You can enjoy dahlias until the first hard frost

A visit to the garden centre this time of year reveals a wide assortment of blooming dahlias. They are easy to grow and provide a show that is unrivalled in the world of annual flowering plants. 

You can plant these in the garden and enjoy their blooms until the first hard frost of fall. Dahlia tubers are not hardy to freezing winter temperatures. You can store the tubers over winter in your basement and start them indoors next year. Let this be your guide to beautiful dahlia blooms in your garden.

This native of Mexico, Central American 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 ‘single-flowered’ and for masses of round clusters of dahlias look for ‘pompom’ dahlias.

Whatever you choose, I predict you will be hooked on them after your first successful season.  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.

Growing dahlias from tubers

The aforementioned tubers are the dahlia roots that you dug up each fall and placed in an insulated craft bag and placed in the basement. Mid-March these should be removed from their hiding place (or purchased from a garden retailer) and potted up into one gallon sized containers using quality potting or container mix.  

Place your dahlia pots in a bright sunny window. 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 5 to 8 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 just to be sure that it is sterile.

Be sure to 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 started

Once the green growth has pushed through the soil a couple of centimeters it is time to fertilize them with half strength 20-20-20 every two weeks. Give the pot a ½ turn every few days to encourage even growth that does not favour the direction of the sun.

Come mid-May your dahlias will be large and strong enough to place 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 4 to 6 weeks earlier than those planted directly into the soil in the garden.

By midsummer 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!).  

Winter storage

Fall is the time of year to harvest dahlia tubers, before or after the first hard frost. If your plan is to dig the tubers early you need to cut the stalks down to 8-10” above the ground. This prompts the formation of nodes on the tubers. If you allow your 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 storage 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 10oC or 48oF.  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 is an expert gardener, author and broadcaster. Get his free monthly newsletter at markcullen.com. Look for his new best seller, The New Canadian Garden' published by Dundurn Press. Follow him on Twitter @MarkCullen4 and Facebook.