Monday, May 30, 2011

Dahlia Diseases: Fungal Infections Pose a Risk

Dahlia leaves with fungal infectionsFungal infections aren't funny when it comes to dahlias. But most can be prevented, and in many cases, you can save those favorite dahlias if your plants develop diseases involving fungi.

Common fungal problems include powdery mildew (Erysiphe polygoni or E. cichoracearum), signaled by powdery white patches on leaves; gray mold (ItalicBotrytis cinerea) which attacks roots, rots like Pythium, Rhozoctonia solani or Sclerotinia sclerotiorum, or smut (Entyloma calendulae f. sp. dahliae). Some symptoms of fungal infection are easy to overlook; sometimes, however, a plant will simply fall over and die.

The best solutions for fungal problems are preventive in nature, according to Bob Romano of the American Dahlia Society. Treating tubers for winter storage with an anti-fungal preparation like Terrachlor can not only prevent problems but result in healthier, more vigorous plants, he says.

Among the causes of fungal infections in the summer growing season are excessive soil moisture and, in the case of greenhouses, overhead misting; overfertilization and the buildup around plants of insoluble salts; low soil temperature (below 68° Fahrenheit) before germination; high soil temperatures after emergence (above 77°F) and overcrowded flats or seedbeds for cuttings or young plants. Plant pathologists at Oregon State University note that plant debris in gardens should be removed and destroyed in the fall to minimize fungal growth. Good gardening practices will help you grow healthy dahlias.

Dahlia Caboose
Fungicides, Romano says, are of two types: contact or protectants, which are applied to healthy dahlias prior to infection, and systemic fungicides, which can not only protect the plant from infection but can actually cure and eradicate fungus present in plant tissue. A recently developed class of fungicides, the Strobulurin group, which are considered environmentally safe. They work by inhibiting the transport system of mitochondrial electrons in plant pathogens but not in other living entities.

Most systemic fungicides are specifically for particular types of fungi, so it's important to diagnose the fungal infection. Your Cooperative Extension office is a good resource. Garden experts there can help you identify the specific type of fungal problem that's affecting your dahlias and choosing the best and safest fungicide with which you can deal with it.

Saturday, May 28, 2011

Dahlia Diseases: Watch Out for Bacterial Infections

2010 LCF field dahliasAt Lynch Creek Farm, the dahlias are all planted — fields are full of thousands of tubers whose sprouts are due to break the soil soon. It's been a late season in the Pacific Northwest, but almost everywhere, it's finally time to be sure all your tubers are in the ground. This weekend is the last for the Farm's tuber booth at the Olympia Farmers Market; the crew will be back at the market when Lynch Creek's first dahlia flowers are ready for picking and selling.

Now that it's growing season across most of the country, it's time to keep an eye out for dahlia diseases. While dahlias are easy to grow, there are some diseases that can cause problems. And in a year whose weather had been problematical, weather-related stresses should be taken into account; stressed plants can be more susceptible to diseases than they would be in a year with fine weather.

Among the problems that afflict dahlias in some areas are bacterial and viral diseases. Bacterial wilt, which causes stem drooping and wilting, can be caused by deep insect damage or spread from other infected plants. Infected plants develop a wet soft rot, usually in the lower part of the main stems. Fusarium and Verticulum wilts are caused by infected soil. Usually the basal leaves turn yellow; then the vascular system darkens. Sometimes only a part of the plant is affected initially. The wilt problems often occur in hot spells after a cool period. The only remedy is to destroy the affected plants and fumigate the soil with chloropicrin-methyl bromide under a tarp. It's best to grow your dahlias elsewhere for a season or two if there has been a bacterial disease problem in a planting area.

Crown gall is another bacterial dahlia disease. It's caused by a soil bacterium, Agrobacterium tumefaciens, which causes abnormal growths on the tubers or stems at the base of the plant. The plants are stunted with spindly shoots. Plants, tuber and all, should be destroyed and dahlias should be rotated to another location for at least three years while soil is fumigated. Grow non-woody plants such as cereals or legumes in the space before returning dahlias to the area.

Dahlia tuber with bacterial soft rot
Bacterial soft rot, caused by the bacteria Erwinia carotovora, claims tubers in storage. They darken and soften, and must be destroyed.

If you suspect your dahlias have a bacterial disease, take a sample to your local county's Cooperative Extension office (associated with your state's land-grant university). If you get a diagnosis of bacterial wilt or gall, get tough and get rid of the infected plant or plants. It's better to sacrifice a few infected dahlias than to risk losing your entire dahlia population.

