Overwintering Dahlias
Dahlias have captured the hearts of gardeners across the world, and though this plant may look right at home in cut flower gardens, its natural range is in the frost-free mountain ranges and uplands of Mexico and Central America. What was once a plant farmed as a food crop by the Aztecs, is now an ornamental garden plant with over 57,000 varieties and 14 different flower types.
How Hardy Are Dahlias?
In USDA hardiness zones eight or higher, dahlias can overwinter in the ground. Areas of Cowichan Valley, with warm microclimates, can grow dahlias akin to a perennial, but if a cold, wet winter freezes the tubers in the ground, the plants will die. It is advised that dahlias be lifted, so their blossoms can be enjoyed for years to come.
Digging Dahlias:
After first killing frost the leaves of dahlias plants will turn blackish-green. Now is the time to cut back the stems to just above ground level. Ideally leave dahlias tubers in the ground for a week or two before digging them up to allow them to cure.
There is no rush to dig tubers up unless temperatures are forecasted that will freeze the soil. Remember, a frost will only damage the top of the plant, not what is underground.
When digging up the tubers start by digging 12 inches away from the plant to prevent injury to the tubers. Using a digging fork is ideal.
Be careful not to damage the neck of the tuber. Don’t worry if you accidentally cut a piece of the tuber.
Avoid washing tubers.
Storage Methods
The most simple method of storing tubers is to simply dig them up, brush some of the dirt off, and pile the tubers in bulb crates, large plastic nursery pots, or milk crates lined with newspaper. Air flow keeps the tubers from rotting so if you choose to use a sealed plastic container poke holes in the sides and lid to allow for good air flow.
Another method is to line a crate or plastic pot with plastic a plastic bag. Fill the plastic bag third full with vermiculite, peat moss, or wood shavings. Put dug dahlia tubers in the bag after brushing off some of the dirt. Add another third of the media you’ve chosen, put more dahlias in the bag, then add the final third of chosen media.
Store dahlias similar to potatoes. These starchy tubers do best when stored with the dirt on them, in a cool, dark and dry place over the winter.
A note on splitting tubers: Tubers can be split in the fall right after digging them, in mid-winter when the gardening season has slowed down, or in spring just before planting.
To split dahlia tubers, start by cutting the dahlia clump in two. Split the clump in two again, and continue to do this until you have individual tubers. It is important to leave at least 1 cm on the crown attached to the dahlia neck. Here is a great guide for splitting dahlia tubers.
Storage Temperature and Humidity
Most importantly dahlia tubers cannot freeze when in storage. Ideal storage temperatures are 4-10 degrees C and ideal humidity is 75-85%.
If storage temperatures are too low, you’ll risk freezing your tubers. If temperatures are too high, plants may begin to sprout prematurely.
If storage humidity is too low, tubers may begin to dry out and shrivel up. If humidity is too high, tubers may develop mold or rot.
The best place to overwinter tubers are in unheated basements, garages or root cellars.
