About your Dahlia Tubers
Dahlia tubers will arrive in a state of dormancy. Since dahlias don’t begin to sprout until in-ground temperatures are above 55°-60°F, you can wait and plant tubers after the threat of first frost ends or you can start them early inside a pot. Tubers take 10-12 weeks to reach buds & blooms. So by starting them in pots indoors mid-March in the Upstate Zone 8, it will allow them to grow and be ready for placing outdoors by April 23 (which was the last frost date in the Upstate Zone 8 in 2021).
Ultimately, you need to decide whether to begin your tubers indoors or store them until planting outdoors. If you plan to store them until Mid-April when planting outdoors is viable, see the section below on storage.
Beginning Tubers Indoors
At Martin Garden Center, prior to planting all our bare roots, bulbs, tubers, or corms, we rehydrate them and activate them using a rooting solution. This involves soaking them for at least 1 hour in a mixture of water and rooting solution (we suggest Bonide Root & Grow), or soaking them in water for 1 hour and then placing tubers in a plastic bag of Bonide Bontone Rooting Powder and shaking the bag until the tubers are coated. The rooting hormone is not necessary but recommended by MGC and many Dahlia societies and organizations.
Dahlias are prone to root rot or crown rot so a well drained potting soil is paramount. We recommend a moistened mixture of ⅔ regular soil mixed with ⅓ soil conditioner for your starting soil mixture. Specifically, using a gallon container, place 3 inches or so of your moist soil mixture in the bottom of the pot and mix in 1 TBS of Espoma Bone Meal to the bottom layer. Insert your tuber so that the crown is up and the tubers are beneath it in a pyramidal shape. This is the natural shape so don’t worry about adjusting … just make sure the crown is up. Then add the additional soil mixture (devoid of bone meal) around the tuber. Ultimately, your crown should be buried 1-inch beneath the top of the soil but the tubers themselves will be buried 5-6 inches.
Place your pot in a warm sunny area. DO NOT WATER until the plant foliage emerges and the plant begins to grow. The tubers will provide the energy for the Dahlia foliage to grow and roots won’t begin setting until the foliage has emerged. Plus, the moistened soil mixture will provide adequate moisture.
You should see growth emerging within 2 weeks. Once foliage emerges, watering once a week should be adequate at this juncture but always check your soil. If it is dry 1-2 inches down (the length of your finger inserted in the soil), then water the plant. Be careful not to overwater. Only water when necessary.
Do not fertilize until at least 1 foot tall (Dinnerplate Dahlias) or 6-8 inches for the Patio Dahlias (the GoGo and Happy Single Series).
Pinching Do’s and Don’ts
Pinch your Dinnerplate Dahlias when 4 sets of leaves have emerged. Pinch the lead tip only, to promote branching. After this initial pinch of the Dinner Plate Dahlias, you may pinch again when branches have at least 4 sets of leaves. However, this second pinch is only recommended if you are attempting to bush and branch your plant, as this second pinch will delay your first set of blooms.
Do not pinch your Patio Dahlias in the early stages of growth. Studies conducted by the Ohio State University have concluded that the GoGo Series blooms will be delayed 2-4 weeks if pinched AND pinching of the GoGo series did not promote branching that was noticeably different from the non-pinched control group.
So save yourself time … don’t pinch your Patio Dahlias to promote branching unless you really want to and then follow the 4 sets of leaves rule as with the Dinnerplate Dahlias.
DO PINCH all Dahlias if they become misshapen and DO PINCH when deadheading.
Planting Tubers Outdoors
Every smart gardener will tell you that proper soil and fertilization are the key components for optimum growing. Dahlia’s are heavy feeders so amend your soil ~ 1 month prior to outdoor planting. For Dinnerplate Dahlias, planting should be at least 24 inches apart, but 36 inches is better. For the GoGo and Happy Single series of Dahlias, 24 inches apart should be adequate. Amend the soil 12 inches deep by 12 inches wide with humic materials. We recommend Mushroom Compost by Black Kow.
