Louis Raymond experiments in his own gardens like

a mad scientist, searching out plants that most people have

never seen before & figuring out how to make them perform.- The Boston Globe

…Louis Raymond ensures that trees can grow in Brooklyn…

or just about any other place where concrete consumes

the dirt and skyscrapers shield the sunshine.- USA Today

 
 

NEW Trips to Take!

Myrtle's easy when the conditions are right.

 
 
 
 

NEW Plants to Try!

Louis tries to capture the exact words to describe the fleeting but deep pleasures to be found in these Summer-into-Autumn incredibles.

 
 
 
 

New Gardening to Do!

Allergic to bees? You can still have an exciting garden, full of flowers and color and wildlife.

 
 
 
 

Plant Profiles

Cut-leaved Alder

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The feathery, finger-like foliage of cut-leaf alder adds unique season-long texture.  And the tree is so versatile it can also be grown as a shrub, a hedge, or a pollard.  I wouldn't be without it.

 

At the base, the sides of the leaf are rolled inward a bit, narrowing the leaf to a shuttlecock profile.

 

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There's plenty of space between the leaves even without their being "shuttlecocked," but the combination of their unusual shape and generous spacing means that the top of the canopy is uniquely open to the sky.  The filigree of Alnus glutinosa 'Imperialis' foliage is one of the garden's more delicate but also enduring pleasures:  The leaves stay in excellent condition right through hard frost.

 

alnus-glutinosa-imperialis-foliage-and-blue-sky-082212-640

 

I'm growing my cut-leaf alder as a pollard, by cutting all growth back to the top of a modest trunk each Spring.  The new growth is plentiful as well as eager, and can lengthen to six or eight feet by September. 

 

The density of new growth highlights the contrast of the foliage with the speckled gray trunk.

 

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Alder trunks are willing hosts to a number of species of lichen; my cut-leaf alder is (apparently) still too young for such subtle ornament.

 

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This moisture-loving tree thrives in my heavy soil and flat terrain.  In a few years, the trunk will have thickened dramatically—and, I hope, welcomed plenty of lichen.

 

 

Here's how to grow this graceful tree:


Latin Name

Alnus glutinosa 'Imperialis'

Common Name

Cut-leaf Alder

Family

Betulaceae, the Birch family.

What kind of plant is it?

Hardy deciduous tree.

Hardiness

Zones 4 - 7 in temperate or "continental" climates, which have hot summers and cold winters, such as those east of the Rocky Mountains, but to Zone 9 in "maritime" climates (with mild wet Winters and mild and sometimes wet Summers) such as those of the Pacific Northwest and Britain.  Also a possibility for Zone 9 locales with Mediterranean climates—mild wet Winters and hot dry Summers—such as those of California and southern Europe, as long as the tree is sited or watered so as to protect from drought stress.

Habit

Variable, developing with single as well as multiple trunks.

Rate of Growth

Medium to fast.

Size in ten years

Twenty-five feet tall and ten to fifteen feet wide.  See both "How to handle it" sections for strategies to maintain the tree at various sizes that are more compact.

Texture

Distinctively feathery and "active."  The leaves are distinctly narrow at the base, with a shuttlecock-like tail of thin lobes only modestly widening at the leaf tip.  The foliage seems to be either caught in a headwind, or to be traveling en masse, and alighting only for the moment on the branches of the tree.    

Grown for

its versatility: Alnus glutinosa can grow in almost any soil that is not prone to drought, from nutritious to lean, acid to alkaline; in atmosphere that's pristine to polluted; in locations that are sheltered as well as those that are fully exposed to moisture-laden wind that roars inland from fresh as well as salty bodies of water.  (The tree is not suitable for dry soils, where Alnus cordata would thrive.)  Alnus glutinosa is particularly at home in soils that are out-and-out sloppy wet, where many other trees would fail. 

