Updated: Sep 25, 2019
If you grow flowers for cutting, you simply MUST grow dahlias. Dahlias are some of the easiest and most rewarding flowers to grow in your cut flower garden. With blooms in every color and ranging in size from huge dinner-plate flowers the size of your head to perfectly round pompom dahlias, there is a dahlia for every taste.
Dahlias take anywhere from 90-120 days to produce flowers. With a frost free growing season of just 140 days this does NOT leave much time for my dahlias to pump out blooms. Since I lift my own dahlias and store them over winter, however, I can pot my dahlias up in late February and start the growing season 10 weeks early. This insures in an blooms up to 2 months earlier and longer than if I waited on dahlias from an online provider.
Late Fall: Prepping Your Dahlias to be Over-Wintered:
If you garden in areas where the ground freezes in the winter, like we do in Boulder, you'll have to pull your dahlias out of the ground. The benefit of this extra step is you can split your dahlias and quickly multiply your stock. On average, I triple my dahlias with every splitting but some plants can offer up to five new plants at splitting! Given how expensive these tubers are initially, even doubling your stock makes the effort of splitting them well worth while!
Every fall, after a few frosts have blackened the foliage but the ground has yet to freeze solid, use a pitchfork to lift your dahlias from the ground.
I group the dahlias by type and wrap them in burlap (you can use brown paper as well or place them in sand in a crate). Store your wrapped dahlias in an area that stays between 32 and 45 degrees. For me, this means storing them in my garage along an interior wall. If you live in an excessively dry climate (like Colorado) it can be helpful to sprits the wrapped dahlias once a month or so so that they do not fully dry out and shrivel up.
Early Spring: Splitting Your Dahlias
In early spring or late winter bring your dahlias out of storage. For me this is usually done in late February. Ideally you will have a warm day on which you can accomplish this outside. This year, unfortunately, we've been hit with a two week long cold snap so I was forced to do my splitting inside. I simply put a large plastic bucket in the kitchen sink and resigned myself to some extra dirt clean up.
Unwrap your bundles and shake any excess dirt from the tubers. Your tubers may have shriveled slightly or they may appear totally plump. If they are significantly shriveled you can try soaking the tubers in luke warm water for up to 24 hours before plating them. Discard any that are mushy, they have rotted and will not grow.
Rinse your dahlias under warm water to remove additional dirt. In this clump of tubers you can clearly see the remnants of at least two two stems, these are a good indication of where to start splitting your dahlias.
Your dahlias may split apart easily with just a little force, and if they do, this is a good indicator that you have found two (or more) separately viable plants.
You may need to use a sharp garden knife to split your tubers. Where you cut will depend on locating developing eyes (see photo below). All viable tubers must have both an eye and a tuber attached.
In this photo you may notice the stolen spot to the side of the word "Eye." I know, it looks like a random bump. Eyes on dahlia tubers are like potato eyes or the very early buds that form on rose stalks and are where new growth develops from. For a tuber to be viable it must contain both an eye and a tuber together. When separating a clump, therefore, search out firm tubers that have not been broken, cut, or sliced, and that have at least one eye attached.
Next, simply pot your tuber in a container of sufficient size and depth with drainage holes and label the variety (if you know it). I re-use the plastic pots from larger annuals I bought from garden centers the year before. Use standard potting soil and filled to the top edge with the eye of the tuber no more than a half inch below the surface.
Store your potted dahlias in a warm, sunny location that stays above 60 degrees. We use our arboretum that just happens to have floor heating, which simulates the earth warming in the spring, but any room above 60 will work just fine. Do NOT water your dahlias.
The tubers were wetted or soaked during the splitting process and should be fine for at least a week. You may lightly water or sprits the soil once a week or so but avoid fully watering the dahlias until green growth appears above the surface of the dirt as dahlia tubers are super prone to rotting.
Late Spring/Early Summer:
When your dahlias reach about 8 inches tall you will want to pinch them. Pinching, like seedling thinning, is one of the most painful things for a gardener to do but pays off in dividend. I will devote a separate article to the process but in a nutshell it simply means pruning about half the plant's initial growth, so in the case of your dahlias, about 4 inches. Cut just above a set of leaves. These leaves will then branch out into two new stalks and double your flower production. It will also result in a bushier plant that is less susceptible to wind damage.
Once the outdoor temperatures are staying above 60 degrees you can begin to harden your dahlias off, eventually moving them outdoors full time. I highly recommend staking your dahlias or planting them inside a tomato cage for support when those big buds start to appear. With any luck you will be swimming in dahlias before long!