A Gardening Journal

Must Have: Bottlebrush Buckeye

Could any shrub in full flower be more exciting? Especially in August, when the flowering woody plants here in New England are so often hydrangeas, butterfly bushes, and roses of sharon. Foot-high spikes of feathery white flowers "candle" the canopy of this very large native shrub. By comparison, the flower spikes of butterfly bushes are too thick and dense—and, besides, everyone already has plenty of them, me included.

 

Aesculus parviflora overall 080517 640

 

Flowers of Aesculus parvifora are so graceful because the stamens and pistils project beyond the cup of white petals.

 

Aesculus parviflora flowers fingers 080517 640

 

In the picture below, the thick pure-white pistil extends an inch farther, while the stamens, tipped with appealing reddish anthers, curve out and around the petals.

 

Aesculus parviflora tip flowers fingers 080517 640

 

The upright stems of bottlebrush buckeye have foliage only at their tips but, even so, the soil surface of the colony is never bare. In the picture below, you can see that shorter leafy stems crowd beneath the taller ones, creating the shrub's own groundcover.

 

Aesculus parviflora groundcover layer 080517 640

 

Besides displayting these significant aesthetic triumphs, bottlebrush buckeye is exceptionally practical: it's very hardy, deer-proof, and happy in deep shade as well as full sun. And it's immortal when in congenial circumstances. Plus, this species is still quite rare in gardens, so its presence guarantees a frisson of discovery each time. 

 

Aesculus parviflora isn't just on my list of top-ten shrubs hardy in Zone 5; it tops it. Truly, no garden should be without it anywhere it is hardy and where it can enjoy reasonable amounts of soil moisture. No surprise, then, that I've used it for decades in projects for clients.

 

Why then, is the magnificent speciment in these photographs not mine? (I chanced upon it by the road in Lenox, Massachusetts.) Why haven't I established bottlebrush buckeye in my own garden? For one, this species is a vigorous colonizer, and can race outward with the speed of any sumac. For another, it's tall: Old colonies can be fifteen feet high and more. Further, it doesn't lend itself to techniques for "stylish compaction" that I use so often with other shrubs and trees; bottlebrush buckeye isn't a natural candidate to cut back to stubs each spring like a butterfly bush, or to grow as a single-trunked tree whose canopy can be kept low and dense. 

 

But if growing full-size and free-range, where could it fit in my own garden? It is ever-more-crowded with choice plants as it is. For a decade and more, I have had a spot all picked out—but in a large and wilder section of the property that will be a project-and-a-half to subdue and transform into ornamental plantings.

 

Meanwhile, there is room for this species' unique compact cultivar, Dawes Dwarf Selection. After thirty years, one grower reports that the original colony is barely five feet high and wide. This fall, then, for the Dwarf and, within the decade, for the full-sized straight species.

 

 

 

Here's how to grow this must-have shrub:

 

 

Latin Name:

Aesculus parviflora

Common Name:

Bottlebrush buckeye. Although there isn't a hard-and-fast rule for common names, the smaller species of horsechestnuts are usually referred to as buckeyes, and the larger ones as horsechestnuts. Because some horsechestnuts can be eighty feet and taller, the distinction is relative: Ohio buckeye, Aesculus glabra, can grow to sixty feet and higher. The common horsechestnut, Aesculus hippocastanum, can mature to one hundred and twenty feet and higher.

 

All of the horsechestnut species that typically mature to thirty feet or smaller, though, are referred to as buckeyes; given cogenial conditions, no species referred to as a horsechestnut would mature to such a compact size.

 

The species that produces edible chestnuts is difficult to distinguish visually from a typical horsechestnut, but is in the Castenea genus. Where it has not been eliminated by chestnut blight, it, too, can become massive: Castenea sativa often grows to about a hundred and twenty feet tall.

Family:

Fagaceae, the Beech family.

What Kind of Plant Is It?

Hardy, spreading, deciduous shrub.

Hardiness:

Zones 4 to 8; some sources say 9.

