A Gardening Journal

Golden Chain Tree

Laburnum x watereri Vossii bud close up 013116 640 

 

The Spring flowers of Laburnum are so spectacular they overshadow the tree's performance the rest of the year. In Winter, olive green stems are tipped with silvery buds. Stems in the background are tipped with a single bud, but the stem in the foreground has five. Does each bud mature to a raceme of flowers, or will some produce vegetative growth instead?

 

With Magnolia buds, you know ahead of time. Those that will mature to flowers are noticeably larger than those that are vegetative. Laburnum buds don't seem to be differentiated, so there's only one way to find out: I'll tie twine around this stem so I can locate it this Spring.

 

Meanwhile, midwinter is another peak in this tree's yearly cycle: It's training time. I'm covering one side and the top of a pergola with Laburnum, but the tree is nothing if not versatile. Its cultivars can be grown as arches, hedges, shrubs, groundcovers, weeping standards, or specimens with strikingly compact canopies. (See the "Quirks" and "Variants" boxes, below, plus the second "How to handle it.") You could even grow Laburnum the traditional way: as a free-range full-size specimen. Just underplant it generously; see "Plant partners," below.

 

So far, I'm using Laburnum on a pergola. For both economy and durability, mine is made of lengths of galvanized fence pipe. Laburnum stems are flexible so it's easy to bend them toward the horizontal and tie in place. 

 

See those slender new stems that have shot up vertically? They function like the whips of Wisteria: They don't flower, but they can be clipped off or used to extend the tree's overall coverage. I was a bit behind on the training a couple of years ago, and let some of these stems become thicker. I had to use clothesline to hold them down to the horizontal pergola pipes. Now that these branches have been in place for two years, they'll have adopted their horizontal orientation permanently; this Spring, I'll replace the line with twine, which will now be all that's needed to keep them in place.

 

Laburnum x watereri Vossii across pergola top 013116 640

 

Notice other stems that are much shorter and branchier. These are the ones I zoomed in on for this article's lead picture. They function like Wisteria spurs: They don't grow fast or far, but their buds usually mature to flowering racemes.

 

I've chosen the classic cultivar for training: Laburnum x watereri 'Vossii'. It's favored because its racemes are the longest (potentially to two feet!) and they bear dense crops of flowers. The racemes of the original hybrid L. x. watereri aren't as full or long, while the racemes of the two parent species, L. alpinum and L. anagyroides, are either shorter or sparsely-flowered.

 

Although this pergola is twenty-eight feet long, I'm training just one pair of 'Vossii' onto it. Even when supported, this cultivar doesn't grow large enough to clothe more than the central portion of the structure. No problem: I'm also using Wisteria sinensis 'Black Dragon'. I've planted one at the front corner of the pergola which, in the picture below, is out of view behind me. The Wisteria could grow to sixty feet or more if allowed; it is already halfway along the pergola, mingling with the Laburnum stems. In a few more years, the Wisteria will reach the back corner of the pergola in the center distance below. (There it will meet up with some stems of Tilia cordata 'Winter Orange' that I've already trained across the pergola's end piece.)

 

Laburnum x watereri Vossii overall side to top from ground level 013116 640B

 

On the side of the pergola frame, I've strung horizontal wires for training the Laburnum into a vertical plane as well, not just the horizontal plane of the pergola top: a Laburnum espalier, in other words. 

 

Laburnum x watereri Vossii overall from side 013116 recropped 640

 

A young yew hedge is developing just outside the wires. In years to come, it will mature to a dense wall of dark greenery, which will be as perfect a backdrop to the leafless Laburnum stems tipped in silvery buds in Winter, as to the cascade of flowers they'll produce in Spring.

 

 

Here's how to grow this ornamental tree:

 

Latin Name

Laburnum x watereri 'Vossii'

Common Name

Golden chain tree

Family

Fabaceae, the Pea family.

What kind of plant is it?

Hardy deciduous tree.

Hardiness

Zones 5 to 7, but is shorter-lived and less vigorous in the warmer portions of Zone 7. Laburnum does best where Summer temperatures are as cool as possible. In eastern North America from New York City south, this means any possible higher elevation or, anywhere, land that borders large bodies of water and, so, has cooler Summers. Regions north and east of New York are probably the most congenial as long as Winter lows don't go below minus twenty Fahrenheit. Laburnum might be best, then, in coastal Maine.

