Two trees with mid-summer blooms: one doomed, one divine

The Sourwood tree's fall foliage, after the lily-of-the-valley-like blooms have faded. The Sourwood tree's fall foliage, after the lily-of-the-valley-like blooms have faded.

The Sourwood tree's fall foliage, after the lily-of-the-valley-like blooms have faded.

The Sourwood tree’s fall foliage, after the lily-of-the-valley-like blooms have faded.

After the lindens finish blooming in early July I quit expecting to see—or smell—another tree in full flower. I’m not entirely sure why that’s so except that it’s usually hot by now, and if I have to be outside under the punishing sun, I hide under a hat that blocks my skyward vision and forces my attention back down at the garden. So even though there was a mature mimosa tree (Albizia julibrissin) spanning the front yard when we moved in, I am always pleasantly surprised to catch its old-fashioned peachy-sweet scent on a hot summer breeze.
Ours was a pretty tree. An ideal example of the species’ broad up-and-out vase shape. Its fern-delicate leaves cast a lovely, lacy dapple; and each July, I got a total kick out of the silken pink poufs arrayed like a chorus line along the top of every branch. That is, until they dropped in tangled smothering heaps all over my front border. No more. Twenty-five or thirty years is about all that can be expected of such a fast-growing tree, and since it was a good 20 or 25 feet tall I would guess it was somewhere near that old when we took possession. And from that day, I could tell it was doomed. It had the fatal, untreatable wilt that plagues the species.
Fusarium oxysporum f. sp. perniciosum is a fungus that attacks and clogs mimosa’s vascular tissue—the very cells that transport water and nutrients to the leaves. It manifests in cracked bark (our tree had a telltale split at a lower limb branch collar that wept black streaks down its trunk), yellowing leaves, premature leaf drop, and sudden death. Kind of a bummer. But on the other hand, for me, it was good riddance. Mimosas are low on my love-list because they’re an exotic species, invasive across much of the country, eager to colonize disturbed ecosystems and able to grow quickly enough to outcompete natives for sunlight and nutrients.
Unfortunately, like the worst offenders, our tree lives on. The few roots I haven’t been able to lever out after the stump was ground still send up suckers, and its seedling progeny continue to pop up all over the garden and in every pavement crack as well as in all of my neighbors’ yards and gardens. But its disease lives too. It’s in the soil, spread by water, air, boots, and insects, and may even have been in the seeds my diseased tree produced. Just this very evening as I was gathering my thoughts to write this, my next door neighbor took an axe to one of the 8-foot tall saplings he had nurtured on his side of the hedge. Over the last month or so, when it might have been blooming, it lost every one of its tiny leaves.
I replaced our mimosa with another mid-summer bloomer. An eastern U.S. native this time. Sourwood (Oxydendron arboreum) will grow more slowly to maturity (25-to 50-feet) and should outlive me. Its habit is much more upright and narrow, better suited to tight confines and small gardens, and its leaves are large and leathery, casting deeper shade. It flowers from mid-July into August, with graceful panicles of tiny cream-colored lily-of-the-valley bells splayed like ballet hands at the ends of each branch. Their scent, if you can nose in around the pollinators, is more subtly sweet but its ambrosial honey, “made by bees and angels,” is highly sought after by those in the know.
Come fall, when neighborhood mimosa leaves are turning yellowish brown, my sourwood’s leaves will salute the end of the season dressed in bright blazing red. Truth is, I chose a sourwood because it has such stunning fall color. As much as I love its mid-summer flowers, I never expect them.

Kristin Green is the interpretive horticulturist at Blithewold Mansion, Gardens & Arboretum in Bristol, where she’s worked since 2003. Follow her garden blog at http://blog.blithewold.org.

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