Monday, May 16, 2011

Garden Bulletin: Lynch Creek Dahlias Are Planted!

Nathanael plantingAfter our stately progression through the classifications of dahlias, we move ahead (whew!) to bring you this bulletin from Lynch Creek Farm: the soil has finally warmed in this clammy South Puget Sound spring, and the crew has spent the last week planting dahlia tubers.

Your blogger is impressed: row upon row of trenches in gently rolling fields, THOUSANDS of dahlia tubers nestling in new beds and then lovingly covered over with beautiful sandy loam. (As one whose gardening areas are all on gravelly glacial leavings, where soil has to be built up with composed leaves and mushroom compost, it's hard not to look over these fields and feel murderously envious. No wonder beautiful dahlias come out of those fields, and thriving tubers emerge at the end of the season.

Andy etc planting
SOME PLANTING TIPS and advice for the care of dahlias came out of that session at the farm as well. Here's advice from the planters at the Farm:

Prepare the soil well. Manure (well rotted), compost, and peat moss all help create the rich, loose soil that will nourish your dahlia tubers and enable them to thrive and bloom abundantly. Good drainage is a must for dahlias; standing water or heavy clay soil will encourage rot or disease, so add sand if your soil is clay-ish or your planting area isn't well drained.

Protect dahlia tubers and the plants, when they emerge, from adverse weather conditions. Be sure the danger of heavy frost is well past before you plant (April 20 to June 15 in most zones). In areas with late frosts, starting dahlias in pots in a greenhouse is a good option. If you are in an area that gets significant wind, be sure stakes are in place to help support the mature plants. In almost any area, staking is necessary for the largest of the bloomers.

Dahlia tuber, eye sprout upDahlia tuber, plantedPosition tubers correctly. Space tubers according to plant size; giants and medium dahlias should be three feet apart and six inches deep; small, miniatures and pompon dahlias can be four inches deep and spaced two feet apart, and mignon singles can grow even a bit closer together for dense borders. Tubers should be laid in the ground horizontally, with the eye sprout facing upward. Cover carefully and do not pack soil hard. (It's a good idea to place a support or identification stake before you cover the tuber so you don't damage it pounding a stake in later.)

Weed often. Dahlias have surface roots, and will suffer stunting from weed competition, so keep them weed-free during the first couple of months (later, they should shade out competition). Weeds also shelter slugs, snails, earwigs, sowbugs and other pests that can damage the young sprouts of your dahlias; you may need to use slug preparations like Sluggo at frequent intervals.

Water only when needed. Unless your region is very hot and dry, don't water dahlias until they begin to bloom; then give the soil a good soaking every week (more often if weather is hot and breezy).

Stay tuned. We'll issue another bulletin when the plants at the farm are up.

Sunday, May 15, 2011

Dahlia Classification: Lovely Color Combinations

Just MarriedParfait
Our final stage in the demystification of dahlia classifications is the mixed colors. While many dahlias are perfectly uniform in their coloration, others combine a mixture of colors, some shading gradually from one color to another, others with distinct streaks, stripes, or zones on the ray florets and/or petioles. These combination colors are great favorites at Lynch Creek Farm, where dahlia tubers in the blends, bicolors and variegated forms sell quickly.

In a blend, the colors shade from one to another from the base to the tip of the petal-like florets. There are three blends recognized by the American Dahlia Society: light (LB), flame (FL) and dark (DB). Light blends involve pastel shades, dark blends incorporate one or more of the deeper colors, and flame blends are the brighter blends of red, orange, yellow and/or gold.

Fire MagicSea FuegoNenekazi
At Lynch Creek, favorite light blends are Parfait, a big dinnerplate dahlia that shades cream to pinky lavender, and Just Married, a laciniated dahlia six to eight inches across. It shades from a lemony yellow near the base to creamy white to pink. A great example of a dark blend is Nenekazi, another laciniated dahlia in the same size range whose ray florets are yellow at the base, blushing to deep pink at the tips. Typical of flame blends are Sea Fuego and Fire Magic, four- to six-inch semi-cactus dahlias. Sea Fuego deepens from golden yellow at the base to a warm red, and Fire Magic is described as a fuchsia-salmon-mauve blend with hints of dusty orange that "defies description," according to the Lynch Creek gang.

