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

Tree Dahlia



I'm completely out of bounds in my giddy pride in my pair of tree dahlias—as out of bounds, actually, as the dahlias themselves:  They're twelve tall and yet they have a good six weeks of prime growing weather before frost.  (Excepting today's Hurricane Irene, of course.)


Will they get to fifteen?  It would be a personal best for them as well as for me.




Tree Dahlias are, indeed, credibly arboreal.  To grow them to this size takes a partnership of gardener and tuber.  (See "How to Handle It" below.)  You'll need to have a strong enough back for the digging, a frost-free place to successfully store the behemoth-but-fussy tubers November through May, rich enough soil and a big enough bed for the planting out later in May, and strong enough sun for a long enough season of growing.  Then your dahlias can be sky-high, too. 


And if you don't get frost until December, your "trees" will be full of flowers, too: Tree dahlias need the short days of Fall to be motivated to form buds.


When I took these pictures yesterday, Irene was only in the forecast.  It was easy to tip all the plants that are growing in containers on their sides, and even to nuzzle a lot of them under the shrubbery.  They'll get through the hurricane just fine.


But twelve-foot dahlias growing right in the ground, with ambitions of fifteen? 




Even if the canes were staked, wouldn't the tops of these thick canes just snap off?




Yes, they're enormous and thick, but still no match for a hurricane.




And all that palmy foliage?  It would just get broken off if the canes can't sway in the wind.




Here's the view across the gardens, with the tree dahlias at the left and dozens of containers waiting to be tipped on their sides.




The containers are horizontal now, and will ride out a Category 1 hurricane just fine.  But the dahlias?




Ah, Irene. 




The canes are leaning terribly—some almost to the ground—but aren't actually snapped.




I'll right them tomorrow, tying loosely to the espaliered Southern magnolias.  It will be sunny and in the eighties; maybe the dahlias will continue upward as if Irene hadn't even happened.


And next Summer I'll consider planting each in a huge terra cotta pot instead of right in the ground.  (Ah, the "need" to get another pair of huge terra cotta pots!  When you garden, there's never, ever, a lack of opportunities, as well as imperatives, for spending money.) 


Thanks to global warming, hurricanes will become more frequent as well as more powerful, so I'll have more and more occasion to protect my tree dahlias from storms that arrive when they're at the peak, literally, of growth.  If the "trees" were planted in huge pots instead of in the ground, I could tip them and their pots on their sides when a storm nears—and right them the next day.



Here's how to grow this remarkable dahlia:


Latin Name

Dahlia imperialis

Common Name

Tree Dahlia


Asteraceae, the Aster family.

What kind of plant is it?

Tender perennial that can also be grown as an annual.


Zones 7-10.


Strongly upright; multi-"trunked" when tubers can be overwintered.

Rate of Growth

Fast—very fast when young.

Size in ten years

An an annual, six to twelve feet tall, four to six feet wide.  As a tender tuber overwintered and planted out each Spring, ten to fifteen feet tall and eight feet wide.  As a hardy in-ground colony, with more and more stems each year, fifteen to thirty feet tall and ten to fifteen feet wide.


Big-boned and tropical.   

Grown for

its rarity: Because dahlias' prolific and colorful Summer and early Fall flowers are usually thought to be their raison d'être, any dahlia grown solely for foliage is, by definition, the odd man out.  Even in their native Mexico—Zones 8, 9, and 10, depending on elevation—tree dahlias don't come into bloom until November.  North of Zone 8, frost usually kills the plants back to the ground before they can bloom; this makes tree dahlias a challenge for retailers to market to the vast gardening public, which is "floral-centric" in taste.  You can just hear them puzzling, "What's the point of a dahlia that doesn't bloom?"   For this crowd, that a "tree" dahlia is so enormous, too, only makes things worse: "It's a space-hogging monster and it doesn't even bloom?"  Understandably, tree dahlias are a tough sell to these folks.


its size: "Normal" dahlias don't get taller than six to ten feet, and then only because they are securely staked.  Bamboo-thick canes of tree dahlias are self-supporting, especially in the interior of the clump.  Even where the immense tubers are dug each Fall and planted out each Spring (see "How to Handle It"), tree dahlias can get ten to fifteen feet tall, and stand proud all on their own.  In Zone 9 and south, where they can survive in-ground year-round without protection (they can survive in Zones 8 and 7, too, but need protection), they can easily top twenty feet.  Skyscrapers to thirty feet have been reported. 


