How to Grow Dahlias
Site preparation
Amend soil with compost and a general organic fertilizer for fruits and flowers. Prepare raised beds as good drainage is important for healthy dahlias.
Soil temperature
Dahlias do best in soil that has warmed to 60 degrees. Use a soil thermometer to evaluate whether it is time to plant your tubers. High Altitude tip: We start our hoop house grown dahlias in small pots ( 4" pots for small tuber and 6" pots for large tubers) in our hoop house nursery in early March to be planted out in the ground in another hoop house by Mid April (protected from frosts with low tunnels and a small propane heater). We start our field grown dahlias similarly in pots by mid April to be planted out in the field (protected from frosts with a low tunnel) in early June.
Location
Dahlias grow best in well-drained soil in full sun (at least 8 hrs).
Spacing
Plant 12" -24" apart and 4" -6" deep
Watering
If you are starting your tubers in pots indoors, water lightly letting the soil dry out for several days before watering again (watering about every 3-4 days depending on how sunny and warm it is and whether you are starting them in your house or in a greenhouse). Dahlias tubers are sensitive to over-watering in the beginning stages of growth. At the same time, starting tubers in pots in the greenhouse can lead to overly dry conditions if you don't water at all.
If you are starting dahlias in the field or backyard, you will want to wait until the soil temperature warms to 60 degrees and not water until you see the first green shoot poking up through the ground. Over-watering before shoots are visible can lead to tuber rot. Once dahlias are up and growing, you will want to water on a regular basis. They will need a good soaking several times a week. Deep watering is best.
Pinching
Once the plants are between 8" and 12" tall, snip the top 3-4" of the central shoot or terminal bud just above a set of leaves. This will encourage branching, increase stems and stem length. Our general rule of thumb is to snip the top once there are 3 sets of leaves on the plant.
Cultivation
Keep dahlia plants weed free. Take extra care around the base of the plants because dahlias grow many surface roots. Avoid using tools to weed around base of plants for this reason.
Insects
Pest pressure can be variable from year to year. Sometimes the grasshoppers are prolific other times thrips, earwigs or aphids can be a problem. The healthier the plant, the fewer pest problems you will have (the stronger your immune system the less likely you are to get sick). To keep thrips and grasshoppers off the blooms, put a small mesh bag (Organza) over the bud before it breaks open. Keep it on until harvest then move the bag to a new bud.
Planting in Pots
Choose low-growing varieties if possible. Plant in a 15”x 15” pot using at least two thirds garden soil. Potting soils can be too porous and will dry out too quickly if you are planning to keep your dahlias in pots for the entire growing season. They can be started in small pots and then moved to a bed in a greenhouse or the field (see above).
Corralling/Staking
Tall healthy plants will require corralling or staking so that your stems don’t flop, losing valuable cut flowers to the wind.
Havesting and Storing Tubers
Before your last hard frost, be sure all your plants are labeled. After the first killing frost it is hard to tell what color the blooms were when it is time to harvest the tubers. Use a digging fork to gently dig up tuber clump, being sure not to dig into the tubers themselves. Brush most of the soil off. Storage is best with some soil left on the tuber clump. Store tuber clumps in a breathable container ( we use large nursery buckets which have holes in the bottom) and cover the clumps with slightly dampened sawdust, peat or vermiculite. Ideal temp for storing is 40-50 degrees. Tuck your tubers into a dark cool area like a basement, or crawlspace with a tarp over them until it’s time to divide them. Check on your tubers a couple times through out the winter to be sure they are not drying out or rotting.
Dividing
When you are ready to divide your dahlias, you will need a place outside where you can hose them off (ideally with a high-pressure stream of water) and get all the soil out of the nooks and crannies. Once they are clean and dry, they are a lot easier to divide. You will need some sharp garden snips, loppers, and a nice work space. Dividing tubers can be tricky and takes experience. Dive in and give it a try. We will provide a skill sheet on dividing in the early spring.
Happy Growing!
