Dahlias are one of the most wonderful plants that you can grow, especially if you like cut flowers. They are classified in CT as TENDER TUBERS. What that means is that they grow from a swollen storage root called a tuber (think potatoes). You can buy dahlias two ways. The first is already started potted plants. This gives you the benefit of not having to pot them and tend them as they sprout, and often you can see the exact color and shape of the bloom. But, this is a more expensive way to grow them.
The most economical way to grow dahlias is to buy the tubers. This also has the benefit of offering you a much greater diversity of choices. Tubers are usually for sale in late winter and early spring. There are two ways to grow them.
- At Natureworks, we recommend that you start your tubers ahead of time, in March or April. Use a one gallon nursery pot and a high quality organic potting soil such as Organic Mechanics Container Blend Soil. Remove the tuber from the package. You will see a growing end with a few green buds or sprouts. This is the top. The tubers will look like long fingers. Fill the container 2/3 of the way. Make a mound in the center and gently place the tubers over the mound so that the top will be 2-3” below the soil surface. Gently add soil, using a stick or your finger to ease it in among the tubers. Add a bit of water to settle the soil in the crevices and then gently add more soil. Continue adding soil until the growing tip is at the proper level. Water well and label with the name of the variety and the date planted. Keep your potted dahlia tubers indoors in a sunny spot. They grow best at 60 degrees or above. Water only when the soil has started to dry out, especially as the tubers are rooting in.
- The second way to plant dahlia tubers is to plant them directly in the ground in late May or early June, after the danger of frost has passed. Dig a deep hole in a sunny spot where the soil is well drained. Amend the soil with compost and an organic flowering plant food. Plant the tubers as described above only place the growing tip SIX INCHES below the soil surface. Water well, label with the name of the variety and the date.
If you started your tubers ahead of time, wait until late May or early June to plant them in the ground. Gradually get them used to the outdoor (harden them off) before planting. Prepare the soil and amend as described above. At this point, plant them a bit deeper, adding an additional 4 inches of soil to the top of the root ball.
As your dahlias grow, many gardeners pinch them in June to encourage fuller, bushier plants.
Most dahlias will grow quite tall and need support. At Natureworks, we love to place our heavy-duty tomato cages around the dahlias as soon as we plant them. You can also use thick, heavy stakes, but be sure not to drive the stake too close to the plant or you will damage the tubers. Do not underestimate how big your dahlias will grow!
We feed our dahlias every couple of weeks with Fish emulsion or Sustane. In late July, we also mix up Coast of Maine lobster compost and granular organic fertilizer and topdress around the plants.
Dahlias grow slowly at first, but as the summer starts to draw to an end, you will see them really take off and start to flower prolifically. Cut the flowers for the house. Deadhead spent blossoms. Keep tying up any shoots that outgrow the cage or the stakes- they are quite brittle in strong rain or winds.
Dahlias must be dug up and stored indoors over the winter. Wait until the tops have been completely killed by a hard frost. Then, wait one week before digging them so the eyes can start to develop a bit. Cut off the frozen growth, leaving 6-12” of stem intact. I try to dig them on a sunny day. I use a digging fork, loosening the soil all around the plant, and gently ease them out of the ground. I ALWAYS use a loop tag or flagging tape and label each one by looping it around the tuber. You really want to know which dahlia is which, especially if you grow different sizes, colors, and shapes! I place them on a sunny, warm surface (in my case, I use my stone courtyard). After drying them all day, I move them under cover to my back garage. After they have been under cover drying for at least 3 days, I then bring them into my cellar to process them for storage. I want them to be dry, but not yet starting to shrivel. Keep your eye on them as they are drying- temperature and humidity will determine how long it takes.
Another alternative is to simply grow your dahlias in big pots and leave them in the pots to dry out. Then you can store them, pot, soil, and tubers, all as one in the cellar. After 2-3 years of doing this, you should plan on dividing the dahlias the following spring, putting half back and replanting the rest in another pot or in the ground.
I am lucky enough to have an old-fashioned root cellar with a dirt floor in my 1800’s house. Not everyone does. You want to store your dahlia tubers at around 40-45 degrees. If you have a cellar with a furnace, find a spot against an outside wall as far away from the furnace as possible.
There are many ways to store them. I used shredded paper. Some folks use peat moss or, wood shavings. Others plunge them into sand. The goal is to keep them happily dormant, not to let them shrivel up and dry out completely nor to let them be too moist and start to rot. I store mine in big plastic bulb crates. Others use onion bags. Many use cardboard boxes. Before placing the tubers in storage, cut off any small roots, damaged or rotten pieces and carefully brush off or remove any loose pieces of dry dirt. I often use a pencil to gently prod the dirt off but I don't worry if some is left as long as it isn't wet.
You can divide your tubers in the fall before storing them or in the spring when you take them out of storage. I prefer to do it in the spring. Every division must have a tuber and an eye (the growing point where new shoots start).
Check your dahlia tubers at the beginning of February. Remove and discard any moldy pieces and gently mist them if they are dry. Then put them back into storage. By mid-late March, when you check on them, you will likely start to see new buds starting to swell. At this point, you can start the process all over again of starting them indoors in pots. If you are simply going to plant them directly in the ground in late May, leave them in storage, checking on them often as the spring progresses.