Habit

The trunk is short, with branches emerging to form a broad inverted pyramid from which the long pendulous flower racemes hang freely. The effect is even better if you reconfigure the tree as suggested in the "Quirks" and second "How to handle it" boxes, below. The process is helped by the tree's propensity to grow somewhat like wisteria, in forming longer whip-like vegetative stems as well as shorter and multi-branched spur stems, which bear the flowering racemes. 

Rate of Growth

Medium.

Size in ten years

Twelve to fifteen feet high, nine to twelve feet wide.

Texture

Full when in leaf; somewhat "bony" and awkward when not.

Grown for

its flowers: The racemes of L. x watereri combine the greater length of those of L. alpinum with the closer-packed array of the flowers of L. anagyroides. The racemes of 'Vossii' can be even longer—up to two feet! The flowers are forsythia yellow; both they and their racemes are produced in such quantity that the intensity of the show rivals that of wisteria. Trees hardier than Zone 8 that have bright yellow flowers are few. Remember how staggering it was when yellow magnolias became readily available? When you first saw a Hamamelis or Koelreuteria in full flower? Laburnum adds to this small group—and is the only one whose yellow flowers are arrayed in profuse pendulous racemes. 

 

its habit: The branching pattern of 'Vossii' is reportedly denser than that of the original hybrid L.watereri, which is, itself, noticeably more compact than either of its parents, L. alpinum and L. anagyroides. Either of these species can mature to twice as large as 'Vossii' and more.

 

its reduced production of seeds: All parts of Laburnum are highly poisonous, but the pea-like pods of seeds could be the most tempting to eat if not for the fact that they taste terrible. L. x watereri sets less seed than its L. alpinum and L. anagyroides parents, which reduces but doesn't eliminate potential risk. But how great is the risk actually? See "Downsides," below.

Flowering season

Mid-Spring; here in southern New England, that's the last half of May. The season is short—two weeks, three at the most—so plan any extended trips accordingly. The flowers of Laburnum alpinum emerge a couple of weeks later than those of L. anagyroides: June instead of May. Judging from the May peak of 'Vossii', it has retained the earlier flowering of its L. anagyroides parent. This is a very good thing, in that the pairing of Laburnum and Wisteria is irresistible because their flowers are borne in similarly pendulous racemes but have shockingly different colors. Neither Wisteria nor Laburnum has a particularly long season of bloom, and it would be tragic if the peak flowering season of each partner didn't coincide solidly: The show of a Laburnum just coming into bloom would be ruined if the racemes of its Wisteria colleague were already faded.

 

Are the flowers of L. alpinum just late enough to dance with those of Wisteria frutescens, which flowers several weeks later than its asian cousins? My W. frutescens 'Amethyst Falls' seems to come into flower by the second week of June.

Color combinations

The strong yellow of Laburnum flowers can cause as much Spring havoc in the general landscape as the equally strong-yellow flowers of forsythia and daffodils. Keep a firm separation between Laburnum and any plants that are celebrating pink or rose at the same time. When clashing is unavoidable, the tree's brief flowering season is a relief.

 

If you find that you've taken a shine to Laburnum, this may be just the nudge you need to incorporate it into a larger garden area in which its saturated yellow flowers will be right at home, and also enjoy a large-enough buffer from surrounding plantings that (almost inevitably) will clash. In addition to other plants that go whole-hog for chrome yellow, you can also incorporate into a Laburnum-friendly garden other plants that can bring to the scene purple, blue, white, or burgundy. See below. 

Plant partners

Unless you have the space and dedication to create a garden that welcomes chrome yellow, the relatively short Spring flowering season of Laburnum x watereri can be more easily handled in reverse. Instead of choosing plants that can provide specific (and attractive) commentary when 'Vossii' is in bloom, use neutrals that harmonize year-round. Broadleaved evergreens with all-green forms include Aucuba, Buxus, Ilex, Mahonia, Osmanthus, Rhododendron x laetevirens, Sarcococca, and the big-leaf bamboo, Indocalamus tessellatus. Taxus would be swell, too.

