Winter Protection for Bigleaf Hydrangeas

I get a lot of questions about bigleaf hydrangea (Hydrangea macrophylla), which people here call blue hydrangea, because that’s its usual color in our climate. In particular, people can’t get it to flower. But they ask me their questions in spring, when it’s too late to do anything. Why not look at the issue when there’s still time to act?

Cards on the Table

First of all, blue hydrangea is not a good choice for northern regions. It’s simply not hardy enough for our climate. Nurserymen often give it a zonal rating of 5 (USDA zone 4), but I find them oddly optimistic. Serious books give it a rating of 6b or even 7 (USDA zone 5-6). That’s a long way from our hardiness zone here in Quebec. In other words, the wisest gardener doesn’t plant bigleaf hydrangea and avoids disappointment.

However, bigleaf hydrangea, like many other borderline shrubs, has a trump card up its sleeve: it is capable of regenerating from the ground if its roots have not been killed by the cold. And with the thick blanket of snow that falls in my region, its roots are protected from the worst of winter’s cold. So, in spring, the shrub grows back from the base, apparently healthy. It’s just a matter of cutting back the branches that have died in the cold. But where are the flowers? There won’t be any! In this species, flower buds are formed the previous summer, on the growing stems. As the branches froze to the ground, the embryonic flowers died too. Hence the desolation of people who can’t understand why their hydrangea isn’t flowering, but yet is producing beautiful new, apparently healthy branches.

A Ray of Hope

And yet, blue hydrangeas can be seen blooming in the region. Not every year, but occasionally. But why? Because, if winter protection is excellent, the branches may not freeze over their entire length. In this case, some flower buds can survive the winter and flowering can take place.

How can we increase the chances of at least some of the branches surviving the winter? First of all, it has to be said that the usual winter protections (jute and geotextile) do absolutely nothing. They offer no protection against the cold, only against the wind (i.e., the temperature inside the protection will be exactly the same as outside). But wind isn’t the problem here.

Styrofoam rose cones, on the other hand, can protect against the cold. They are insulated on the sides and top, but open at the base, so the heat from the soil (the earth is warmer than the air in winter) can rise to the inside of the shelter. Unfortunately, there’s no air circulation in a rose cone, and the hydrangea can rot. We need to find another solution.

Snow and Fallen Leaves

It’s worth noting that in our region we have two great natural protections against the cold: snow and fallen leaves. In fact, unprotected hydrangeas often bloom when protected hydrangeas do not. Why is this? It’s because, when the blue hydrangeas do bloom, we’ve had beautiful snow flurries, starting early in the season and lasting throughout the winter. As a result, the branches are well protected from the extreme cold and can flower the following spring. Just planting the blue hydrangea where the snow accumulates makes all the difference.

But we can do even better! Dead leaves also protect against the cold. Shred them (otherwise they’ll blow away) and pile them on top of your hydrangeas. Don’t be afraid to pile them thickly: 15, 30, even 60 cm of dead leaves. Shredded leaves have one advantage over all other winter protection: they give off heat! Indeed, even in the middle of winter, they decompose… and decomposition produces heat. What’s more, whereas snow can melt too early, leaving the plant exposed to late frosts, and “protections” need to be removed early in the season to prevent the plant from suffocating, you don’t need to remove dead leaves, as they disappear on their own, at their own pace. By the way, bigleaf hydrangea is an understory plant in its native habitat. By lining it with leaves, you’re simply replicating Mother Nature!

But even leaves and snow can’t always guarantee flowering. If it’s -35 or -40°C (-30 to -40?) for weeks on end, the cold will certainly penetrate and kill the flower buds, but in most winters, under a thick layer of leaves, and with the snow usually abundant in our region, bigloeaf hydrangea will flower without too much trouble.

The Ideal Location

For best results, plant blue hydrangea in part shade (especially not in full sun, where snow melts much too quickly!) in an area where snow accumulates, perhaps on the north or east side of the house. It prefers soil rich in organic matter and moist, but not soggy. Avoid fertilizers that are supposed to make flowers bluer: they are often toxic to hydrangeas! The best fertilizer for this plant is leaf mulch, which decomposes and enriches the soil.

No Pruning!

In our climate, bigleaf hydrangea should only be pruned to remove dead branches. Their presence will be obvious in spring, as they have no leaves! Above all, don’t prune them in autumn, or you’ll remove all next spring’s flower buds!

Good luck!

Blue or Pink?

Did you know that large-leaved hydrangea can be blue-flowered or pink-flowered, depending on the soil? In acid soils, their flowers are blue; in alkaline soils, they’re pink. However, in our region, soils are always acidic and large-leaved hydrangeas always bloom blue.

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Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This article was originally published in Le Soleil on November 13, 2005.