spring-flowering shrubs will come though just fine.  “Nevertheless, this year will be a test. I’m going to watch my  ‘Arnold Promise’ witch hazel. It’s the first plant to flower in my garden and could be an indicator. If it doesn’t bloom, it might mean trouble ahead for other plants.”

I remember a frigid winter about 20 years ago when even the hardy forsythia didn’t bloom above the snow line in spring. (Ohio State’s Pocket Gardener web site says forsythia buds don’t survive below -5 to -15 F [about -20F to -26C].) That was an odd sight—shrubs covered with brilliant yellow blooms for a foot or more of their base, and bare branches above. The thick layer of snow we’ve had this year will be a saviour for low-growing plants, an insulating blanket protecting them from the cold.  Maybe the thick layer of ice that encased the magnolia and wisteria branches insulated and protected their buds, too.

There’s another factor–our early winter after a seasonably coolish fall. “Plants can handle extreme temperatures if they’ve acclimated slowly over a few weeks, as opposed to sudden shocks,” says Douglas.  Maybe the buds on the magnolia, the wisteria and rhododendron hardened enough to be protected against the extreme cold. The plants themselves should be fine–wisteria is hardy to -25 F or Zone 4 in Canada, although it may not bloom; my P. J. M. rhodos were bred in Minnesota  and the U of Minnesota site says the buds can handle -35F  (-37C.) It lists several rhodos and azaleas with buds that are hardy to that temperature.

But as far as I can tell from some cursory research, cold temperatures alone don’t damage plants. It’s the intracellular ice crystals that form inside the protoplasm of the cells that damage the cells’ structure.   How much damage depends on how fast the temperature drops and to what level the plant’s cells supercool, which means their temperature goes below the freezing point without solidifaction. Some dormant buds supercool to surprisingly low temperatures–certain azaleas, for example, down to -41 degrees C.; some peach blossom buds to -25 C.

The length of time plants stay frozen (all winter, perchance?) apparently has little or no effect on their longevity.  And the temperature at which freezing occurs depends on to what extent the plant hardened off as the growing season wound down, confirming Douglas’s point.

That’s how I understand it, anyway. You can see it’s a complicated area with many influences. Botanists out there can feel free to correct me, please.

The real bud killer is easier to understand–it’s a late spring frost, or a spell of early warm weather followed by a heavy frost, as we had in March a couple of years ago. Many buds started to open prematurely, including some hardy fruit trees.  They thought spring had come, they stretched their lovely filaments and the juices started flowing. Then a hard frost hit ….

The next morning the very pregnant white buds on our magnolia were drooping like dozens of used Kleenexes. The frost got the wisteria buds too, although they weren’t nearly full term.  Our next-door neighbour’s apple tree didn’t bloom, like hundreds of peach and apple trees in the Niagara region.

“Northern plants evolved to handle cold temperatures, but now with the generally warming climate we have Carolinian species growing in areas of the country they never grew before,” Douglas says.  “Have our northern species also acclimated to warmer weather? All we can do is be observant, gather data and compare notes.”

And so I invite you to observe and record how your spring flowering shrubs fare this year. Did they bloom? Were some buds killed? Were the flowers  better than ever?  Let’s talk about it in May or June, and maybe we can understand the whole process better.

Which doesn’t mean we can do much about it. It’s the weather, after all.















7 thoughts on “Will your spring-flowering shrubs survive the frigid winter?

  1. Hi there — I’m curious whether your wisteria has managed to make it through the winter and whether you are seeing any signs of growth yet? We have a 50-year-old wisteria vine over our front door that is normally showing signs of life well before now but this year it still looks dormant. I’ve snipped a few tendrils and they are pale green inside so the vine can’t be completely dead, but it sure looks it. How’s yours doing?

    • Hi Karen and Alison:
      We’re all in the same boat–my wisteria isn’t going to bloom this year either. And it’s usually a conversation stopper for it’s great flowers and its intoxicating perfume.
      But severe winters like the done we just experienced can kill the plant’s flower buds. This is the second time it’s happened here in Ontario in three years (the first was a late hard frost that killed visible flower buds) and I’m getting discouraged.
      But what can you do?
      First, Karen, don’t take your wisteria out–it would take a week to cut it down and dig out your established vine anyway. Scratch the outer bark with your thumbnail in a couple of places. If you see pale green inside, as Alison did, your vine is alive, and it will bloom again. Maybe even this year, if you feed it right away with superphosphate (0-20-0). You might encourage it to bloom later this summer, although the flowers won’t be as full and lush as they would have been in an normal spring. I’m going to give that a try and see what happens.
      Wisteria normally flowers before the leaves come out, so I’m not expecting to see foliage for a couple of weeks.

      Whatever you do, be patient and hope for a better winter this coming year.
      I didn’t think my neighbour’s forsythia hedge was going to bloom this year, but it did, although about a month later than usual. My witch hazel was spectacular, too, again late. Around here, boxwood took a big hit, although my little established hedge looks pretty good.
      I’ve also lost a half dozen perennials planted last spring, an last fall they looked healthy and established.
      Too young and vulnerable I guess.
      The climate is changing, there’s no doubt about that.
      Sorry for my delay in replying….I was in California, where it was hot and droughty. See my upcoming blog on a great Westlake Village garden tour, with roses we die for here in the north.

  2. A depressed gardener — help. I have a 14 year old wisteria over my arbor on our decked that has provided unlimited hours of shade and enjoyment during hot Ohio summers. Every guest that visits our home comments on its beauty not to mention numerous years of trimming and training. We experienced temps this winter of -11F with wind chills of -50F. The result, it is now May 21st and not a leaf on the wisteria and buds appear to be dead. Will it revive over time or is it destined to become a demolition project no matter how optimistic I am.

    • I am in toronto and experiencing the same disappointment with my wysteria vine which is showing no signs of life at all. There are no active buds and we are entering the last week of May. Given the size of it’s trunk and age I cannot believe it is dead completely and so will have patience and see if leaf comes later. I have had a similar problem with old lilacs…no flowers but leaf and burning bushes have been hit on one side or another with some green. These I will prune and cut back as I know they can re grow even if cut back to ground level. I can sympathize with wysteria growers but suggest some patience for now.

      • HI Deb:
        My neighbour’s lilac is blooming near my bedroom window, wonder of wonders. It looked pretty dry and dead till a couple of weeks ago, when a few leaves reluctantly made their appearance. Then…flower buds! Yay! Not as many as last year, but enough to prove that life goes on. As for our wisteria, which grows over the arbour at the side of the house (see the photo above), right next to the lilac, it looks dead.
        But my husband (the official pruner here) assures me the branches closer to the main stems are showing signs of green when he scratches them with his fingernail. He plans to prune the plant as far back as he needs to in a few weeks. He’s giving it time to rejuvenate.

  3. My 20 year old wisteria has been challenged by the ice storm in Toronto last December as well. I am holding my breath as it looks dead. However this week from the second floor of the house we can see several leaf buds forming. It is late May and we should being seeing bloom formations so I suspect it will not bloom this year as was noted in the article. Was wondering if trimming the vine right now would also be a good idea to promote growth?

    • Well, you have a point, Andrea–wisteria does seem to respond well to severe pruning. However, I’d wait a couple of weeks to see how much leaf-out occurs then prune out trailers and, of course, the dead stuff.

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