 

Another "in reverse" priority is to avoid companion plants that, whatever their other charms, are at their worst by late May. The floral display of 'Vossii' is so powerful that the tree will commandeer the attention of you and your visitors. You will all savor the tree's flowers at as close a range as possible. Avoid bulbs that might just have passed their blooming peak in late May, or whose foliage will already have begun to look shabby.

 

There is no sin in embedding 'Vossii' within such a neutral context, but any such response is, in effect, only solving a problem: those insanely bright yellow flowers for just two weeks a year. It's also possible to respond to the delirious but brief Laburnum flowers with pleasure and even inspiration. What plants are colorful at the very same time and don't faint when a nearby plant is, in effect, shouting? And are there any that (with a smile) shout right back?

 

Laburnum x 'Vossii' and Chinese wisteria, W. sinensis, are usually both in flower at the same time. (Japanese wisteria, W. floribunda, can flower a week or two earlier, so might not coincide. American wisteria, W. frutescens, flowers several weeks later, so never coincides.) Laburnum and Wisteria are both easy to train onto arches, espaliers, and pergolas, so why not train them onto the same one? (See my combination of 'Vossii' with Wisteria sinensis 'Black Dragon'.) Train the Wisteria directly onto the supporting frame, instead of letting it twine around the Laburnum, lest its rapidly-thicking stems strangle the tree. If you are gardening at the southern end of hardiness for Laburnum—Zone 7a— consider growing the Wisteria as the pergola canopy and the Laburnum as an espalier along the pergola's east or north sides: Laburnum welcomes a bit of shade in Zone 7a whereas Wisteria will bask in strong sun as well as high heat and humidity.

 

Although some forms of Allium also flower when Laburnum does—Rosemary Verey used 'Purple Sensation' under her Laburnum pergola—unless the pergola crosses open lawn, the Allium might need to be replaced every year: Although the foliage of the Laburnum (and, if you use it, Wisteria) doesn't emerge until the Allium foliage is going dormant, the base of the pergola would still receive some shade from any bulky companion plants on either side.

 

Permanent underplantings for Laburnum that provide color when the trees are in flower include Aucuba japonica 'Picturata', any number of forms of Hosta, Leucosceptrum japonicum 'Gold Angel', and Rubus idaeus 'Aureus'. And what about letting Hedera colchica 'Sulphur Heart' climb the Laburnum trunks? If you're not up to the (admittedly) high-maintenance pairing with Wisteria, what about Group A Clematis? They flower in April or May, not the June and July that begin the season for the Groups B and C forms. Clematis macropetala is usually in flower in April, which is too early. Clematis alpina, though, can be in flower in late May and early June, and forms with deep blue flowers would be sizzling with chrome yellow nearby.

 

And what about rhododendrons? Many flower in May. While typical broadleaved rhodies flower in shades of pink and rose that would be painful near the bright yellow of Laburnum, flowers of the deciduous Exbury hybrids and their descendents are often in saturated shades of yellow and orange and, so, could be just the thing. Flowers of 'Klondyke' are incendiary orange; those of 'Admiral Semmes' are bright yellow.

 

Pairing or contrasting with colors is always a delicate matter when the shade in question is so strident as the yellow of Laburnum. Without question, it is simple as well as exhilarating not to contrast at all but, rather, to bring in only white, neutral green, and other shades of yellow. There's all the more reason to restrict the colors because the range of yellows is uniquely large, in that there's a multitude of partner plants whose foliage features yellow, too. If the main color were, say, pink or red, the pickings for foliage would be exponentially fewer.

 

The temptation to add Wisteria or Clematis is so strong, though, that there's no point in trying to shut the doors after those cows have gotten out. Here's the challenge: Laburnum flowers lack contrasting colors themselves. Keel, wings, and banner are all the same shade of pure yellow. The suitability of any contrasting color you would add needs to arise from (so to speak) first principles in color theory, not a theory-be-damned alliance from, say, the crazy presence of pink at the tip of the keel. Alas, no. Think of that color wheel, w/ contrasting colors opposite one another. For yellow, that means only blue to purple—or, rather, violet as it is identified on the wheel.

