Potting up dahlia tubers in mid-March for earlier blooms

Bob Dluzen
The Detroit News

Dahlias are generally thought of as fall flowering plants in our area. When planted directly into the ground, they generally don’t hit their blooming stride until late summer.

Dalia gardeners who want to extend their flowering time usually pot up their tubers sometime around mid-March.

By potting dahlia tubers now, you can gain a month of flowering time before the usual dahlia blooming period. You get to enjoy more flowers until the first frost finally freezes them back.

The mid-March start time holds true for both tubers saved from last year or for those purchased from dahlia growers.

Mail order dahlias are shipped as single individual tubers. Home-grown, overwintered tubers are sometimes left connected together in a clump and then separated when it comes time to plant. 

A dahlia tuber separated from the crown and ready for potting up now or planting in the garden later on.

Because of the size of whole dahlia tubers, pots that are at least 6 to 8 inches in diameter are needed. Larger-size tubers need even bigger pots, such as 1 gallon nursery containers.

I always save my containers from year to year. I can get several seasons out of them before they finally deteriorate. Extra pots always come in handy for projects such as early starting of dahlias, so don’t throw them away.

In order to grow, each tuber needs an eye. Like potato tubers, the eye is the growing point from which the plant develops. Unlike potatoes that have several eyes scattered all around the tuber, dahlia eyes are located very near or on the crown of the plant where the stem meets the tuber.

The crown of a dahlia plant

Some varieties have dormant eyes so small that it’s nearly impossible to see them. If that’s the case, place your clump in a bucket or box of warm, moist potting mix and set it in a warm spot for a few days. That will cause the eyes to come out of dormancy and begin to swell up. At that point it’s much easier to see the eyes.

If you have saved your own tubers from last year, take extra care separating them. 

When a tuber is growing away from others, it's very easy to cut the tuber from the clump. But more often than not, the roots are bunched up together and are more difficult to separate.

With tightly tangled tubers, you may have to split the old stalk in order to get a good separation. That's where a sharp knife comes in handy. Line up your cut so that a tuber is on either side of the cut. That usually takes care of the tangle. 

Splitting the old dahlia stem with a knife will make separating tubers easier

It is good practice to dust the newly cut areas with powdered sulfur. Sulfur is a mild fungicide and anti-bacterial and will prevent microorganisms from infecting your tubers through the fresh cut. That  being said, I have cut and planted many tubers without dusting and most of the time have had no problems.

Match the pot to the size of the tuber.

It’s simple to pot your dahlia tuber. Pick an appropriate size container and partially fill it with a potting mix. Any kind of mix will do. Place the tuber eye up at an angle in the potting mix and backfill until the eye is an inch or so under the surface. Water it well and allow any extra water to drain from the pot. 

Cover the tuber with potting soil

Place the pot in a room-temperature area indoors. In a couple of weeks or so, you should see new growth peeking out of the soil. Once that happens, the shoots grow fairly fast and that’s when they will require direct sunlight. 

After the last frost in spring, about the same time you would plant your tomatoes or peppers, carefully unpot your plant and place it in a sunny spot in your garden.

Sometimes the roots will have developed enough to hold onto the potting soil. Other times the potting soil falls off. But that’s ok, just plant it.

Dahlias are hungry feeders and like fertile soil so be sure the spot you chose is fertile enough to grow good vegetables. It should also be well drained so the plants are not sitting in damp soil.

Many dahlia varieties need to be staked in order to help hold up their tall foliage and large flowers. It’s a good idea to drive the stake into the ground first and then dig your planting hole next to it. If you put the stake in later, the stake might hit a tuber and damage it.

You don’t have to pot up all of your tubers. Pot up a few and see how much difference there is in blooming time.