Habit:

When established, strongly suckering. In my experience, the rhizomes don't travel as far underground before popping up like those of Broussonetia, Rhus, or Tetrapanax. But spread they do. Outward exploration is also fostered by fast-growing prostrate stems—the "horizontals"—that snoop along the surface of the soil.

 

Other stems soar upward so quickly—two to four feet their first season—that they rightly belong in the category of growth termed "shoots." Side branches eventually emerge, but they, too, maintain a generally upward angle. The contrast with the ground-hugging horizontal stems is striking. Still other vertical stems—are these side shoots of the horizontals, or do they arise from the rhizomes?—are quite short and form profusely at ground level. These give the colony a self-groundcovering capacity. See "Quirks," below.

 

Left to its own, a colony of Aesculus parviflora will cover many square yards; old colonies in large-scale settings can be forty feet across and more. See "Where to use it in your garden" for contexts that can welcome such vigor; see the second "How to handle it" box, below, for ways to use Aesculus parviflora in more compact settings.

Rate of Growth:

Basal shoots can soar two to four feet their first season. Mature stems grow much more slowly.

Size in Ten Years:

Twelve to fifteen feet high and wide. Potentially to twenty feet or taller, and with an indefinitely wide spread.

Texture:

Lush when in leaf, thanks to the large foliage. Mid-fall to early spring, colonies are leafless, but they then have a contrasting spareness and integrity.

Grown for:

Its rarity in gardens: despite being discovered in the wild in its native Alabama by American and English botanizers in the late eighteenth century, and universally acknowledged as superb—whether in flower or in summer or fall foliage—this native, very hardy, disease-free, shade-tolerant shrub is not well known. Nearly 250 years later, it is still a joy to encounter it. In my experience, only in the last twenty years has Aesculus parviflora become more generally available; see "Sources," below.

 

Its flowers: most people are familiar with chestnuts only via their tree-sized forms, whose spectacular vertical panicles are borne in spring. Aesculus parviflora flowers in summer, after the majority of hardy woody plants are long done. The vertical spikes can be a foot long and longer, and are crowded with small white-petaled flowers; parviflora means small-flowered. Each blossom is enhanced by its stamens and pistil, which project beyond the petals an inch or so and give the entire panicle a feathery and, indeed, bottlebrushy character. Flowers toward the tip of the panicle are more likely to be complete—to have both stamens and pistil. The pistil projects even farther than the stamens and, so, further enhances the feathery texture. Flowers farther down the panicle are more likely to possess only stamens. The pistil is thicker, pure white, and more rigid; the slender stamens are tipped with creamy-red, showy-only-at-close-range anthers.

 

Its foliage, which is typically chestnut-like in form (relatively large for a hardy tree, and with five to seven round-ended, palmately-held leaflets), size (each leaflet can be three to eight inches long, so an overall leaf could be a foot across), and color (mid-green). Leaves of Aesculus parviflora provide three additional thrills: first, they are not troubled by diseases that can disfigure foliage of horse chestnuts. Second, the leaves are retained through the summer; leaves of horsechestnuts can scorch, turn brown, and be shed in August. Third, Aesculus parviflora leaves usually adopt a terrific butter-yellow color in the fall.

 

Its hardiness: in Zone 6 and colder, any summer-showy woody plant that is not a hydrangea, buddleja, or rose of sharon is automatically of great interest.

 

Its shade tolerance: provided it isn't subjected to drought stress, Aesculus parviflora succeeds in full sun. (See "Culture," below.) Seen across an immense lawn or meadow, a happy huge colony in full bloom is a horticultural coup de théâtre. But the shrub doesn't just seem to tolerate shade; it revels in it. Growth is likely to be (even) taller, somewhat more open, and even more laterally rambunctious—but not at the expense of the dense groundcovering of the "pachysandra" layer of stems. (See "Quirks," below.) There are more large-scale, wide-colonizing ornamental woodies for sun—think Clerodendron trichotomum, Elaeagnus x 'Quicksilver', Rhus, and Sorbariathan shade, for which only Calycanthus floridus comes readily to mind. (Sambucus canadensis, true, can do either.) In terms of shade tolerance, then, Aesculus parviflora is particularly desirable; in terms of aesthetic impact, it triumphs in both sun and shade.  