 

The only side trip permitted would be to define burgundy as the far reach of violet. I'm all for it, so that burgundy foliage could come out to play, too. Is there a Laburnum pergola that is flanked by hedges of purple beech? Please, someone: Get on it! To tie this combination together with style as well as intimacy, I would add a pot (or pots) of a coleus that explicitly marries burgundy and yellow: 'Dipt in Wine', say, or 'Saturn'. Or, if tulips work for you (my voles, squirrels, and groundhogs rule them out for me), then pots of late beauties such as 'Black Parrot' mixed with 'West Point'.

 

Note that it would be too much of a good thing to add burgundy to the blues of any Wisteria and most Clematis alpina. With colors this powerful, two is party enough: the yellow of the Laburnum flowers and just one other. If you want burgundy, omit Wisteria (which doesn't have any burgundy-flowered forms) and switch your Clematis alpina to 'Helsingborg'.

Where to use it in your garden

Yes, the olive-green stems and silver buds are modestly interesting if you are able to backdrop them during the Winter with dark evergreens (see "Plant partners" above and the second "How to handle it," below). But the Fall foliage doesn't provide any pleasing color, the seed pods aren't ornamental and, when growing free-range, the tree's architecture of branches isn't particularly graceful or striking either.

 

On its own, 'Vossii' has only one peak season that will intrigue more than a geek: the flowers that are so bright and showy for just a couple of weeks in Spring. So, locate free-range Laburnum where it is partly hidden or, simply, in the distance. Then, you can take the hint from the flashy flowers and stroll over to enjoy them close at hand. If the tree is trained as a pergola canopy, an espalier, a hedge, or an attentively manicured coppice or pollard, it will have assumed a shape distinctive enough to warrant placement where it will be seen year round. Handily, 'Vossii' is very amenable to just such tricks. See the "Quirks" and second "How to handle it" boxes, below.

 

If underplanted with shrubbery mounding three to seven feet high—or sited at the back of deep beds, where the front plantings achieve the same mass—then 'Vossii' can be an unassuming and structural small tree when out of flower. It can therefore be sited where seen frequently regardless of whether it's in flower or not. See the shrubby evergreens in "Plant partners," above.

Culture

Almost any reasonable soil, including those with high pH, as long as drainage is prompt. Laburnum does not tolerate standing water, even briefly. Full sun in Zones 5 and 6; in Zone 7, especially in more southerly portions, light shade from late morning to late afternoon would be preferable. See the second "How to handle it" box, below, for how to train Laburnum to keep it compact enough for easy shading—and "Plant partners," above, for a suggestion for companionable shade-providing horticulture for trained Laburnum.

How to handle it: The Basics

Plant in Spring, ensuring enough water for establishment. If growing free-range, 'Vossii' needs little attention thereafter. If pruning is needed—perhaps to control size if you've planted the tree too close to a building, say—do so either right after flowering or in midwinter.

 

Although the earlier pruning allows any resultant growth to mature enough to begin formation of the buds that could mature into flowers the next Spring, 'Vossii' is so floriferous that I wait until midwinter to complete the majority of the work. Yes, there's some potential loss of flowers with mid-Winter pruning, but this loss is far outweighed by the ease of manipulating the stems when they are leafless. There's also the pleasure of an additional opportunity to get out into the garden in the depths of Winter regardless of whether the ground is frozen and the snow is deep.

How to handle it: Another option—or three?

Although the outward-and-upward angle of the tree's branches displays the long pendulous racemes of flowers beautifully, the floral performance lasts just two weeks out of the year, three at the most. When out of flower, let alone out of leaf, this tree is rarely interesting when growing on its own. Plus, for so many years the tree has been planted as an isolated lawn feature in the middle of compact front gardens that it has acquired the curse of being too easy: bullet proof, actually, as long as the modest requirements in "Hardiness" and "Culture" (see above) are met. The unrelentingly colorful flowers are part of the problem, in that they provide so much color for so little effort. Like 'Kwanzan' cherry, free-standing Laburnum became one of the arboreal signs that the garden's owner couldn't see beyond two weeks in Spring, and didn't care, either. Both trees became indictments.

 

But Laburnum is easy to train, and then looks irrepressibly joyful in flower and cleverly dignified the rest of the year. While free-standing ones need careful placement and generous companion plants (see "Partner plants," above) if they are not to look like quick-and-dirty stunts, a well-shaped Laburnum deserves a place in any garden that can provide the required degree of sun and good drainage. That all the training options reduce the size of the tree's already-modest footprint even further means that even tiny gardens can welcome one. And because some options can incorporate multiple trees, a group of well-trained Laburnum can become the star of even the most extensive and elaborate gardens.

