Dear Dot: Does Wildfire Smoke Affect the Color of Hydrangea?

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Dear Dot,

I noticed on the Vineyard this summer that the usual bluey-purple hydrangeas were mostly pink or white. We were affected by wildfire smoke from Canada, and it was a rainy July. Did that impact the color of hydrangeas?

–Raymond

Dear Raymond,

Dot, too, noticed that there seemed to be more pink and white hydrangeas than usual. Indeed a number of Vineyard-based Bluedot staffers nodded their heads in agreement when I asked if your question resonated with them. And so I fully expected Tim Boland, executive director at Polly Hill Arboretum, to affirm our observations and offer up his usual expertise as to why. Was it the wildfire smoke, eh? The rain?

Nope and nada, Tim said. In fact, he explained, “the mop top hydrangeas — aka Hydrangea macrophylla; aka big leaf hydrangea — set their flora buds by the fall of the previous season for the next year’s bloom.” In other words, the stage was set for the hydrangea blooms we saw in July last fall, long before wildfires or rain.

And autumn 2022, Tim reminds us, was particularly warm, which lasted into January. In early February, however, a cold snap took temperatures down to negative 8 degrees Fahrenheit, along with 68 mph winds. “The floral buds were not cold acclimated,” Tim says. Because the buds hadn’t cooled down slowly into dormancy, the floral buds were killed by cold temperatures. Massacred by February, Raymond. Dot wants to report a crime against nascent loveliness!

All was not lost, however. While those hydrangea that set their buds the prior season were mostly winter killed, Tim explains, a few stalwart stems nonetheless formed new buds. Tim wants to remind us, too, that the Vineyard boasts a number of hydrangea species. Hydrangea panniculata  — aka limelight hydrangea — develops buds on the current year’s growth and it managed just fine. Similarly, smooth hydrangea — aka Hydrangea arborescens and native to the eastern United States (though not Massachusetts) — also blooms on new growth and so avoided February’s wrath. 

“I grow a close relative of smooth Hydrangea in my garden and at Polly Hill,” Tim tells Dot. “I collected it in Georgia — Hydrangea radiata — similar to smooth hydrangea but has silver undersides to the leaves.” Sounds beautiful. Dot has made a note in her calendar to visit Polly Hill next summer and take it in.

But to your question Raymond: it seems that those of us convinced that we were seeing more pink and white blooms and fewer blue are simply imagining things. Soil pH affects flower color by affecting the availability of aluminum in the soil. Acidic soil (pH 5.5 or lower) makes aluminum more available to plants’ roots. Neutral or alkaline soil (pH 7.0 or higher) decreases aluminum levels. And while ash from wildfires can temporarily alter the pH of soil (making it more alkaline), Tim says there’s no evidence that there was enough on Martha’s Vineyard to create a color change in hydrangea. Vineyard soils are so acidic — 4.5 to 5.8 — the amount of ash would have to be considerable, he says. 

Whatever color hydrangea we favor most, given that they aren’t native to the Vineyard, should we be avoiding them altogether? Not at all, Tim says. While he and the folks at Polly Hill encourage (and sell) lots of native plants, hydrangea arborescens is beloved by pollinators. “My garden is a mix, and it’s very alive and ecologically active. So, yes, you can plant non-native and still have abundant ecological services.” That’s horticulture speak for ‘plant … and they shall come.’

Acidically, Dot

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