Our Guide to Growing Dahlias
Choosing the right time to plant is the key to a successful dahlia crop. You shouldn’t plant dahlias too early, when ground temps are still cold. Dahlias prefer warm soil, at least 55-60 degrees. If the forecast is predicting a prolonged stretch of wet weather, hold off on planting as tubers are prone to rot. In the Northeast, early to mid-May is typically the earliest we can plant outside, usually around the same time you would plant tomatoes. If you want earlier flowers, you can start your tubers indoors in pots about a month before planting time.
Dahlias thrive in full sun here in the Northeast and should be planted in a location that receives a minimum of 8 hours of sunlight a day. Dahlias planted in an area with less sunlight will grow taller and won’t yield as many blooms. Dahlias are heavy feeders and like soil with lots of organic matter. Because tubers are prone to rot, it’s important to make sure that your soil isn’t too heavy and drains freely and easily; if not, amend it with peat or sand.
Dahlia tubers will grow and multiple quite substantially over the course of the season so it is important that you prepare your soil well to accommodate such growth. Start by digging a hole at least 6” wide and 12” deep. We choose to incorporate an organic balanced fertilizer (ProGrow 5-3-4 with additional added bonemeal) at planting time, but depending on the results of your soil test, you may need to amend your soil with more/less nutrients (nitrogen, phosphorous, potassium, etc). Make sure to mix the fertilizer in thoroughly with some of the soil you removed before planting tubers whole, with the eyes facing up, about 4 to 6 inches deep. We space dahlias at 12" apart, with dinnerplate and larger-headed dahlias spaced at least 18-24” apart. All dahlias that we sell will require additional support (tomato cages or bamboo stakes work well).
Once you plant your tubers, wait to water until you see the first shoots emerge. There should be enough moisture in the soil to get your tubers to start growing. If you are planting in a container, water well once at planting, then hold off on watering again until shoots emerge. Young dahlia plants do not require too much water. Once your dahlia plants are established (12"-18” tall), water regularly and deeply. A good rule of thumb is that your dahlias should receive 1” of water every week. In the heat of summer, dahlias will require more water especially if it is warm and dry. When then are actively growing and blooming, dahlias will need more water. Dahlias grown in pots will require more frequent watering no matter what.
We amend our soil with a general purpose (5-3-4) organic fertilizer at planting time. Once dahlias begin to bloom, we foliar feed every other week with a fish and seaweed formula (low nitrogen). Avoid high nitrogen fertilizers -- these will make your plants large and green but you will have small (or no) blooms with weak stems.
Topping (Pinching Dahlias)
A regular practice for dahlia growers is to pinch or “top” your dahlia plants to promote lateral growth and a more evenly grown plant (one that isn’t top heavy). When your plant is about 12” tall, snip out the growing tip leaving 3-4 pairs of leaves. This will encourage the plant to send up low basal growth that not only will give you more flower to cut but distribute the weight of your plant better so they do not topple in summer winds and rains. Note: supporting your dahlia is paramount. If you are a home gardener, tomato cages or bamboo stakes work well. If you are growing in large amounts, corralling your plants with T-posts spaced every 6-8 feet or using "‘Hortonova’ netting is recommended.
The best time to cut dahlias is in the cool of the morning. Using sharp pruners, cut the stem at a 45 degree angle just above a leaf node. (A good stem length is from the tip of your finger to the crook of your elbow). The plant will branch just below the cut and produce additional stems. Harvest dahlias when the flower is nearly or fully open. Closed buds won't open after the stem is cut. Strip any foliage that will be below the waterline in a vase. Place the stems in fresh water and add flower food as desired. Replace the water and recut stems daily for a vase life of 3-5 days (vase life of dahlias varies depending on variety and size).
Harvest dahlias regularly to promote additional flowering. Once flowers go to seed, the plants will slow down flower production.
In the Northeast, our winter temperatures are just too cold, and dahlias’ thin-skinned tubers will freeze if left in the ground over winter. Tubers can be pulled each fall, cleaned, and stored in a cool, dark room until next spring. Once the frost has killed the plant, cut the dahlias down close to the base leaving some stem as a handle to help you pull the tuber clump up. We like to let our tubers “cure” in the soil for up to 2 weeks after a hard frost so the skins have thickened and tubers hardened off. This is not necessary however and tubers can be lifted the same day they are cut down. Starting at least 6” away from the heart of the plant, use a digging fork to gently lift your tuber clump out of the ground being careful not to break the tuber’s necks. We find that you will need to “dig” 2-3 times to get the whole clump out intact.
It's important to keep tubers clean and maintain a balanced level of humidity in storage over the winter. Tubers should be kept in a cool (40-45 degree), dark place with 85-90% humidity. Depending on your location and winter weather conditions you may need to store tubers in airtight containers (if located in dry climates) or in no medium at all (wetter, rainy climates). Regardless, you will have to check on your tubers frequently. Tubers that have properly hardened off will maintain their firmness in storage and are free of rot or mold.
Dahlia tubers are readily available from many growers online. Tubers are essentially the root stock of the dahlia plant and, once planted, will multiply in the ground every season. A dahlia plant grown from one tuber will yield multiple tubers once dug up in the fall. We sell our excess tuber stock in late winter/early spring. Join our mailing list to be notified when our dahlias will go on sale!