 

Without question, the most spectacular training for Laburnum is to change it from a three-dimensional tree to a two-dimensional plane of growth supported by a metal frame. Laburnum branches are fairly flexible, especially in youth. Plus, the tree responds well to pruning, both in tolerating removal of out-of-bounds stems and in producing new stems that, either, continue development of the permanent framework of branches or bear the flowering racemes. Tying in branches to a frame is as easy as cutting off any that just aren't pointing in the desired direction.

 

If the frame is vertical, then you're espaliering. If it's broadly arched or (as mine is) flat-topped, then you're covering a pergola. Build the frame first. Galvanized pipe is an economical and durable choice although, yes, the durability is achieved via the brightly metallic finish. If you are blessed with a high masonry wall, your espalier "frame" can be just a series of horizontal wires anchored to the wall. Plant young trees immediately alongside your frame, considering which side of the canopy to orient toward the frame so that the majority of its branches can be most easily tied in. If you are espaliering multiple trees in a row, they could be spaced every ten or twelve feet: You'll have little difficulty extending the growth six or more feet out on either side of each trunk.

 

To tie a branch to a rung of an espalier frame, first lower it so it doesn't angle upward quite as steeply. (Tying branches to arched or flat-topped pergolas inherently necessitates the same helpful lowering.) If a branch becomes somewhat more horizontal than before, it is likely to flower even more heavily. After flowering is complete, there will be stems that need handling one of three ways: to be tied in to extend the tree's coverage on the frame; to be cut off entirely if they are redundant or projecting outward from the frame at an inconvenient angle; or, if you've extended growth far enough across the frame such that you can now also foster the floral performance, to be cut back to about six inches without being tied in at all. In midwinter, all of the branches are now bare and easy to review. Reposition and retie them as needed to even out their spacing and extend coverage, plus remove any ties that have grown tight and replace with twine tied more loosely. Then, cut off any remaining lengthy stems that are "pronging" outward from the overall plane of growth and aren't needed as tie-ins to extend it or fill it in. 

 

Then, look at all the stems that are short, sometimes also with short side branches. These will be tipped with silvery buds and, in Spring, some of those will mature to flowering racemes. Don't cut any of these short bud-tipped stems if you can avoid it. (They usually do not need to be tied-in, either.) Finally, locate the stems that you shortened to six inches back in early Summer, and see if you can't give them a second trim that leaves just two or three side buds. This helps those stems channel new growth into flowers instead of foliage or further lengths of stem.

 

One benefit of espaliering Laburnum is that the tree's overall height is usually reduced. This could be a help where Summers are so warm that the tree would appreciate dappled shade: If you include the espalier as the east or north enclosing sides of a pergola—and select the plant providing your pergola canopy so that it can be trained to keep the coverage lacy and light, not dense—then the espaliered Laburnum can be protected from sun that is too powerful for too long during lengthy Summer days. See "Plant partners," above, for a suggestion.   

 

Laburnum can also be trained as a free-standing soloist. The result is a free-flowering tree whose canopy is strikingly compact. This might be necessary if Laburnum were planted in a compact city garden where beds could be small and nearby boundaries or walls close by. But even in roomier settings, a more compact canopy casts less shade on underplantings while also giving the Laburnum a tailored and more unusual profile that is an asset in itself.

 

As with Spring-flowering shrubs and trees in general as well as woody Spring-flowering vines such as Wisteria, pruning needs to occur either right after flowering or in midwinter. (Do these later cuts with full knowledge that at least some of them are likely to remove stems that would otherwise contribute to the upcoming floral display.) Laburnum is similar to Wisteria in that flowers are produced from buds borne on shorter and branchier stems, not long wand-like stems that formed over the previous Summer. To control size while also fostering flowering or, at least, not unduly diminishing it, try these tactics: The first season or two after planting, shorten all the branches by half or even more if they already show young stems emerging closer to the trunk. As new stems emerge directly from the trunk, or closer and closer to it on the remaining sections of the original main limbs, cut back those original limbs even more. The goal is to reduce the canopy's original architecture to stubs six or ten inches long while also forming plenty of younger stems that can more easily be kept as short as desired.

