“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.