In such ideal conditions, the canes can be four inches in diameter.  But the outer canes of the colony—which can get crowded—can flop right out to the ground, too, so it's not all perfect in paradise.


its foliage: The leaves are compound like a normal dahlia but many times the size—to three feet long and more.  They're in pairs up the stems, each set 90 degrees off the pair below.


its flowers: typically single and a rather blah pale lavender, they are in huge clusters that, overall, are very showy.  White and pink forms exist, too, plus a cactus-flowered double-white.  Even in Zone 8, they can develop too late—November into December—to escape killing frosts.

Flowering season

November into December.


As for any dahlia, full sun and rich fluffy soil with excellent drainage.

How to handle it

As an annual, tree dahlias are no different than any other dahlia, apart from size, of course.  Provide full sun and rich soil that is well-worked and open so that the tubers and roots can expand into it easily.   Water only during drought: Dahlias prefer soil whose top inch is dry.   The plants are killed to the ground by any serious frost.


As an in-ground perennial in Zone 8 and 7: mulch the colony heavily after frost; don't cut the canes all the way to the ground after they've been frosted; leave a few feet standing until Spring.  (For those perennials that are only semi-hardy for you, it's always best to hold off on cutting dead stems completely off until Spring.)


In Zone 9 and 10, which can be frost-free for years at a time, cut the oldest canes to the ground in Spring to encourage fresh young canes, which bloom better and look better:  If the tall canes aren't ever killed back by frost, they eventually get tattered and tired from wind and heavy rains.  Unless your spot is particularly sheltered, it can be a help to grow the dahlias alongside a building, so you can (I guess) lean out second- and third-story windows to do a bit of tying-up to anchors directly in the building walls.  Or just bring out a tall ladder.


To overwinter below Zone 7: Even in a single season, tree dahlia tubers can easily grow larger than anything that can be dug up without breaking into separate "fingers."  In my experience, the tubers' huge size doesn't really help them overwinter more easily.  Instead, they seem more susceptible to rotting because of their size, not more protected from drying out.  "Tuber trauma" during the Fall dig-up seems to increase vulnerability.


The solution is to grow the plants in large porous boxes that are sunk into the ground.  Roots and young tubers grow out through the openings in the box's sides—and, yes, are pretty much sheered off during the Fall dig-up—but the massive core of the tuber cluster remains intact, which seems to be the key to survival.  I've seen custom-built wood crates, but I just use the black plastic crates that (if you ask nicely) any nursery will be thrilled to give you. They've often have accumulated a small mountain of the crates, which are used for shipping by bulb wholesalers.  Grow lots of tree dahlias and do the nurseries a favor.


Now that you've got your tree dahlia tubers safely out of the ground, the next challenge is the actual overwintering. 


Because the tubers will have grown out through the holes in the bottom and sides of the crate—and many or even most of those extensions will have remained intact despite the disturance inherent in the digging up—don't even think about removing the tubers from it.  This would only create more disturbance and "tuber trauma."  Leave undisturbed, then, the growth that's sticking out, stand the crate on the one of its sides with the fewest such "out-pokers" tip out much of the soil that's still in the crate.  If you planted the crated tubers using great compost (see the Spring planting routine, below), this will be easy anyway:  Good compost is inherently friable, so it will tumble out of the tilted crate all on its own.  (More than likely, you're going to be filling the crate with somewhat drier and even more friable material than the compost, but it's fine to leave the soil that, in any event, you couldn't remove without getting to personal with the tubers.) 