 

Pruning to encourage flower production is next. Go over the tree as soon as flowering is through with an eye toward clipping back to six inches any long wand-like or sparsely-branched stems that began forming the previous season. Then evaluate the entire tree in midwinter. First, clip those six-inch branch stubs even farther—to just two or three buds if possible. Leave alone shorter, branchier stems, which are the ones whose buds will bear flowers in Spring. 

 

The result so far will be a Laburnum with a remarkably dense and compact canopy just a couple of feet in diameter. If you want, let the canopy increase in spread by cutting back wand-like stems to just a foot for a year or two instead of all the way back to six inches. If ever a smaller canopy size needs to be recaptured, achieve this gradually by cutting off one or two of the main limbs back to stubs, midwinter after midwinter, until the desired dimensions are achieved.

Quirks and special cases

Being a downside is sometimes a matter of context. When Laburnum is allowed to grow free-range as a small tree, or is trained atop pergolas or onto espalier frames, often-profuse shoots from the base of the trunk are a messy distraction even though, when allowed to mature a year or two, they will also flower with the same abandon as the tree's canopy.

 

But if the tree is grown as a hedge, those basal shoots can be just the solution for dense growth at the hedge's bottom. There are a couple of ways to "hedge" Laburnum, but they share the priority for good formation first and, only later, attention to the quantity of the floral display.

 

For quick but expensive coverage, plant saplings in a line, spacing them at two feet. You'll have created a quick screen of their upper growth, but there won't yet be any coverage around their trunks. Right after flowering, cut the branches back by half to force plenty of new side stems and also control the overall bulk of the line-up's developing canopy, which would otherwise cast increasing shade lower down, where you want those basal shoots to establish vigorously. Thereafter, prune the canopy annually, but just lightly: The initial severe pruning removed outer portions of major limbs that had taken several years to grow, but subsequent prunings need be concerned only with each year's new growth. Pruning heavily is likely to cause new side stems to lengthen too vigorously so, instead, cut off just the tips—the last six inches or foot, say. Then, new shoots will grow only modestly.

 

A couple of years after planting, the basal shoots will begin appearing. Allow them to grow their first year, but thereafter cut them back by half at the same time that you prune branches of the canopy. This encourages additional basal shoots while also helping the extant ones send out vigorous side branches. Hedges grow best when widest at the bottom, and you'll encourage the most vigorous new growth by pruning more severely, not less, so keep pruning the lower shoots back harder than the shoots from the canopy. Allow the height of this basal growth to increase gradually until it merges with the canopy growth.

 

Thereafter, the hedge can be kept in a steady state that encourages flowering while maintaining the mature size. Do this by modifying the annual clipping that you do right after flowering: Cut back any of the fast-growing wand-like stems to about six inches. In midwinter, revisit those pruned stems, cutting them, as well as any side-stems they may have produced, back to two or three buds. This second cut encourages the remaining buds to produce flowering racemes, not fast-growing vegetative stems.

 

A simpler and far more economical way to create a Laburnum hedge is to use small starter plants. Their first season, let them establish without any pruning, but in midwinter, cut off the tips of the branches to encourage side stems. In a year or two, some stems should begin to grow quickly enough to be cut back by half in early Summer. As the young hedge becomes thicker with side stems, allow overall height to increase by cutting just the tips up the topmost stems back right after flowering (if any). As the desired mature height is reached, then switch to the two-stage pruning, above, that fosters flowering.

 

If you plant just one such small starter Laburnum and handle it the same way as for a hedge, you can grow it as a compact and floriferous shrub.

Downsides

From bark to foliage to pods to flowers, all parts of Laburnum are poisonous. It's probably not excessively cautious to eschew the tree if you are raising children, who might find the pods too temptingly similar to those of snow peas.

 

All forms of Laburnum are propagated by rooted cuttings except the dwarf and weeping cultivars, which are usually grafted atop vertical understock of one of the species to form instant standards. So, even though the trunk of any Laburnum is growing on its own roots, the base often sends up suckers as enthusiastically as if it were grafted onto rootstock. Cut these shoots off any time the urge strikes—except if you are growing Laburnum as a hedge, in which case such low shoots will only further formation of the dense-to-the-ground habit that is the hallmark of a great hedge. Plus, because they are genetically identical to the higher-up growth that produces the glorious flowers, these shoots will do the same if allowed to mature sufficiently. See "Quirks and Special Cases," above.

