A Gardening Journal

Tree Sorbaria

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No, this isn't the largest and most graceful plume of white astilbe.  This is better:  Tree Sorbaria: Bigger, later, and less well known.  Those minute white buds, in pointed clusters of, oh, a thousand, are a lacey show in themselves. 

 

Even when younger and greener, they are a delicate show against the ferny green leaves.

 

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Tree Sorbaria's particular talent, though, is to bloom for such a long time that the early flower clusters are still a-bloom when the buds of adolescent clusters are just turning white, the buds of juvenile clusters are still light green, and still other clusters are just being dreamed up.

 

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The plant blooms with a thoughtful sense of progression and pacing.  Not that all-at-once "kaBLAM" of dogwood or daffodils, which is great for Spring, when we welcome a slap in the face after Winter's long dreary stasis. 

 

But by Summer, it's better to prolong things instead of racing to the finish line.  And sorbaria is one of the champs of a measured response to the heat.  Full-on blossoms here, just-ready-to-pop buds there, still-in-diapers green buds to one side, and Hey-waddaya-say-we-think-about-making-a-bloom-cluster branch tips everywhere else.

 

 

 

Here's how to grow this well-paced Summer-blooming shrub:

 

Latin Name

Sorbaria kirilowii

Common Name

Tree Sorbaria

Family

Rosaceae, the Rose family.

What kind of plant is it?

Deciduous shrub.

Hardiness

Zones 5 - 9

Habit

Mounding; spreading from roots like sumac, as well as self-layering where stems touch the ground.

Rate of Growth

Fast.

Size in ten years

Depending if the spread is controlled or not, a mound ten to fifteen feet tall and ten to twenty feet across.

Texture

Despite its overall size, delicate and ferny, like cut-leaf sumac or, indeed, really huge ferns.

Grown for

the flowers are the real show here, despite the charms of the pinnate, ferny foliage.  The flower clusters are huge, up to two feet long and over a foot wide.  They are much-branched and pointed, and make a blooming shrub look like the biggest white astilbe you've ever seen.  Individual flowers are quite tiny, as small and tight as BB's in bud, but contrastingly fluffy in bloom.  Unlike the vertical plumes of astilbes, the flower clusters of tree sorbaria nod outward and even downward, with a demur grace that belies their impressive size. 

 

the Summer blooming season, always welcome in any hardy shrub or tree.  The season is long enough that full-bloom clusters are side-by-side with clusters that are in bud or even just forming.

 

the tough and tolerant constitution.  Tree sorbaria thrives even in lean soil, and after it's established doesn't need supplemental watering.  It blooms acceptably even in part shade but is denser and more floriferous in full sun. 

 

the unusually early leaf-out in Spring.  Sorbaria is the first woody plant to start leafing in most gardens—in early April for me, when all the other deciduous "woodies" are still just sticks.

Flowering season

Summer.  Depending on pruning—see How to handle it, below—in July or August.

Culture

Almost any soil, part shade to full sun.  In my experience, sorbaria is the shrub for people who claim they kill everything.  I have colonies thriving within spitting distance of open ocean, in blazing sun and sandy soil; in heavy poorly-draining soil inland; in shade in average soil; and in full sun in lovingly-composted beds.

How to handle it

Despite the "tree" name, tree sorbaria is just a big haystack of a shrub.  It doesn't make trunks or, even when you squint, remind you of a tree.  But it is several times larger, potentially, than the other sorbaria species, and calling it "tree" sorbaria is quicker and more engaging than, say, "bigger-than-the-regular-sorbaria" sorbaria.

 

Conceivably, you could select one main branch to stake and limb-up to make a small (and lovely) tree out of tree sorbaria.  Growing as a free-form bush, it's lovely at the back of large beds, or as a giant haystack all to itself in the middle of lawn.  But the easier and lovelier option is to grow the bush like you would a butterfly bush:  Cut it to the ground in Spring.  It resprouts lustily, just like the buddleia, and blooms at the tip of each and every new sprout, too.  Sorbaria is even more flexible than buddleias in that you could just as well do the pruning in the Fall or Winter, whereas buddleias should only be pruned in Spring after they've already started into growth.

 

This annual coppicing also delays the flowering a few weeks, so you can have yet one more shrub in bloom in August—well, early August at least—when all possible garden interest is desperately needed.

 

Sorbarias sucker profusely, which makes them great candidates for large-scale groundcovers, especially for banks in full sun or island beds in the middle of deathlessly-hot parking lots.  In mixed plantings, though, you need to make your peace with having to chop all around the plant each Spring lest it overrun everything else.  The runners are very shallow, and in well-worked soil you can even pull them up by hand.  Either way, there's never an excuse for not being able to give all comers their own hunk of this beautiful and easy-going shrub.

Downsides

If only it didn't really want to take over a thirty-foot patch of ground. 

Variants

The regular-size species, S. sorbifolia, is widely naturalized along roadsides, and, for a too-brief season a few weeks earlier than tree sorbaria, really does look like a huge colony of white astilbe: Its bloom clusters are held vertically.  It too can be grown in garden or big-scale-groundcover settings, and is also best when coppiced annually, either in Fall, Winter, or Spring.  S. sorbifolia 'Sem' is a bit more compact (but, alas, not a whit less interested in racing all over) than the species, but its real charm is that its new Spring foliage is a strong pink.  Perfect for unusual early-season interest in your Pink Garden.  I grow it proudly.  

Availability

On-line.

Propagation

By digging up the running sprouts in Fall, Winter, Spring, or even Summer.

Native habitat

"Temperate" (as opposed to tropical) Asia, from Siberia to Japan.

 
 
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