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Dahlias: Indescribable

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

Here's how to grow these enthusiastic and colorful tender perennials:


Latin Name

Dahlia

Common Name

Dahlia

What kind of plant is it?

Tuberous perennial.

Hardiness

Zones 8 - 10

Habit

Dwarf dahlias are naturally multi-stemmed and bushy, and don't need support.  Medium and taller-height varieties, though, have fewer canes and can be distinctly upright and even awkwardly bare from the ankles to above the waist.  And they need staking if they're not to flop miserably.

Rate of Growth

Fast when happy: to full size in just one season.


Size in one season

Only tree dahlias, Dahlia imperialis, require several years to achieve full size.  All the rest can grow from seed (or a single tuber) to full size in just one season.  Depending on the cultivar, that size can be from one foot to ten.  Tree dahlias are the gigantic exception, getting only six to eight feet tall their first year.  Each Spring thereafter, though, the new shoots mature higher and higher, from twelve to thirty feet tall! 

Texture

Most dahlia foliage is thick and forgettable, looking more like forage than anything ornamental.  Bronze- and purple-leaved cultivars, though, can be an out-and-out thrill even before they flower.  'Bishop of Llandaff' is one of the earliest of the purple-leaved dahlias, and in this regard is still the best:  The foliage is not just dark, it's amost ferny, providing a lively texture from the get-go.  Its sizzling red, semi-single flowers ramp up the excitement even farther.

Grown for

Their flowers:  Dahlias come in every color but true blue.  See "Variants" below.

Their foliage:  A precious few cultivars have bronze or purple foliage, which is an invaluable distraction over the (potentially) three months from starting up the dahlias (by seed or by tuber) in the Spring to when they start flowering.

Their energy:  Once dahlias start into bloom, they'll continue through frost provided they get dead-headed.  (See "How to Handle It" below.)

Their economy:  If you master the tricks of overwintering the tubers, that one dahlia tuber you bought and planted one year can be divided into a dozen or more when you go to replant the clump next year.  Dahlias can be saved year-to-year indefinitely.  Conceivably, you'll only need to buy one tuber of any one dahlia, and then you can have it—and as many individual plants of it as you could want—forever.  Dahlia tubers are inexpensive in the first place, too; even from die-hard mail-order dahlia specialists, many tubers are only $3 or $4 each; only the newest-and-weirdest might be as much as—gasp!—$8 to $15 each.  

Flowering season

Early Summer to frost.  Depending on whether you start your dahlias indoors or buy plants that are already in bloom, you'll either have dahlias in bloom from August through frost, or right from the frost-free day you plant them out in late Spring.

Culture

Full sun and loose, rich soil that is well-drained.  Dahlias do best in soil that is crumbly enough that their tubers can penetrate it easily and don't have to "fight" their way along, as they would in heavy or compacted soil.  Dahlias don't need much supplemental watering, either, so rich, loose soil that also gets dry between waterings is, actually, ideal.

How to handle it

Plant the tubers on their sides; normally one end is more knobby or complex than the other, and that's where the tuber will sprout.  Plant three times as deep as the tuber is thick.  Do not water: Dahlias are very good at pulling the water they need from the soil all by themselves.  After sprouts emerge, you can hill soil up farther and farther, which provides more support.  Full-size dahlias can be "hilled" to six inches or even (if you've got enough of the loose and light soil they crave) a foot.


For dahlias that you know will need staking, "plant" the stake right when you plant the tuber.  Plant it an inch away from that knobby or "complex" end of the tuber.  If you wait until the plant clearly needs staking, you'll inevitably shove the stake through one of the new tubers that the plant has formed all Summer, which is rude even if not, really, harmful.  Dahlias that are four feet and shorter can usually get away with just pea stakes, but the big-flowered and full-height cultivars need serious support.  I'm partial to the one-inch square wooden "tomato" stakes that my local lumber yard (believe it or not) cuts to order.  Six-foot bamboo stakes also work.

Tie the dahlia stems to the stakes gently and loosely, with wide and soft material; wire or even twine can cut into the stems.  If you have the patience, cut old pillowcases into one-inch strips.  Given how unattractive a lot of bright-white pillow-case ties would be, though, it would be worth it to buy a pillow-case, or even an entire sheet, that was tan or even brown.

Soon after the plants begin to flower, it's time to start dead-heading.  The buds of flowers-to-come are round, whereas dahlia flowers that are done flowering and are now hoping to go to seed are pointed; it will soon be second nature what to clip and what to leave.

Dahlias are often at their best the last weeks before frost; September and October are always the peak of the dahlia season.  Let the plants get thoroughly frosted (which may take several nights of sub-freezing temps), and then don't do anything!  Let the dead plants contemplate their fate for two weeks or so before you even think about digging up the tubers.  This interval helps the tubers themselves go dormant. 

