Overwintering Dahlias

By Marilyn Homewood, Master Gardener

A riddle, wrapped in a mystery inside an enigma.  Winston Churchill’s famous quotation is the way that I as a novice dahlia grower feel about this subject.  So many variations exist. Can they all be correct??  The answer to this mystery seems to be fine tuning a storage method to suit your own situation, which means some trial and error. So, expect some losses at first.

Pumpkin centrepiece with year end flowers

When to dig? Conventional advice says to wait for the frost but this year’s fine weather made other alternatives a consideration. Dahlias originate from the mountains of Mexico where the fall is semi-arid.  It is the lack of water that causes the plant to go dormant.  Here that happens either with a killing frost or by cutting the plant down. Both cause the onset of dormancy and once begun, the tubers underground start to set “eyes”.  Leave the tubers in ground for 1-2 weeks before digging (this also helps the thin skinned tubers to toughen up, which helps them store better).

Divide now or in the spring?  This is entirely personal preference.  Dahlia are easier to split in the fall as the stalk hardens over winter.  However, the eyes are easier to see in the spring.  If you choose to split in the fall, tubers will need washing and drying before splitting.  For plants being overwintered as a clump, knock off excess soil and let dry before storing.  Some sources conjecture that the fine covering of soil helps to protect the tubers from shriveling over the winter. 

Successful dahlia storage is a balance between the right temperature range and the relative humidity.  Ideally, dahlias should be stored around 45 to 50 F and at a RH of 75-85%.  The method you use should try to ameliorate the conditions you are storing in.  For example, the dryness of the air in winter in Ontario means that shriveling of tubers is more of a problem than rot.  Use of a packing material such as vermiculite or wood shavings can provide a more stable environment, absorbing excess moisture when necessary and giving back when needed. 

Last of dahlias on a sunny October day

Specifics of various techniques are referenced for your information.  I have decided to try 3 methods. I am going to split some this fall and store using the saran wrap method as well as in vermiculite in plastic tubs. I will also leave some in clumps with a slight covering of earth, pack in vermiculite in a large plastic tote.  I lean towards the plastic tubs as my basement in quite dry in the winter so am concerned with moisture retention.  Don’t forget to check your tubers over the winter and remove any ones with rot or spritz with water if they appear to be shriveling. 

Good luck!!

Resources

https://summerdreamsfarm.com/dahlia-care

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