Variants

Laburnum alpinum and Laburnum anagyroides can both mature at twenty to thirty feet. Racemes of alpinum are longer, to ten to fifteen inches. Their flowers are more widely spaced, too, giving each raceme a more slender and graceful look than those of anagyroides, whose racemes are only six to ten inches long and appear more congested with bloom. There is little reason to plant either species when you can have the best of both in their hybrid cultivar 'Vossii'.

 

These two Laburnum species yield a small but tantalizing range of cultivars other than 'Vossii', few of which, alas, are available in North America. Nearly all of the variance is in branching habit or overall size; to date, there is just one floral variant, x Laburnocytisus adamii, a chimera whose flowers can be yellow (from Laburnum anagyroides), pink (from the shrub in the related Cytisus genus, C. purpureus), or a combination of the two. If you want to grow anything in the Laburnum genus itself, then, you need to make peace with the bright-yellow flowers that all the forms bear. There is only a single foliar variant: L. anagyroides 'Aureum'. Its foliage is bright gold, and the combination with the yellow of the flowers must be thrilling.

 

'Pendulum' cultivars of both species of Laburnum have been identified. Either one of them can be formed into a small weeping tree by staking up a starter plant so it can form a trunk. Any weeping laburnums for sale at garden centers will have been formed into already-weeping standards by grafting the 'Pendulum' form atop its straight species' understock, which will serve as a pre-formed trunk. Weeping Laburnum has particular Winter interest in that the small silvery buds could be displayed much more easily against evergreen groundcovers and background shrubs than those borne by the non-weeping forms, which are much larger and whose stems aren't nearly as close together. Because the branches themselves are weeping, not just their flowering racemes, the flowers can be a bit hidden by dangling foliage. This can be partially solved by clipping off the lower portions of the branches to shape the canopy into a dense rounded head of growth. (As the weeping branches emerge from their point of grafting onto the straight-trunked understock, they form the top half of the sphere automatically.) With the bottom shaped so as to complete the sphere, lower flowering racemes will dangle freely below the canopy. Racemes farther up will be more or less hidden amid the foliage of the weeping stems. As is usual with Laburnum, prune in Fall or midwinter. If left to themselves, both of these 'Pendulum' forms will develop a more irregular and, sometimes, multi-headed canopy.

 

L. x watereri has also produced a pendulous cultivar, 'Alford's Weeping'. L. anagyroides var. alschingeri is a shrubby dwarf. Both of these cultivars are usually available only as grafted standards.  

 

Laburnum x watereri 'Sunspire' is also known as 'Columnaris'. Judging from pictures, its branches grow in the shape of a more narrow and dense inverted pyramid than the straight watereri hybrid or its parent species, but in no way could the habit be termed columnar. Growth of L. alpinum 'Pyramidale' is reported as being erect also, but not as tight or upright as 'Sunspire'.

 

I would leap at the chance to grow L. anagyroides 'Aureum' as well as L. anagyroides var. alschingeri but, apart from the two Laburnum species and the ever-available 'Vossii' cultivar, the only other forms available from any North American source seem to be L. alpinum 'Pendulum' and L. x watereri 'Sunspire'. I predict I'll find a place for that source's 'Pendulum', especially as it is sold as a rooted cutting not a standard already formed through grafting. The cutting-grown plant could be staked up into a standard of whatever height I wanted, trained onto a frame as an espalier, left alone to cascade out of one of my large troughs, or planted directly in the garden to ramble along as a mounding groundcover.

Availability

Online as well as at retailers.

Propagation

By cuttings taken in early Summer.

Native habitat

Laburnum alpinum and Laburnum anagyroides are both native to central Europe and, although alpinum flowers a bit later, there's enough overlap in their seasons of bloom that the two species hybridize naturally. In the 19th century, Waterer's nursery in Surrey, England, brought one such hybrid into commerce with the name Laburnum x watereri; later that century, its 'Vossii' cultivar was selected in Holland.

 
 
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