By amending the soil, we mean dig 12 inches wide and 12 inches deep. Then add back a mixture of ⅔ natural soil (lots of clay) with ⅓ amendment (Mushroom Compost). Some purists will even recommend adding in about 4-8 cups of Soil Conditioner for drainage.
In Zone 8, you should plant your Dahlias in 4-6 hours of afternoon sun. Plants can tolerate all day sun (8 hours) but they can also tolerate less than 4 hours of sunlight. Both excessive sun and less sunlight can make for less blooms.
Staking and Planting Deep
Patio Dahlias should not require staking if they are well maintained. By well maintained, we mean pinched when deadheading and pinched when misshapen as this pinching helps keep the plants from growing out of control.
Dinner Plate Dahlias must be staked. Planting these tubers along a fenceline will give you a perfect place to later tie plants. Bamboo Stakes are available at Martin Garden Center. Rebar is also well used in the Dahlia world. Finally, tomato cages (though pricey as a stake) make excellent supports for Dahlia flowers. We also recommend Bond Garden Tape to secure stalks and larger flowers.
Some Dahlia enthusiasts recommend starting Dahlias indoors and then, when planting, removing the lower two sets of leaves and planting the stalk deeper in the ground. Plant in this way at your own risk … crown rot is always a concern when planting in this manner. However, Martin Garden Center does recommend mulching the dahlia planting area but instructs all you mulch-a-holics not to let the mulch touch the stalk … keep it 3 inch diameter away from the stalk. And, don’t over mulch. Layer mulch 2 inches maximum for the spring. Please. Then in late fall, add more mulch for protection but remember to remove it by early April before the Dahlia emerges
Martin Garden Center DOES recommend snipping, at the stalk, the two lower sets of leaves when planting. These lower sets of leaves are generally non-blossom bearing. And they tend to sag over time and create opportunities for pests and powdery mildew to become problematic.
Wow are there a lot of debates about fertilization of Dahlias. All agree that phosphorus rich fertilization is necessary to maximize flowering. Most professionals agree that you don’t need to provide fertilization until buds appear.
Martin Garden Center recommends use of Bone Meal when planting tubers, whether inside or outside, for a phosphorus source. To provide fertilization when planting outside, MGC also provides the following suggestions:
- For a one time fertilization, MGC recommends Fertilome Garden Cote 6, which provides a 6 month feed. This is the simplest and easiest fertilization method. MGC has used Garden Cote 6 for 6 years and we believe it is the best slow release fertilizer on the market.
- If an organic solution is preferred by organic gardeners, we instead recommend Espoma Flower Tone granular. It is a little more maintenance as it should be applied every 4 weeks until mid-September.
- For supplements, we recommend Jacks Blossom Booster 10-30-20 every 2-4 weeks. It can be dissolved in water and sprayed on foliage as well as used for irrigation. Plants love it and respond well to a phosphorus boost.
We highly recommend ending fertilization by mid- to the end of September because you don’t want plants trying to bloom when the temperatures are approaching frost.
As with all flowers, Dahlias should be cut in the early morning hours. Cute the flowers that are ¾ open for maximized longevity. Cut the flower stem just above the main stem to avoid unsightly stems sticking up everywhere. Take care to not cut the new emerging growth of the future buds.
Once inside, you can further cut your stems to desired length and discard your debris to your compost pile. The best method for longevity we’ve found is placing the bottom 2 inches of the newly cut stem in a pitcher of hot tap water until cool (about an hour). This helps seal the stem but it does discolor the stem so if you are using a clear vase, try a cold water approach. Specifically, place the stems in 50 degrees water for several hours and then place.
Move your cut flowers to their respective vases after the stems have “cured.” As with all cut flowers, remove any foliage that may be below the waterline of your vase.
Dahlias are also great dried flowers. Consult the internet for information on how to dry your dahlia blooms.