 

its intrepidity: As is often the case with alders, Alnus glutinosa fixes nitrogen directly from the air, so is self-fertilizing.  As such, it can be planted as a pioneer plant in nutrient-poor or recently-ravaged landscapes such as those created by strip mines, floods, landslides, and forest fires.

 

its flexibility: Alnus glutinosa can be grown as a free-range tree, a hedge, a pollard, or a coppice.

 

its foliage: The sides of the leaves at are rolled in at the base, narrowing the width of the leaf overall and gathering the slender lobes at the leaf edge into a smaller volume than otherwise.  One result is that there is more open space between leaves, which helps silhouette their shapes all the better against the sky or a contrasting background.  The other result is the "active" texture described above. 

 

its flowers: Alders are monoecious, bearing separate male and female flowers on the same tree.  The male flowers are in pendulous catkins.  Before opening, those of Alnus glutinosa are yellow, green, and pink cylinders an inch and a half long; they lengthen as they open.  The female flowers are small and cone-like.  The catkins are welcome details in early Spring, but the cone-like female flowers are not as large or showy as those of Alnus cordata, which could be grown for those alone.

 

the bark of young branches, which is shiny green.  The bark of mature branches is shiny brown.  That of the trunk is typical for alders: grey with lighter speckles as well as horizontal striations. 

 

its rarity in North American gardens: Alnus glutinosa 'Imperialis' is puzzlingly scarce in United States' gardens.

Flowering season

Early Spring, before the leaves emerge.

Color combinations

Alnus glutinosa is not coloristically showy, so goes with anything.

Plant Partners

Alnus glutinosa 'Imperialis' associates stylishly with foliage that's either significantly smaller or larger.  It grows in such a diversity of habitats that the full range of possibilities would be chapter-length.  Assuming normal soil and moisture, but with decent drainage, there are many choices for smaller foliage, such as needled and fan-spray conifers, smaller-leaved bamboo, ornamental grasses, and box and Japanese holly.  Larger foliage for the same middle-of-the-road circumstances includes species and cultivars of hosta, magnolia, viburnum, catalpa, and hydrangea, as well as big-leaved tropicals such as canna, banana, elephant ear, and ginger.

 

The more and more eccentric the habitat, the more specific and, sometimes, oddball the partner plants will need to be.  Alnus glutinosa is comfortable with wet feet, and can be grown as a bog plant.  Fellow boggies include species of iris, willow, grass, reed, fern, privet, baccharis, poplar, and the unique hardy mangrove.

 

I'm training the loose-limbed climbing rose, 'Darlow's Enigma', up into my 'Imperialis', which gives its sprays of small white flowers even more height and visibility than if the rose were growing on its own.

Where to use it in your garden

Alnus glutinosa 'Imperialis' is so versatile that it can be grown as a grove by a sheltered swamp as well as by open ocean; as a manicured quartet of pollards in a severe courtyard; as a tall and thick hedge at the edge of your property to buffer the worst winds; or as a green-twigged shrub in a mixed border or by a pond. 

 

The tree's visual appeal doesn't have the "blatancy" of fancy flowerers like cherries or magnolias, but the unusual leaves provide striking texture.  If you grow the tree as a coppice or pollard—see "How to handle it," below—you'll want to site it so that the green first-year twigs can be appreciated at close as well as medium range.  If you grow the tree as a pollard, those twigs' contrast with the speckled-gray trunk is one more subtle detail that you'll appreciate being able to savor at close range. 

 

The feathery foliage shows up particularly well against open sky, so try to site 'Imperialis' where it isn't backed by taller trees.  If you grow your 'Imperialis' as a pollard (see "How to handle it," below), the tree will always remain comparatively smaller and lower to the ground and, therefore, needs even more careful siting to keep the view through its foliage to the sky unimpeded.

 

In whatever form you choose to grow it, cut-leaf alder will be a tree of subtle and sophisticated charm.     