 

Its lack of appeal to browsers: Aesculus parviflora foliage and bark is not palatable, although its seeds no doubt are.

 

Its disease resistance: chestnuts are often at risk for fatal or at least disfiguring ailments, but not Aesculus parviflora.

 

Its longevity: Aesculus parviflora is immortal as long as cultural conditions are favorable. Because new stems emerge from the ground, even complete loss of above-ground growth, say from a forest fire, isn't necessarily fatal. Bottlebrush buckeye, then, can be thought of as a garden citizen with every bit, or even more, potential for longevity as any major tree. When possible, site this shrub for the very longest term. See "Where to use it," below.  

 

Its quality four-season presence: Aesculus parviflora is satisfying season after season. In spring, new foliage emerges; summer brings the dramatic show of buds then flowers; fall brings colorful foliage and (sometimes) a crop of horsechestnut-like seed capsules; winter reveals dignified if not colorful branching architecture that is impervious to snow and ice. See "Quirks," below, for a discussion of factors that can encourage seed-capsule production.

Flowering Season:

Mid-Summer: long before the flowers emerge, the whip-like spikes of buds are a show in themselves. Here in New England, the display begins in early July; the flowers themselves begin emerging in late July, and are effective into mid-August. Farther and farther south, the entire show happens as much as a month earlier: in its native range, the Deep South, flowering occurs June into July.

 

Interestingly, this earlier window also seems to occur in some colder inland locations, e.g., at the Morton Arboretum, which is west of Chicago. It could be that the high heat of a typical Midwest summer more than balances the deeper dormancy of a frigid Midwest winter. Coastal New England is, after all, a string of summer resorts precisely because the summer temperatures aren't so intense as they are farther inland—which could also mean that flowering is slower to occur, too. Having bottlebrush buckeye as part of the August contingent is yet another reason for me to be grateful to be gardening where I do.

Color Combinations:

Aesculus parviflora is colorfully flexible year-round. Spring to early Fall, the mid-green foliage goes with everything. In mid-summer, when the flowers are out, their sherberty-red anthers are perceptible only at very close range. Otherwise, the flowers read as pure white, allowing Aesculus parviflora also to harmonize with plants of any other shade while it's in bloom.

 

By fall, I always suspend my sense of proper color harmonies: at that time of year, I'm grateful for all liveliness in the garden, regardless of the hues and juxtapositions. That said, you and your garden might be able to coordinate nearby plantings so as to harmonize with this shrub's butter-yellow fall foliage.

 

As colder weather causes release of the colorful fall foliage, the gray-tan of the stems' bark is revealed. Like the green warm-weather foliage and (at any distance greater than two feet) the "pure" white of the flowers, the bark also goes with everything.

Plant Partners:

When you combine the almost inevitably sizable mature size of a colony of Aesculus parviflora with the near impossibility of establishing smaller-scale partners near it, let alone within it, and the buckeye's stunning display when in full flower, you have a formidable hog for any spotlight. If near neighbors are not to be left in the dark, they had better be not just monumental, but also maximally contrasting as well as showy.

 

Combinations with sufficient intensity as well as sheer size often involve a grove of an already-classy narrow and tall conifer. Bottlebrush can function either as its gigantic groundcover or as a counterweighting broad mound that is separate but nearby. Because Aesculus parviflora can grow to twenty feet over time, such conifers had better mature to fifty feet and taller. Select your candidate for a grove from these massive possibilities: Cryptomeria japonica, Metasequoia glyptostroboides, Sequoiadendron giganteum, Taxodium distichum, Thuja plicata.

 

It's more effective during the winter if the conifer is evergreen, but the foliage of the two deciduous conifer species in my list—Metasequoia and Taxodium—is so extreme in its feathery delicacy that the contrast with the Aesculus foliage would be definitive. Both trees display exciting russety fall foliage color, which would be in striking contrast to the butter yellow of the bottlebrush. With both of these trees, use Aesculus parviflora as an adjacent colony, not a groundcover: Metasequoia trunks are strikingly buttressed at the base, while Taxodium roots can produce bizarre, unique knees. It would be criminal for Aesculus to interfere with either eccentric display.