The single tuber you planted in May will have matured to a starfish-like clump when you dig in October.  Start digging a foot out from each stake until you get the feel of how large your tuber clumps will be and how easy they'll be to extract from the soil.  The clumps are fragile but not maddeningly so.  Cut off the stems several inches above the tubers, not right down at them. 

As soon as you've dug up a clump, gently tie a water-proof label around it; dahlia tubers can rarely be distinguished down to the level of cultivar and color.  If you've dried the tubers enough, you can write directly onto the tuber with a waterproof marker, such as a "Sharpie."  If you don't label, all you will know is that the smaller tubers are, indeed, the tubers of your dwarf dahlias.  But you won't have a clue about which cultivar they actually are.

 

Gently remove most of the soil, but don't be dogmatic about it.  Leave the clumps in the sun (or store them frost-free and under cover for a few days until there is sun) to dry for a few hours; don't leave them out overnight, though:  If they don't get hit by frost, they'll get covered with dew, which defeats your effort to dry them in the sun.  Then load them loosely into cardboard boxes or wood or plastic crates—upside down so any water in the stems drains out, not down into the tuber.  

The tubers don't like to feel lonely during their long Winter in the crates, but neither do they want to be packed together.  A gentle association, so to speak, is best.  Try loading them in no more than two layers in your crate, and only as many as will fit with absolutely no nudging or twisting or packing-in. 

Unless your frost-free basement or garage is unusually humid, then cover the tubers gently with loose dry leaves or vermiculite or sawdust.  The goal of any of these covering materials is to slow down transpiration from the tubers but without trapping the water vapor that does leave even so, lest it condense and encourage growth of mold.

Especially your  first year or three of overwintering dahlia tubers, check your tubers monthly to see if they're drying out or rotting.  If the former, sprinkle with water, add more coverage, and see if you can't move the tubers to a slightly cooler location.  If the latter, reduce or remove the coverage entirely, and store in a single layer on newspaper with no tuber clumps touching any of their neighbors.  I have a client who stores his dahlia tubers—naked!—in their wine cellar, which, indeed, is the perfect combination of temperature and humidity.

If you have the space and energy and warm sunny windows, you can pot up your tubers again in late Winter, as much as two months before the last frost.  Divide the clumps into individual tubers in that case; you'd need huge pots, indeed, to pot up whole clumps.  When dividing, be sure that each tuber retains a portion of the central mass of the clump, which is where the sprout will arise.  Use a steak knife for your surgery, or buy a purpose-built "soil" knife, instead.

Or you can just wait until the soil is warm and the weather's balmy, and plant your still-dormant tubers right out into your garden in Spring.  The second week of May works for me.  In that case, you can skip dividing the clumps into individual tubers.

Downsides

Dahlia tubers, admittedly, can be tricky and even frustrating to over-Winter.  Even I lose a few each year.  Deer will eat dahlias, as will ground-hogs; spray regularly with an organic and safe repellent.  Rabbits, though, seem to ignore them.  

Variants

With at least 36 species, plus thousands of hybrids and cultivars, there are as many different dahlias to grow as you have room for.  Plants range in size from a foot to thirty feet, the ultimate towering height of the aptly-named "tree" dahlia.  Flowers range in size from a couple of inches to over a foot.  Flower color can be almost any single or multiple of hues from white, pink, rose, and purple; to yellow, orange, red, and oxblood; to lavender and near-blue.  Flowers can be single, semi-double, or fully-double.  Flower habits can be tight and orderly spheres, shaggy and even wild "cactus-flower" shapes, or flat simple daisies.  Foliage can be broad or thin and dissected, sometimes with exciting purple highlights or even outright deep-purple color.  Only orchids, mums, and iris have as wide or wider color possibilities.

To learn more about the world of dahlias, deep and broad as it is, visit The American Dahlia Society.   

Availability

The most interesting and obscure varieties are available on-line; there are many specialist dahlia growers, and almost any vendor that sells bulbs will also list a few dahlias, too.  More generic cultivars can be bought in Spring at any garden center or big-box retailer.  Garden centers often sell potted plants that are already in bloom.

Although the cultivars don't come true from seed, there are "lines" of dahlias—chiefly the bushy dwarfs—that can be grown from seed as annuals.  They'll all have the same habit and height, but will be in a variety of colors.

Propagation

By seeds in Spring, by division of the clumps of tubers, also in Spring, and by rooting stem tips, in Summer.

Native habitat

All the dahlia species are native to the New World, from Mexico to Columbia. 











 
 
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