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Annual Showstoppers – Jun/Jul2015

Annual Showstoppers - Jun/Jul2015

This seasonal guide to dahlias will ensure a gorgeous repeat performance year after year.

Easy to grow, providing a show that is unrivalled in the world of annual flowering plants, you can plant dahlias in the garden now and enjoy their blooms until the first hard frost of fall. Dahlia tubers are not hardy to freezing winter temperatures, so overwinter the tubers in your basement and start them indoors next year.

This native of Mexico, Central America and Columbia was first exported to Europe in the 1500s. Since that time it has experienced extensive hybridization and improvement. For daisy-shaped dahlias, look for singleflowered varieties, and for masses of round clusters of dahlias, look for pompom dahlias. Whatever you choose, you will be hooked on them after your first successful season. In fact, you will be giving them away to friends and family come fall and taking pictures of them. Go ahead, give it a try and prove me wrong.


If you’re a seasoned dahlia gardener, you know the tubers are the dahlia roots that you dig up each fall and place in an insulated craft bag to winter in the basement. In mid-March, remove them from their hiding place [or purchase new from a garden retailer] and pot them up into onegallon sized containers using quality potting or container mix.

Place your dahlia pots in a bright sunny window. The floor in front of a sliding door out to the deck works too, since you’re not likely to use it much until the heat of mid-spring hits home. The sun will intensify through the glass door or window, warming the pots of dormant tubers, encouraging them to put down roots before they push new green growth through the surface of the soil.

Before planting your dahlia tubers divide the hefty ones that you stored. Look for finger-like tubers about five to eight centimetres long, with an eye at the stem end. Cut each tuber using a clean, sharp knife. Pass the blade over an open flame before using it just to be sure that it is sterile.

Plant the tuber with the stem end up and the tapered end down. If you get them sideways or upside down, it is not the end of the world as they are smart enough to find their own way, generally speaking. They have been programmed genetically to push roots down and green growth up without any help from us.


Once the green growth has pushed through the soil a couple of centimetres, it is time to fertilize them with half strength 20-20-20 every two weeks. Give the pot a half-turn every few days to encourage even growth for the side that does not favour the direction of the sun.

Come mid-May your dahlias will be large and strong enough to be placed outdoors in the garden. Choose a sunny, sheltered place where they will not blow over in the wind as they mature. Dahlias started indoors in this way will bloom four to six weeks earlier than those planted directly into the soil in the garden.

By mid-summer you will likely have to stake your dahlias with a sturdy 2 x 2-inch wooden stake or using one of the new Mark’s Choice link stakes that are much easier to work with and to look at (as you do not see them).


Harvest dahlia tubers in the fall, before or after the first hard frost. If you dig the tubers early, you need to cut the stalks down to 8-10 inches above the ground. This prompts the formation of nodes on the tubers. If you allow the dahlias to experience a hard frost the stalks will dieback naturally.

After several hard frosts, use a garden fork to loosen the soil around the tuber. Carefully lift the tuber out of the soil and wash gently to remove remaining dirt. Allow the tuber to dry for 24 hours in a cool, dry location. This is a good time to divide large dahlia tubers into smaller sections. Each new division must have an eye (bud) to produce a new plant.

Place tubers in a cardboard box with sawdust, dry peat moss or vermiculite. Label the container to help you identify the tubers next year. Be sure to dust the tubers with Green Earth garden sulphur powder to prevent rot and disease while in storage.

Choose a storage location in a dry area where the temperature will remain near 10C (48F). Check on the tubers periodically during the winter. Look for signs of shriveling. If the tubers are beginning to shrivel, I recommend that you moisten the storage medium to beef them up again. But be sure to check weekly for mildew or rot.

Mark Cullen appears on Canada AM every Wednesday morning at 8:40. He is spokesperson for Home Hardware Lawn and Garden. Sign up for his free monthly newsletter at