Tubers do best in the cool, humid but not moist environment of a wine cellar.  (I have a client who uses part of their wine cellar itself for the dahlias.  More likely, you'll use your basement or an unheated (but frost-free) garage or porch.


In ideal conditions, the tubers can be stored uncovered; it's more likely, though, that you'll want to provide a fluffy layer around them: perlite, almost-dry potting soil, sawdust, wood shavings, even dry leaves.  Just sprinkle handsful into the crate so the tubers are covered but not truly buried.


Check once a month January, February, and March (and April if you're in a really cold climate).   Are the tubers shriveling?   Sprinkle on some water and add some more "fluff."  Are they starting to get moldy?  Carefully tip the crate on its side so more or most of the fluff falls out; shift the crates to a location that's a bit warmer as well as better ventilated.


If you have room, the best way to help your tubers overwinter is to start them into growth as soon as you can. Set the crate on a water-proof surface in a sunny window, replace as much the fluff with potting soil as you can do easily, and—without watering—let the sprouting begin.


Don't plant outside until the weather is truly steadily warm.   If you're not planting tomatoes yet, you shouldn't plant dahlias yet, either.  Dig a hole wide enough to set the entire crate into—it will look like a shallow grave for a pet, truth to tell—working in plenty of compost at every opportunity.  Set the box gently into the hole and fill around it; no need to tamp down: Dahlias want the feel of and the easy tuber access to truly fluffy soil.


As the dahlia shoots grow (as quickly as bamboo, it seems), carefully mound more of the compost atop the crate, burying it and the bases of the new shoots with up to six inches. The shoots themselves will develop roots (just like corn stalks do) and send them out into the compost mount to anchor themselves all the better.


Unless your site is windy, staking is usually not necessary for tree dahlias. Thank goodness: it would be a challenge, indeed, to stake something twelve to fifteen feet tall. Just remember that there's a tree dahlia under that suspicious low mound, and let the monster sprout into growth in the gentle late-Spring warmth.


Tree dahlias' leaves can get three feet long, and arranged as they are in opposite pairs up the thick stems, even a single tree-dahlia "cane" can take up a six-foot circle of bed.   A multi-stemmed clump that you've overwintered year after year can take up even more room.  Allow plenty of room for your tree dahlias.


Plants of such stupefying size are naturals for the back of huge garden beds.  But the canes are so startling in their own right it's even better to plant where you and your visitors can see them close-up.


Because the dahlias don't bloom in climates (such as mine) where frost comes long before December, there's always the thought of a companion flowering vine, also annual, that could scramble up the dahlia as it grows. The goal is to have this gigantic architectural presence swagged or gossamered with ornamentally textural growth, and spangled with contrastingly-colorful flowers, too— smothered under a dense cover of, say, morning glories.  Scarlet runner bean?   Its purple-leaved cousin, Lablab purpurea?  Perhaps best: cardinal climber (Ipomoea quamoclit), with definitively ferny foliage that belies its intrepid speed of growth—up to fifteen feet— and all the while with scores of brilliant scarlet trumpets no bigger than impatiens' flowers.  I'm looking forward to experimenting in 2012.


Overwintering the tubers where the plant isn't hardy in-ground is tricky (but a triumph when you succeed), and involves lugging and digging on a much larger scale than anything you'd face with a normal-sized dahlia.


In climates mild enough to allow tree dahlias to bloom, it matters what the flowers look like.  So pink as well as white-flowered variants are sometimes available in those markets.  The double-flowered white variety is supposed to bloom several weeks earlier but, even so, I doubt it would flower by mid-October.


On-line, as well as at "destination" nurseries.


By seed, by Spring division of the tuber cluster into individual fingers, and by stem sections, which sprout platelets.

Native habitat

Dahlia imperialis is native to Central America, from the higher elevations of Mexico to Columbia.  

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