Been bitten by the dahlia bug? We’re there, too. These beauties are hard to beat for bold garden color, cutting flowers galore, diversity of bloom form and delicious color blends. With lots of gardeners growing dahlias for the first time, we’re often asked “are dahlias perennials or annuals?” Which is usually then followed by “can you leave dahlias in the ground over the winter?”
The answers depend on where you live.
Are Dahlias Perennials? Sort of . . .
Dahlias are considered “tender perennials”, plants that return year after year when the growing conditions are favorable.
Tender perennials re-sprout in the spring and deliver repeat performances in regions where winters aren’t too cold for their survival. So, what’s too cold for dahlias? If the soil in your area freezes, dahlia tubers left in the ground will freeze, too. And that’ll be the end of their story.
However, dahlias can overwinter outdoors where temperatures never get cold enough for the ground to freeze. It’s usually safe to leave dahlia clumps in garden beds in hardiness zones 8-10. Dahlias may also survive mild winters in zone 7b, sited in a sheltered location and/or with the help of protective mulch.
If your winters are mild and you want to leave your dahlias in the ground, here are a several tips.
• Cut the stems to about 4″ long. This helps clean up the bed.
• Because the stems are hollow, they can fill with water. This encourages rot at the stem junction. Avoid by covering the top of stems with little squares of plastic wrap, secured with rubber bands.
• If your region gets lots of winter rain, months of wet soil can rot tubers. Either dig the tubers or cover the bed with a plastic tarp to keep much of the water out. Remove any plastic coverings when sunny days arrive in spring to avoid heat build up underneath.
Do Pot-Grown Dahlias Come Back Every Year ?
Dahlia can overwinter in containers if you keep in mind that pots typically freeze before the ground does. Cold air circulates on all sides of containers. So the soil, and tubers in the center of the pot, lose their retained heat and freeze more quickly. Soil temperature in the garden is buffered by, well, more soil. And because of this, garden beds take longer to heat up and cool down than does the soil in pots.
Typically, your container plants have hardiness ratings 1-2 zones colder than the rest of your landscape. If you garden in zone 7, your container residents experience winters more like plants in zone 6 or even zone 5.
(This same logic applies to early spring planting, too. Refrain from planting too early.)
Want to leave your dahlias in their containers? Where night temperatures dip to 35 degrees and colder, move those pots into an unheated garage or breezeway. Temperatures of 40 to 50°F are ideal for holding your dahlias over the winter. Keep the pots sheltered until spring’s warmth has arrived, then move them back outside.
What About Perennial Longevity?
The other thing gardeners are thinking when they ask “are dahlias perennials?” relates to longevity, i.e. how many seasons will the plants grow when left in the same spot. Perennial plants can be counted on to sprout each spring and add beauty to the landscape for years.
Dahlias lose vigor and flower less robustly when left in the same site year after year. Even in warm regions, it’s advisable to dig the clumps every two to three years.
Dig To Revitalize
Many dahlias develop a sizable tuber clump each season. Some cultivars develop huge, soccer ball size clumps. After a few years, the clumps become overgrown and congested. They can be reinvigorated by removing dead tubers and broken pieces.
Dig your dahlia clumps at the end of the growing season. Snip off any tubers that aren’t firm and solidly attached. Then cut the clumps into two to four pieces, making sure each has a piece of the collar and a growing eye. Allow the cut surfaces to dry, dust the cuts with fungicide and store for the winter.
This video shows you how to dig and store dahlias.
Whether dahlias are treated as annuals or perennials in your part of the country, if you’re keeping them for several seasons, they’ll benefit from being dug.
So, Dahlias Are Annuals in Cold Regions?
Technically, no. Annual plants complete a full life cycle in a single season. They sprout, grow, flower, set seed and die in the same year. Dahlias don’t fit that definition.
While dahlia top growth may die back each fall, the plant is still alive. Stored energy in the tubers allow dahlias to regrow the following spring.
However, many home gardeners treat dahlias as annuals, leaving the clumps in the ground in fall or adding them to the compost pile. This works well if you have no place to overwinter tubers, or no time (or desire) to dig them. Or if you simply love to experiment with all the options dahlias offer.
Starting fresh every spring means you can change up your dahlia varieties. You can add some of the newest cultivars and grow colors, forms and sizes that you love. And let’s face it, sometimes it’s just fun to grow something new!
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