AmorousHulin's Carnival
A variegated dahlia incorporates more than one color in streaks or stripes on the ray florets or petioles. Miniature formal decoratives Hulin's Carnival and Amorous, both with two- to four-inch flowers on five-foot plants, are typical, with streaks and stripes of darker color running lengthwise on the ray florets. Hulin's Carnival has streaks of purple, burgundy and deep red on pale lavender-white petals while Amorous has crimson spatters on a light orange-apricot base.

A bicolor dahlia (BI) incorporates contrasting colors, often a dark tone and white, in separate areas of the ray florets and/or petioles. Lemon Candy and Candy Cane, small formal decorative dahlias with four- to six-inch blooms, feature white tips on lemon-yellow and bright red flowers. Then there's Double Trouble, a collarette dahlia; its ray florets are bicolored, with pink tips on raspberry red florets, and the petioles of its collar are variegated fuchsia and pink.

When a dahlia is identified in American Dahlia Society catalogues, the size comes first, then the form, then the color. Sea Fuego, for example, would be listed as B SC FL, with B the size designation for six- to eight-inch flowers, SC the code for semi-cactus flower type, and FL the listing for flame blend coloration. If you're just now picking up on these blogs, follow us back for the earlier descriptions of size and flower type listings.

Lemon CandyCandy CaneDouble Trouble

Saturday, May 14, 2011

Dahlia Classification: They're Classed by Colors, Too

White NettieHissy FitzMonarch of the EastThe first dahlias discovered by late 16th-Century explorers in Mexico and Central America were Dahlia zimpanii, small single flowers in the red and orangy-red ranges. When dahlia tubers were introduced to Europe via Spain toward the end of the 18th Century, and plant hybridization was a new science, additional dahlia species were added to the gene pool, and variations in color and form began to flourish. Now dahlias are considered to have one of the widest color ranges among garden flowers.

It wasn't long, once hybridization and competitive exhibition began, before the Royal Horticultural Society and, later, the American Dahlia Society began developing means of classifying dahlias. We have explored classifications by form and by size; to make matters more complicated, dahlias are also classified by color. Color classification is helpful when buying dahlia tubers and planning flower beds and borders.

ExcentricCanby Centennial
The dahlia societies have decreed that the official dahlia colors are white (W), yellow (Y), orange (OR), pink (PK), dark pink (DP), red (R), dark red (DR), lavender (L), purple (PR), and bronze (BR). If a dark red dahlia has any hint of blue in it, as with many of the burgundy tones, it is classified as purple. For the single-color classifications, all the colors taken into account occur on the forward facing surfaces of the ray florets and petaloid structures except in the case of the orchid dahlia, whose ray florets are virtually tubular and roll toward the center; for orchid dahlias, the color is that which occurs on the reverse of the petals and shows up on the outside of the rolled florets.

Baby RedShadow Cat
But wait. We haven't got quite complicated enough. There's a whole separate code for bicolors and blends. We'll deal with the blends, which are listed as separate colors, in the next blog. For bicolors, the predominant color(s) on the ray florets appear first, in capital letters; then the color or colors on floret tips, petaloids, domes and eye-zones are listed in lower-case letters. So a bicolor with a yellow ray florets with orange tips, for instance, would appear color-coded as Yor. On Lynch Creek Farm's website, you'll find color listings below the list of forms on the left side of the home page.

Staying, for simplicity's sake, with one-color dahlias, we present a palette of colors among the dahlias at Lynch Creek Farm: White Nettie, a miniature ball dahlia; Hissy Fitz, a lemon-yellow laciniated cactus dahlia with four- to six-inch blooms; Monarch of the East, a dinner-plate-sized orange semi-cactus eight to 12 inches in diameter; Excentric, a pink informal decorative miniature whose blooms are four to six inches across; Canby Centennial, a dark pink formal decorative dahlia six to eight inches across; Baby Red, a mignon single with one- to two-inch flowers on 12-inch plants; Shadow Cat, a dark red formal decorative miniature with four- to six-inch blooms; Hugs and Kisses, a lavender informal decorative with four- to six-inch flowers; Ripples, a burgundy-purple informal decorative whose blooms are six to eight inches across; and Ginger Snap, a waterlily dahlia with four- to six-inch flowers (actually, we can't say for sure that its classification is with the bronzes, because we haven't found it listed in any of the color classes, but we'd definitely call it bronze).
Next time around: the blends and bicolors will finish off the classifications.