Where to Use It in Your Garden:

Site Aesculus parviflora with these considerations in mind: it's a major spatial and aesthetic presence that will probably outlive you. Unless controlled, it will spread vigorously over a large area. Its unusual "pachysandra" layer of growth combines with the shade that its higher branches cast, and the competition for moisture and nutrients that its roots mount, to rule out companion plantings that are smaller. Even so, it's a terrific mixer—when all the other plants are much much taller. With good soil and a reasonable amount of water, it can thrive in full sun as well as deep shade—but overly moist soil, let alone boggy conditions, wouldn't be tolerated. Lastly, the usual pruning to maintain good form, promote heavy bloom, and further overall health isn't needed, so you won't have interventions like coppicing, tip-pruning, or dead-heading to rely on to limit colony height. Unless you're gardening in Minnesota, you're unlikely even to need to groom the colony in spring to remove winter-killed stem tips.

 

Where, then, to site a spreading shrub that will want to become twelve feet tall and more? For one, in the heart of as vast a lawn or meadow as you can manage; mowing around the colony, which will happen as a matter of course for that lawn or meadow, will also control the outward spread of bottlebrush buckeye's rhizomes and "horizontals."

 

What about a location that borders a pond, stream, or bog but that is high enough to avoid anything more than rare and brief flooding? The colony won't extend into the lower always-wet areas, so you need control further spread only on its drier sides.

 

In these regards, siting bottlebrush buckeye is similar to siting bamboo, which also can't tolerate saturated soil, but needs unstinting and industrial-strength control elsewhere. Can a colony be established behind curbing, which usually extends eighteen inches or deeper into the ground. A "wet-laid" wall—meaning one that has a poured-concrete core, or is made of blocks that are mortared together, and all atop a deep concrete founcation—is also effective. (A dry-laid one is likely to fail at stopping the colony's rhizomes and horizontal stems, which could extend into the wall's air-filled interstices and, likely, right out the other side.) Backing a colony up to a building whose foundation extends to a basement level will provide similar blockage.

 

The colony's eventual height will likely affect your choice as much as its potential spread. In a compact garden, any woody plant twelve to fifteen feet tall may well qualify as a major tree. In any garden, a dense (at least when in leaf) colony of anything that high and tens of feet long can function as a screen or even barrier. In an open setting, any mounding plant of such a scale that is spectacular in bloom will automatically be a focal point and, because the colony could become as large as allowed, it can command acreage of almost any extent.

 

Because Aesculus parviflora has a naturally irregular roundy-moundy profile, it could be just the relief needed for contexts surfeited with clean geometry. I'm developing a site with a more-or-less square parking court that is partly bounded by a five-foot-high hedge of purple beech, and fully outlined with curbing that, impressively, is formed from thick slabs of bluestone. One thrilling counterpoint of transcendently-disruptive irregularity will be a forty-foot beam hosting wisteria. Another will be a large colony of Aesculus parviflora (safely beyond a long retaining wall and a monolithically-built flight of masonry steps) that will (someday) billow high up behind the one length of the beech hedge.

 

See "Plant partners," above, for other large-scale combinations that bring even mammoth colonies of Aesculus parviflora into the same frame of view. These neighbors' habit (very tall and vertical) and scale (reasonably tall to extraordinarily so) make them safe from becoming overwhelmed, either visually or literally, by anything the size of the buckeye.

Culture:

Aesculus parviflora is not a plant to be drought-stressed, so site in moisture-retentive soil. There, bottlebrush buckeye will tolerate full sun. That said, the shrub seems even more vigorous—both in height and in spread—in dappled or even fairly dense shade, or with morning sun but full shade from mid-day on. As I've proved with many colonies thriving in often baking-dry full-shade backyard gardens in Manhattan, established Aesculus parviflora is remarkably drought tolerant as long as the foliage is protected from the sun. In normal soil, and with the amount and pattern of rainfall typical of eastern North Ameria, established colonies do not need supplemental water even during the dog days of summer.

