Nothing inspires awe and good cheer like a dahlia. Native to Mexico and Central America, the dahlia is a member of the Asteraceae family. Its garden relatives include the sunflower, daisy, chrysanthemum, and zinnia. They are gorgeous flowers that bloom from midsummer through autumn and come in a rainbow of colors.  Dahlias’ size can range from petite 2-inch pompoms to giant 15-inch “dinner plates”. Many varieties reach 4 to 5 feet tall. They do best in rich, well-draining soil with a pH level of 6.0 to 7.5. 

Beautiful dahlia flowers by Marie Woodward, UConn

Dahlias can bring color to any garden, and dwarf varieties can be grown successfully in containers. Of course, they simply rule supreme in a cutting garden. (Growing vegetables? Put a row of dahlias on the border, where they will not shade your edibles.)

Dahlias are hardy to zone 8 and will survive in warmer climates if cut back and mulched heavily.  In colder zones, like Connecticut, dahlias can either be treated as annuals or dug up after the first frost and stored indoors for winter.  Dahlia tubers don’t like cold soil. They prefer a soil temperature of 60 degrees. A good rule of thumb many gardeners use is to plant dahlia tubers a few days after tomatoes are planted in the ground.  Some gardeners start tubers indoors in containers a month ahead to get a jump on the season.

Tuberous root of dahlia before planting. Photo by dmp2009.

To get the most blooms from dahlias and for a plant with a more bushy and rounded shape, it’s recommended that dahlias get “topped” when the plant gets to about 18 to 20 inches tall.  Topping means pruning the main stalk or trunk of the plant back to the third or fourth set of leaves. Even if there is a bud present at the top of the plant, it still needs to be removed, something some gardeners find hard to do especially after going through the effort of planting the tubers and waiting with anticipation for blooms to appear. But, doing this step will stop the main trunk of the plant from growing upward, encouraging the plant to develop lateral stems, making it a fuller, rounder shaped plant that is more stable and aesthetically more pleasing to the eye. And, of course, it will encourage more blooms.

When topping dahlias, one can use fingers and pinch the stem, use a knife, or a pair of small secateurs.  Location of the cut is important. Dahlias have sets of leaves that create a layer around the plant. The cut should be made above the third or fourth set of leaves from the bottom.

Use pruners or a sharp knife to make your cut. Photo by Marie Woodward.

It’s recommended that all dahlias that will grow to maturity above three feet get topped at the beginning of the growing season. Dahlias that don’t reach that mature height don’t need topping.  These dwarf varieties of dahlias will naturally produce lateral stems on their own. 

Some dahlias develop more than one stalk. This means the tuber has more than one eye on the tuber. Some dahlia growers believe that it is best not to have more than two stalks per plant. In any plant with more than two stalks, the extra stalks should be pruned down to the soil leaving only one or two main stalks. 

The most important thing to remember when topping dahlias is to watch the weather. Topping should not be done if there is a heavy rain in the forecast. Avoid watering dahlias heavily after topping, too. Why? because the main stalk on dahlias is hollow. If water gets into the stalk, this can cause the plant to rot. If water is present in the stalk, the dahlia will start to show signs of wilting. To fix this, take a clean sewing needle or pin and prick a hole on the side of the stalk to see if water runs out of the stalk. Then allow it to drain.

Stake dahlias or grow through wire mesh to keep top heaving plants from toppling over. Photo by Marie Woodward.

Topping dahlias, though counterintuitive to some, should be viewed as a “one step back, two steps forward” approach. It will benefit your dahlias, rewarding you with fuller, healthier plants with an abundance of blooms to enjoy from mid-summer to the first hard frost. And who wouldn’t want that?

Marie Woodward