Hugs and KissesRipplesGinger Snap

Thursday, May 12, 2011

Dahlia Classification: Stunning Singles

The first dahlias discovered in Mexico were singles, a flat corona of rounded ray florets surrounding a center of disk florets. While growers have worked over the years to develop larger and more double dahlias with varied floret or petaloid shapes, some of today's most popular dahlias are those which echo that early form.

Single dahlias, according to American Dahlia Society standards, are open dahlias with a single row of uniform, evenly spaced ray florets in a flat plane surrounding the disc flowers. Dahlias of the single class are two inches or more in diameter. Mignon single dahlias are the minis of the species. The standards are the same as for the singles, and the mignon singles have been a separate ADS classification since 1977. Since it was developed in Holland in the late 19th Century, the mignon single has been variously called miniature, Tom Thumb, Lilliput, top mix, dwarf single, bedding dahlia and dwarf mignon.

Early single hybrids originated with Dahlia coccinea varieties crossed with one or two other dahlia species. These dahlias were called "simplex" and were described as two- to three-inch blooms with long, tapered rays that gave the flower a starry appearance on bushes only two or three feet high. All such varieties in the first decades of the 1800s were red, crimson or scarlet; only when growers added Dahlia crocea and D. barkerae to the gene pool toward the middle of the century did the hybrids that resulted show a range of colors more like those we expect in dahlias today, and that color range was incorporated into both the single and double dahlias that were developed by the thousands as dahlias grew in popularity.

An example of color variation in single dahlia flowers is Union Jack, a British introduction hybridized in the late 1800s and registered early in the 20th century. It is a great garden dahlia but unlike many of its class, has poor stems for cutting and arranging. Popular at Lynch Creek Farm is Kelly, a single blend of pink and red with two- to four-inch flowers on a three-foot plant.

Pretty mignon singles include Baby Yellow, a bright yellow dwarf dahlia that grows just a foot high and makes a great pot plant, and its counterparts Baby Red and salmon-orange Inflammation. Another tiny mignon single dahlia is Lupin Tori, a white dahlia with tapered ray florets and dense foot-high plants that look like they're spangled with stars.

By the later decades of the 1800s many growers referred to "garden dahlias," the single dahlia flowers, and "show dahlias," the doubles. Although some single dahlias like Union Jack were popular since their development, many American Dahlia Society shows didn't have a single class until the 1970s. Now the single forms are popular for floral design and border gardens, and single dahlias are showcased in many exhibitions and flower shows.

Tuesday, May 3, 2011

Dahlia Classification: Love Those Colorful Collarettes

Collarette dahlia La BombaCollarette dahlias are among the most colorful varieties of the genus. The colarette dahlia has an open center with a single row of uniform, compound florets, composed of petaloids and ray florets, surrounding the disc flower. The effect of these compound florets is that of a single row of flat petals, nicely rounded, and an inner row of slightly elevated multiple petals, sometimes in contrasting colors, about half the length of the flat-planed petals.

Collarette dahlia Double Trouble
These popular plants are easily grown from dahlia bulbs or tubers, and generally bloom profusely on four-foot plants. They have a long season, beginning in midsummer and often continuing to produce beautiful flowers until the first frost. Typical of the collarette classification is Double Trouble, a raspberry red dahlia flower with a variegated fuchsia and pink collar. It produces an abundance of blooms, as does La Bomba, a similar collarette that blooms dark red with lighter streaks in its collar florets.

The collarette dahlia type was developed by hybridizers in France and Germany in the early 1900s. The first such flowers were introduced in the U.S. in 1912, and the American Dahlia Society listed the collarette as a form in 1915.

Collarette dahlia Alpen CherubAmong the major developers of new collarette hybrids is dahlia guru Bill McLaren, whose introductions often incorporate interesting color variations. His Alpen Lois, for instance, has a corona of flat-plane petals blushing yellow to pink, while the inner petaloids are a blend of white and lavender. Not so colorful, but one of the most striking collarettes, is his Alpen Cherub, a pure white dahlia whose collar petaloids are lime-green at the base. It's a stunning landscape dahlia.

Collarette dahlia Caboose
At Lynch Creek Farm, another favorite is Caboose, a colorful collarette whose flat-plane petals are bright red, sometimes streaked with a paler hue, while its collar petaloids are streaked shades of red and yellow. Another prolific bloomer, it provides stunning late-season color bursts in the garden. Like other collarettes, it has nice stem length for flower arranging or bouquets, and it boasts healthy compact foliage. Collarette dahlias are a great choice for the mixed flower bed or border, and they're great show-offs if you grow for exhibition as well.