When to PlantPlant around the same time as you would tomatoes, or in pots a month ahead of when you want to transplant outside. Dahlias don’t like the cold, sopping soils of early spring, when they are liable to rot in the ground, so wait a few weeks after the last frost before planting outside. In much of the northeast, middle-late May is a pretty safe bet.
Where to Plant
Choose a sunny location - the more sun it gets, the bigger and more floriferous your dahlia will be. HOWEVER, that does not mean you need an open field to plant dahlias, nor that you must clear several feet in every direction, so your dahlias float alone in a sea of bare soil or mulch. Any semi-robust plants that are 18”+ tall can even help prop up your garden dahlias, so be creative.You CAN plant dahlias in places where they are fully or partially shaded here and there throughout the day. Just know that there is a tradeoff. More sun = bigger plants, more blooms. Less sun = weaker plants, fewer blooms.
How to Plant
Choose a location where soils drain well and are rich in organic matter. Wait to work up the soil until it is semi-dry; never when it’s wet. Amend with compost and/or sand as needed, well incorporated into the soil. Prepare a much larger hole than the tuber needs, because it will produce many more tubers over the course of a growing season - much like a potato. About 6-12” wide and 6-12” deep should be ample. Your dahlias will appreciate this, and will reward you handsomely with healthy plants and lots of tubers to increase your stock.
Tubers should be planted 4-6 inches deep. Horizontal or vertical orientation does not matter - just make sure the eye(s) on the tuber are facing upwards, as this is where the aboveground part of the plant will grow from.
Tubers may be planted into *damp* soil (not wet! There’s a difference! Damp feels like a light, well wrung out sponge; wet feels like a heavy, dripping sponge), and will likely not need watering unless the soil dries out completely and there is no rain in the forecast. If planting into dry soil, water thoroughly once and not again until they sprout out of the ground. From then on, only water as needed to keep the soil damp (but not wet!).
Tubers may also be planted into pots sized 1.5 gallons and up and spend the whole season in there, though they will typically not grow to be as big and robust as those planted in the ground. Still, they can be a great addition to your gardens and patios. The same “damp-but-not-wet” rules apply as above. Water them in thoroughly at planting and not again until they’ve sprouted and the soil seems dry.
Physical support, such as bamboo stakes, tomato cages, or other plants in your garden, are recommended to help keep your plant upright all season long. In addition, pinching out the growth tip of your dahlia when it is about 12” tall will cause it to branch from lower down, giving you a stronger foundation for your plant.
How to grow
Young plants <18” tall need a modest amount of water, and shouldn’t be allowed to get swallowed up by weeds. Large plants 18” tall and above need thorough, regular waterings, and can tolerate some competition around the base of the plant. At this point, the soil around them should not be allowed to dry out completely. When they begin to flower, they can take even more water. In pots, dahlias will need frequent watering once they are growing, and they will always need more watering than those planted in the ground.
A good dose of compost plus balanced fertilizers when they are planted, and small doses of diluted fertilizer later in the season, should give your dahlias all they need to be happy.
Harvesting flowers promotes a longer bloom window for your dahlias. They will keep trying to go to seed, and once they are successful, they will put out fewer and fewer flowers. If you prevent this from happening, they will keep flowering until frost.
How to save your dahlias - forever!
In the northeast, if we want to keep our dahlias year to year, we have to dig up and store our tubers indoors. It is simply too cold and too wet outside for these natives of the Mexian highlands.
If you want to save your dahlia tubers, or increase next year’s planting stock, then you’re going to want to dig them out of the ground. To do so:
- Wait until frost has killed the top part of your plant.
- Wait another week or two.
- Cut the top part of the plant away, leaving 6-12” of the main trunk to give you something to hold onto.
- Dig up your clump with your favorite digging tool - we prefer digging forks. This clump will be much bigger than the single tuber you planted, so dig a 1’-diameter circle around the base of the plant. These tubers will also likely be deep, so try to get under the whole clump with your digging tool to loosen it; then it can be lifted from the soil by the main trunk of the plant.
At this point, you have some choices to make. Either clean up and divide your tubers, or leave the tuber clumps with lots of soil still attached and store them like this until the spring, when you will divide them. We’ve done it both ways, and prefer to clean and divide tubers in the fall. We mostly do this because of our scale - dealing with thousands of clumps would be a nightmare. But for a home gardener, the ‘easy way’ is a perfectly acceptable way to deal with your dahlias.
The easy way (less work, fewer plants next season):
Leave soil attached to your clump of dahlias, and place the whole clumps in a basement or other cool, dark location. Keep the clumps off the floor and loosely covered with a plastic tarp. You want it cool in there - around 40 degrees is ideal - but not freezing. And you want relatively high humidity, which is normally the case in basements anyways. The plastic covering is to hold in the moisture that the attached soil contains. You’ll want to check on your tubers about once a month to make sure no rodents are eating them, and that conditions are still dark, cool, and humid. In the spring, you can either follow the instructions below for dividing tubers, or you can just take a sharp shovel or spade and slice clumps into 2-4 chunks, and plant those. You will almost definitely get dahlias growing from each clump if you’ve followed the instructions.
The hard way (more work, more plants next season):
Remove soil from the clumps of dahlias by hand, and then use a strong hose blast to remove the remaining soil. You want to remove as much soil as you can - this will make dividing easier, and give you cleaner dahlia tubers less likely to rot over winter.
Using a big sharp knife, a small sharp knife, pruning shears, or snips, break the clump into 2 or three equally sized parts, each containing tubers and some of the crown of the plant (the crown is the plant tissues that all the tubers connect to). At this point you can get rid of the old plant trunk that was left on for handling purposes.
Then, take each chunk and look for places where tubers meet the crown of the plant. Find the small round bumps that often have faded concentric circles around them. These are the “eyes” - the only part of the plant that will be able to generate a new plant next season. These eyes cannot survive on their own all winter; they must remain connected to a tuber, or part of a tuber, which will provide energy to the plant all the way until next spring. By the same token, a tuber with no eyes will not be able to produce a plant next season.
Then, making clean cuts, you want to separate as many tuber-eye combinations as possible, being careful to not injure the thin ‘necks’ that connect them, nor injure the eyes themselves by trying to cut too closely to them or by trying to make oddly angled cuts when trying to keep a tuber with an eye. It’s an art, and it takes some getting used to.
Then, cut the rooting tails off of your tubers (just because they are liable to break off when moving tubers around, and can be a source of rot). Then you want to leave your tubers somewhere with some warmth and good air flow so that their wounds dry and callus over. This is called ‘curing.’ Tubers can be cut severely and still survive through winter without rotting, as long as you allow them to cure first. It normally just takes a couple days on our greenhouse benches with fans moving air around the structure.
Finally - store your clean, divided tubers. Some growers dip their divided, ‘cured’ tubers into a disinfecting solution. This is fine to do, as long as you then dry the tubers again before storing them. We do not do this, and have good results anyway. We just take our cured tubers and pack them in wood shavings in bulb crates, label the crates, and stack them in a basement on pallets. It would be a good idea to keep these loosely covered with some plastic if you can. We did not do this last winter and everything is fine this spring.
The number of tubers you can divide from a clump varies widely by variety and size of the plant. Some tubers are big, others are small, but all are capable of growing worthy plants next season.