Culture

Full sun and almost any soil that doesn't become dry enough to subject the tree to drought stress.  Alnus glutinosa 'Imperialis' grows fine in average soil and moisture, but is at its quickest and most vigorous when it never lacks for water.  Heavy and even compacted soils are more appealing to this tree than many others, then, because these soils' poor drainage usually ensures the steady moisture the tree craves.  Alnus glutinosa will be especially grateful if sited alongside ponds and streams, or when growing directly in marshes, where its roots are permanently submerged.

How to handle it: The Basics

Plant in Spring.  The only requirement is for full sun and ready moisture. 

 

Alnus glutinosa 'Imperialis' is nothing if not accommodating to your individual interest in—or aversion to—pruning.  Although you can let it grow into a free-range tree if you never want to get out the loppers, the plant rewards prune-o-philes by producing more young twigs, which have shiny green bark.  Not only does pruning concentrate them in a plant of much smaller size than a free-range specimen, it also removes much of the older growth.  Older bark is interesting on the main truck, but less so on branches that aren't pruned.

 

To maximize the contrast between oldest bark (speckled gray and covering the main trunk) and the newest bark (green and covering the first-year twigs), grow Alnus glutinosa 'Imperialis' as a pollard, by pruning all top growth back to the top of the trunk and pruning off any branches that sprout from the trunk below.  I recommend letting the tree form a trunk that is no shorter than eight or ten feet tall.  

 

Prune in late Winter or early Spring, cutting all top growth back to nubs, and removing any side growth entirely.  (You can clip off side shoots any time throughout the year; I usually find that a second such clipping is needed by August.)  New growth will emerge as Spring develops and, depending on the ready access to soil moisture, could be four to eight feet long by September.

 

New growth can reach six to eight feet by September, so you might try giving your 'Imperialis' pollard a second pruning in mid-Summer to maintain a smaller profile.  Cut any new stems that are out of bounds back by half; typically, this will entail cutting back only the three or four of the branches that are at the very top of the canopy.  Be careful that the head of the pollard isn't now so low to the ground that the view through it to the sky is blocked by surrounding vegetation.  You could grow the pollard on a higher-than-usual trunk (ten feet, say, rather than six or eight), but that would mean that you would also need to have a taller-than-usual ladder.  (Which I recommend!)  It's easier to plan neighboring vegetation so that it stays low enough naturally. 


Because the display of the male and female catkins isn't as showy as those of Alnus cordata, you don't need to take them into account when pruning a pollard of Alnus glutinosa

How to handle it: Another option—or two?

Alnus glutinosa 'Imperialis' can also form a hedge.  Plant small young trees two feet apart in Spring.  In their second Summer, cut the young trees back by half, to encourage new branching as low as possible.  In the third Summer, prune new growth back by half in July or August.  In following Summers, prune new growth back as needed to allow the hedge to gain height but also to control width.  When mature dimensions are reached, you can prune new growth off almost entirely, again in mid-Summer. 

 

The trees grow so fast that if you pruned in Spring, new growth could be a yard or two long by October.  Pruning in mid-Summer gives new growth time to get started that season but maintains a less shaggy look through the Winter.  The pruning also provides a cleaner profile for high Summer, when all gardens welcome fresh growth that also helps maintain overall geometric integrity. 

Quirks or special cases

None.


Downsides

Like willows, alders have a long list of potential ailments and insect pests, but I've never seen them on any of my own plants.  (At present, I'm also growing Alnus incana 'Aurea' and Alnus cordata.)  As usual, healthy and well-grown plants are often more resistant to attack.  That said, there is probably a strong regional variance in alder problems.  It might be worth it to check in with the local office of the USDA Cooperative Extension Service to see whether alders are unusually prone to this or that pest where you garden.

Variants

'Pyramidalis' is, actually, distinctly columnar, and narrow enough to be grown as a substitute for Lombardy poplar, Populus nigra 'Italica', which, in eastern North America, almost inevitably contracts a disfiguring canker. 

Availability

On-line.

Propagation

By cuttings.  

Native habitat

Alnus glutinosa is broadly native to Europe and the British Isles. 

 
 
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