 

Although this shrub enjoys good moisture, it is not happy with waterlogged soil. See how to turn this limitation into an advantage in "Where to use it in your garden," above.

How to Handle It: The Basics:

Plant transplants or divisions in early Spring, before the foliage has emerged. If your plant is potted or balled-and-burlapped, you can plant it at almost any time the soil is workable. Whenever planting, ensure enough water for establishment.

 

Aesculus parviflora is striking among deciduous shrubs or trees in that it needs no formative pruning, and little to no maintenance pruning. An upright "shoot" branch isn't normally susceptible to snow or ice damage and, even after it develops side branches, they maintain in collegial and graceful relationship with the shoots and side branches around it. In contrast to, say, any fruit tree, there is never a need to prune growth that is too jarringly vertical, nor branches that cross awkwardly. And because flowering is generous regardless of whether old limbs remain year after year—and those old limbs remain handsome, so why not leave them?—there's no need for the annual cut-backs that are needed for buddleja. Even deadheading isn't necessary.

How to Handle it: Another Option—or Two? 

The only pruning that might become important is controlling a colony's lateral spread. If your colony doesn't enjoy whatever head-and-shoulder room the side branches of its vertical shoots might desire, don't hesitate to prune any offending "sides" off entirely. Don't hesitate, as well, to cut any out-of-bounds major shoots back to ground level, so that overall colony height and width is reduced, and colony density from ground-level to canopy top is increased.

 

If for some reason you need to cut the entire colony to the ground to start over with its size and character, go right ahead. You'll likely eliminate flowering that season, but the colony's ground-level and rhizomatous growth will provide fresh vertical shoots that begin the ramp-up for full long-term impact. Such renewal pruning is best done in late winter or early spring, so that emerging growth has the longest season to maximize its height.

 

At ground level, you may want to cut back outward-pointing horizontal shoots to reduce or at least slow the spread of the colony's footprint. If any portions of the perimeter of your colony border grass, its regular mowing should discourage the spread of underground rhizomes.

 

Are there ways to grow Aesculus parviflora that affect its habit, not just its size? Could this normally many-stemmed haystack-forming shrub be grown as a low flowering groundcover, or a small multi- or single-trunked tree?

 

The first is easy: Plant the dwarf cultivar en masse. See "Variants," below.

 

The second and third options involve arborization: turning something that isn't a tree into one. With Aesculus parviflora, this is a multi-part project.

 

   -- Excess vertical shoots would need to be cut back to the ground so that the retained one reads as a young tree. For large colonies, the retained ones would read as a grove of young trees as long as there were enough space between them such that their canopies didn't touch.

 

    -- For a small multi-trunked tree, the retained vertical shoot would be allowed to branch naturally, and to increase in height ad libitum.

 

    -- For a small single-trunked tree—a buckeye standard, in other words—the retained vertical shoot would likely need to be tied to a permanent stake as tall as the intended trunk. (Providing support to a woody stem as it lengthens normally enables it to grow longer—in this case, higher—than it would on its own.) Any side branches that form below the top of the stake would be pruned off, whereas any that form at the tip of the shoot after it's higher than the stake would be left alone to form the tree's canopy.

 

    -- The "pachysandra" layer would be retained as the groundcover it is; it would be Sisyphean to keep it clipped back to soil level.

 

The purest way to grow Aesculus parviflora as a tree or trees is to begin with small starter plants and select and train the desired vertical shoots from birth, as it were. Then, any trunk can be perfectly straight, and in just the right location. The quickest way, though, is to begin with a mature colony. Select the main stem or stems that will be the trunk(s), and prune away the others. This latter tactic is also the more organic one, in that you're highlighting a main stem or stem that's already in place. 

 

Without question, changing the habit of this shrub so dramatically would be a victory of high-concept beauty over natural tendencies, and at a steep cost of ongoing labor. Only you'll know if the decision to embark on such a transformation is right for you, your garden, and your Aesculus.

