Anatomy of Dahlia Tubers
Dahlia tubers vary in shape and size but all have the same three important parts that work together to make a viable tuber: the eyes of the crown, the body of the tuber, and the neck.
The eye is a growing point where vegetative growth will begin. The body provides the eyes with the nutrition and energy they need to get the plant going, just as with a seed potato or other kind of tuber. And the neck is the point where the crown and the body meet.
Being able to correctly identify and assess these three parts, especially the eyes, is what you need in order to successfully divide dahlias and increase your dahlia stock. Not all tuber bodies will connect to a neck with an eye above it, making it quite easy to end up with “tubers” without eyes. All three elements must be present, intact and healthy in order to produce a robust plant.
Getting a Head Start
To increase success in the garden, dahlias can be pre-sprouted before planting. To do so, bring them out of storage and into your house, where they will experience temperatures above 60 for a couple of weeks. You’ll see the eyes begin to swell and that’s a sign that they have woken up and are ready to grow.
If you have the space, dahlias may be potted up a month ahead of being planted outside. At our farm, we pot up roughly two-thirds of our dahlias each year to get a head start on the growing season. To do this, we fill pots with soil, add the tuber — leaving the eyes above the top of the soil line — add more soil and water lightly. After about a month, the plants have robust, healthy leaves and the temperatures outside allow us to move them outside to harden off before going into the field. At planting time, we bury a bit of the growing plant so that the tuber is fully buried.
Planting Dahlias
Plant tubers on their side 4” to 6” beneath the surface of the soil, 12” to 18” apart. We plant most of our dahlias at 18”, with dinnerplate dahlias like Cafe au Laits spaced at 24”.
Dahlias should be planted in a well-drained site rich in organic matter that receives a full eight hours of sun.
Dahlias are not frost-hardy so even if the immediate forecast looks good, we always wait until mid-to-late May before planting them outside. The soil temperature should be close to 60 degrees.
Dahlias are prone to rot, especially in cool soil when it takes longer for them to emerge. It is important to make sure cool spring soils are not saturated at the time of planting. Once in the ground, unless the soil is unusually dry,do not water the tubers before the sprouts break through the soil.
Pinching and Topping Plants
Once dahlias are about one foot tall, we pinch back about half the plant so that it has between 3 and 4 pairs of leaves remaining. This encourages the plant to branch and send out a greater number of flowering stems.
Preparing your Site
Ahead of planting, we amend our beds with compost and a mix of balanced 5-4-3 organic fertilizer. At the time of planting, we add a couple of tablespoons of bonemeal to each planting hole as this helps with rooting and flowering.
Support and Trellising
Dahlias have hollow stems and need to be trellised. Without support, there is a risk that they can collapse under the weight of their own flowers, or that brittle stems will break during gusty wind storms.
If you have long rows of plants to support, you can use horizontal netting or basket weave twine between hardwood stakes. The later has become our preferred trellising method. If you are growing plants in the garden, a single bamboo stake is a great option.
Foliar Feeding
Because dahlias are a long-season crop and need more than the initial dose of nutrition from the general fertilizer, we use fish emulsion/seaweed as a foliar feed every two weeks from June through September. This provides supplemental phosphorus and potassium without adding significantly more nitrogen, resulting in more flowers.
Harvesting Dahlias
Harvest dahlias in the morning before the heat of the day sets in. Place stems into clean water immediately. Dahlias should be harvested when they are almost fully open, the petals on the backside of the flower should be nearly flat or still tucked toward the bud. If petals on the backside are fully flayed out or desiccated, the flower should be deadheaded to promote further blooming.
Cut deep into the plant just above above a leaf node/growth point so that stems are between 18” and 24” (or proportionately less if the plant is shorter). If you cut the stems too short, the plant will respond with more short stems. If you cut stems long, the plant will respond with more long stems.
Vase Life
Vase life for dahlias is between 3 to 5 days. Vase life depends on the size and shape of the flower. Ball types are the longest lasting (for example: Sylvia, Cornel Red, Cornel Bronze, Jowey Winnie) and are great for bouquets and farmers market. Dinnerplate dahlias and other varieties with flat wide petals are showstoppers but tend to lose moisture quickest, making them most suitable for custom arrangements and wedding work.
Post-harvesting handling is just as important as cultivar selection when it comes to vase life. After harvest, we transfer the dahlias into new water with a transport/holding solution before putting into the cooler. If dahlias are to be sold at a retail location, we also include a Gerbera chlorine tablet in the water as cleanliness is paramount for maximum vase life.