To keep your dahlia blooming strongly, you must deadhead your spent blooms because spent blooms suck energy from your plant that would otherwise be used in creating new blooms. You can deadhead at the same time and in the same method as you would cutting your Dahlia blooms for vases. Do not cut off only the spent flower but instead cut the stem down to the next main stem. Again, this avoids unsightly stems sticking up everywhere. By deadheading your plant regularly, your plant will continue to bloom regularly. Win – Win.
With emerging growth, watch for slugs or snails and control with Bonide Slug Magic if a problem occurs.
Aphids are a common pest for Dahlias, usually landing on young stems and buds in the early spring. Preventatively, weekly spraying with a cocktail composed of neem oil with pyrethrins (Triple Action) and Insecticidal Soap can prevent aphids. If infested, use the cocktail every 4 days for 12 days. Spider mites may also be problematic, causing mottling of foliage. If spider mites are found, neem oil with pyrethrins (Triple Action) is recommended. However, you cannot spray oil once temperatures are over 90°F, so control any outbreaks of spider mites before the summer. Or, spray at night and rinse plants thoroughly in the morning.
Caterpillars and worms (particularly cut worms and army worms) can descend upon and decimate dahlias very quickly, eating both the foliage and disfiguring the blooms. These pests appear later in the growing season, mid-May. Regular (weekly) spraying with Bacillus Thuringiensis (commonly known as BT) controls these pests once noticed.
Japanese Beetles generally emerge late May early June. They tend to appear just when blooms are ready to cut. They are hatched over a one or two day period and they descend upon the unprepared gardens to munch upon all the beautiful dahlias and other flowers that have been nurtured for months. We recommend crushing them by hand or spraying them with Bonide Japanese Beetle Killer. This spray is a contact-killer only and will not work for prevention.
The above chemicals for pests are all organic solutions for organic gardening. However, at times, it is worthwhile to consider some other chemicals if organic chemicals are not working. One such chemical is Bonide Systemic Insect Control, which contains Acephate. Acephate will kill worms, caterpillars, japanese beetles and aphids. And, because it is a systemic, it will remain on the plant for several days and consequently is not a contact-only killer. Unfortunately, acephate does little to control mites.
We do not recommend any Imidacloprid use on Dahlia plants due to the effects of this chemical on the bee population.
Fungal diseases can also affect dahlias. The most common is powdery mildew which causes leaves to have a grayish cast. Powdery mildew can be prevented by making sure dahlias are well spaced so that air flow is maximized. Watering in the mornings and avoiding moisture on the foliage also helps. But, if powdery mildew arises, Dahlia’s can be inexpensively treated with Bonide’s Copper Fungicide. Again, this is an organic chemical for organic gardening. Martin Garden Center also offers other fungicides that help control powdery mildew that can be discussed if organic gardening solutions do not produce the desired result.
Root rot and crown rot can be prevented by amending your soil to make sure it is well draining, avoiding overwatering, and by keeping mulch away from the stem of the plant.
These are zone 8-10 Dahlias so consequently, they are zoned to survive our winters in the ground except in the mountains of the Upstate, which are Zone 7. If you plan to allow your tubers to overwinter in the ground, it is very wise to generously (3+ inches) mulch these tubers at the end of the season.
If you plan to dig up your tubers and overwinter them inside, you will need a cool space that remains above freezing but below 50°F.. Remove tubers after the first frost kills the foliage and cut foliage away at the crown. You can tie on an identification to the crown to keep track of each dahlia cultivar. Thoroughly rinse and remove dirt from tubers. Check tubers for rot or soft spots and cut such away. Consider adding a coating of sulfur. Then store your tubers in your breathable container (cardboard box -yes; plastic box – no). You can wrap tubers in newspaper or tissue, or place them in slightly moist vermiculite or sand. Check your tubers midway through the winter (about 2 months from harvesting) to determine if they are shriveling. If shriveling, mist well and then repack for storage.
Remember, keeping Dahlia’s below 50°F will keep tubers from sprouting. But, you must keep your tubers above freezing. It is a tough storage dilemma but gardeners all over the world have been doing it for centuries.