Quirks and Special Cases:

In Zone 6 and colder, Aesculus parviflora is a very sparse producer of any chestnut's characteristic large-seeded capsules. In Zone 7 and warmer, production is reported as being much heavier. It isn't known if this is due to a lack of an ideal pollinator in Zone 6 or colder, or because the growing season there is shorter.

 

The overall growth of Aesculus parviflora is strikingly two-tiered, with strongly vertical and, often, very tall older growth arising from a low pachysandra-like groundcover of emerging young growth. Are these groundcovering stems just first-year, and they'll shoot up the next? Or do upright stems declare their tall nature their first year by shooting right up through the "pachysandra" layer?

 

The groundcovering ability of this species is powerful. Even if colonies didn't produce the pachysandra layer, almost nothing else will thrive beneath the taller shoots even though they are foliaged only higher up. It isn't known if this inhibition of competitors is from the shade of the foliage canopy, the overwhelming competition from the established root system, or—more deviously—an alleleopathic effect, meaning that the plant gives off (either from the roots or from the falling leaves) a chemical that inhibits the growth of plants that aren't buckeyes. Black walnuts are famous for their alleleopathic prowess.

 

No matter how it is that Aesculus parviflora out-competes would-be companions, it's a distinct rarity among tall and generally open shrubs in not needing underplanting with a weed-fighting groundcover. This ability, however, also affects how Aesculus parviflora can be used with companion plantings. See both "Partner plants" and "Where to use it in your garden," above.

Downsides:

Aesculus parviflora suckers enthusiastically and, unless contained, will grow to a vast (if thrilling) colony. See above, in the "How to handle it" and "Where to use it" boxes, for strategies for successfully growing the species even in compact gardens. Yet another tactic is to plant its compact cultivar; see "Variants," below.

Variants:

The racemes of Aesculus parviflora var. serotina 'Rogers' are considerably longer—eighteen to thirty inches!—and (but) are not held in the orderly fashion of those of the straight species. Instead, they loll, droop, and swoop, which may or may not be as appealing. To me, not.

 

Rogers also flowers a couple of weeks later than the straight species, which here in New England usually shifts its floral peak well into August. (The straight species is still in flower here in early August.) Rogers is reported to be even larger, too: to twenty feet. 

 

Regardless of the increased lateness and size of Rogers, A. parviflora [Dawes Dwarf Selection] is even more exciting. Its flowering isn't particularly late for a bottlebrush, but its growth is distinctly compact: After thirty years, the original colony is still not taller than five feet. Lateral spread via suckering also seems to happen in baby steps. This supplier indicates that in the first decade, a height of just two to four feet is typical, and a spread of just four. For bottlebrush buckeye, this is teensy-weensy.

 

With such large foliage that remains in great shape the entire season, if Aesculus parviflora ever produced a variegated form, it could be a sensation. Goodness knows other chestnuts with colorful foliage are intriguing. Here's a variegated form of horsechestnut, Aesculus carnea. Here's another horsechestnut whose emerging forliage is pink. Next year, I hope to profile a striking variegate of the Japanese horsechestnut, Aesculus turbinata.

Availability:

Ah, the mystery: Aesculus parviflora thrives all over eastern North America, and should be on the top-ten list of any gardener. But a generation ago, it was far harder to find than (I hear) seriously powerful pot: you had to know someone who had a friend who knew someone who might be getting some. At least as far as this shrub is concerned, civilization has advanced nicely since: Aesculus parviflora is now available online, especially in its cultivar forms, and also at quality retailers. Here in southern New England, I know of three major wholesalers who grow at least the straight species; there must be plenty of retailers that are stocking it, otherwise the wholesalers wouldn't bother. Hooray!

Propagation:

By division in early Spring, before the new leaves emerge. Seeds that are collected when ripe (September to October) and planted immediately, so that they don't have the chance to dry out, are reported as germinating readily.

Native Habitat:

Aesculus parviflora is native to the southeastern United States, which is zones 7, 8, and 9. It is strikingly hardy north of that: throughout Zone 4, in fact, which means it should thrive from Vermont to southern Minnseota, let alone Quebec. I love greeting it in the croquet garden of this drop-dead country hotel in Ontario.

 
 
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