The garden dahlia is one that a lot of us take for granted, since it has been a part of our industry for so long. There are currently more than 57,000 different cultivars registered and about 45 different species identified. Dahlia blooms range from 1 inch in size to more than 12 inches across. Plant size goes from a few inches to more than 20 feet tall. Thanks to breeding efforts all around the world over the past 200 years, there are currently more than 20 different flower forms from simple daisy types to double, anemone forms, cactus types, pompom and many others. That is some serious plasticity in flower size, form and plant habit.
Dahlias became popular in the U.S. as a seed-produced annual flower with a wide range of good performance. It was only with the emergence of the vegetative market that clonal lines were commercially produced in North American greenhouses. Along the way from 1791, dahlia breeding took a sidetrack, and they were bred for flowering pot production, not consumer performance. What is called flowering potted plant genetics in dahlias refers to primarily European breeding for cold preference and compact, dense plants that flower evenly across the top of the plant all at one time, similar to the way pot mums grow and flower. However, in a summer flowering plant, like dahlia, this type of genetics is a real drawback for consumers and a problem for growers, as well.
The biggest issues in dahlia production lie in avoiding powdery mildew and botrytis. The flowering potted plant genetics were designed for plant habit, and all the resistance to disease went away. Botrytis became a problem when these greenhouse lines of dahlia were planted outside and all the flowers bloomed all at once (which looks good on a bench), but when the flowers begin to fade they all deteriorate at once, and botrytis becomes a serious issue. You can still find these genetics at retail, and they have great retail appeal, but their use has really damaged the reputation of dahlias in the U.S. Now you are beginning to see a new generation of dahlia breeding where emphasis is moving to staggered blooming and a resurgence in disease resistance or tolerance, and the dahlia is coming back into the market as a strong consumer performance plant.
pH: 6.2 to 5.8
EC (2:1 Extraction Method): 0.8 to 1.2
Fertilization: 150 to 200 ppm
Like most plants with very large flowers, dahlias need a lot of feed, but the trick is to go lightly on newly transplanted liners and then to begin to increase feed as plants move into buds and blooms. A balanced fertilizer with micronutrients is best, but if you do not normally use this, make sure the crop gets a dose of micronutrients about two weeks after transplanting. Too much fertilizer when liners are young can cause more stretch than you want to make a strong young plant. If top dressing with a slow release fertilizer, use a medium rate since you lose the ability to cut back on fertility after applying slow release fertilizers.
Light requirements: High light levels are essential to reduce stem stretch and also to provide the energy needed for forming large flowers.
Water: Keep moist at all times; drought stress and wilting cause lower leaf drop.
Temperature: From transplant through grow out, dahlias prefer a slightly cooler temperature range between 65˚F to 75˚F daytime and cool nights. For the grower, where possible, cooler nights provide stronger stems and higher quality plants.
Planting And Finishing
Planting and timing: Early spring through fall finish.
Pot size: Plants per pot time to finish
4- to 5-inch, quart pot: one liner per pot for five to six weeks
6-inch pot, gallon pot: one liner per pot for six to eight weeks
7- to 9-inch pot: one to three liners per pot for eight to 10 weeks
10- to 12-inch pot, 2 gallon pot: three liners per pot for 10 to 12 weeks
Pinching and growth regulators: Pinch one week after liner planting. A second pinch is recommended for 8 inch or larger containers. Pinch when liners have produced three or four sets of new leaves and remove only the bud (soft pinch) for fastest regrowth. Expect pinched crops to be delayed about two weeks compared to unpinched liners. PGR’s should not be needed, but B-9 or a tank mix application can be used if necessary in small pot production. If used, work with rates around 2,500 to 3,500 ppm Daminozide and if tank mixed, add 1,000 to 1,200 ppm chlormequat chloride. Follow label instructions for application of all PGRs.
Photoperiod for flowering: Dahlias are long-day plants. Provide 12 to 14 hour day length or night interruption lighting through week 13 (April 1) to allow the plants to bulk up, and to prevent premature budding and tuber formation. In very early season, be careful that day length is correct as the combination of colder temperatures and short days causes dahlias to stop growing and turn their energy to root development, which means plants stop growing and yellow.
Pest And Disease Management
Good sanitation practices include keeping greenhouses clean at all times, eliminating weeds and debris, disinfecting equipment in greenhouses as often as possible and keeping hose ends off of the greenhouse floor. Good water management practices are critical for good, healthy plant growth. Treat with a broad-spectrum fungicide at transplant. Good air circulation is the key to healthy plant growth. Whitefly, aphids, thrips, spider mites, cyclamen mites and leaf miners can be problematic. Scout aggressively and treat as needed. Do not allow populations to get out of control before treating the crop. Powdery mildew is more likely to be a problem under low-light, high humidity and cool temperatures. Good cultural practices should alleviate most issues with the disease, but scout and treat as necessary.
Main Considerations For Succeeding With Dahlias
1. Cultivar selection: Make sure you are choosing a series or collection that is what your customers want. The pot dahlia genetics are much like potted-mum genetics in that they look good for a brief time and then are designed to be disposed of.
2. Disease resistance: Look for cultivars with some resistance. Botrytis and powdery mildew are the biggest disease issues. Spider mites when the greenhouse is too dry and whitefly are the likeliest insect risks.
3. Photoperiod: Keep daylength in your facility above 12 to 14 hours a day. This is especially key for early spring crop cycles, otherwise plants may shift their energy away from developing flower buds. Remember dahlias like high levels of light and long days for best growth and performance.
So consider adding a Dahlia collection to your lineup next spring, they are easy to produce and very tolerant of a wide range of conditions both in the greenhouse and in the field. Improvements made in the vegetative production process have reduced seed set, improved disease tolerance and provided better habits than older genetics. Dahlias can be a great fast crop, especially later in the season when spring annuals are less tolerant of warmer weather.