Some people say plant dahlia tubers when you plant your tomatoes, but I have already planted my tomatoes and the soil is still only 52-53 degrees. Maybe another week will do it!
Pick a sunny spot as dahlias love sun. If you have some shade, your dahlias won’t flower as much and the plants not as full and bushy.
Frost will hit your plants sometime in October or November and they will go from glorious specimen plants to blackened wilted skeletons overnight. Check your weather report and before a hard frost is forecast, cut off every flower and bring it in to enjoy for another week.
Once the plants are frost killed, you can start digging around the root ball carefully to remove the shrunken starfish-like tuber that is nestled a few inches under the soil. Wash off any soil with a hard stream from your hose and dry in the sun. If you leave you tubers in the ground, I have found that some even come back if the winter hasn’t been too cold. Some people don’t save them, preferring to buy new ones every year.
Cut the stems a few inches above the tubers and store them in a container full of peat moss and perlite. I only place two layers of the tubers in a container, as I find that the bottom layers tend to rot more often than the top. If the tubers are too wet, they might rot anyway, so I check them after a couple of weeks of storage to see how they are doing. If they are moldy, I scrape off the mold and add some dry peat moss. You are going to lose some of the tubers, but I have a success rate of about 75% saved tubers.
Alternative Method of Planting/Saving
Another method is to plant your tubers in 1 gallon plastic pots early in the spring. When the weather warms up, plant the whole pot in the garden and cover with soil. Leave the tuber in the pot and roots will come out the bottom drainage holes. When frost hits, dig up the entire pot, cutting off roots that are outside of the pot and bring the pot inside and place in a cool dark place for the winter. When shoots come up in the spring, top dress with compost and plant outside for another season of bloom. I read about this method on Old House Gardens and want to try it next season.
Another method which a friend swears by is to dig up the tubers and shake the loose soil off and place in a large trash bag, leaving all the clinging soil attached to the tubers. Store the trash bag in an unheated garage that won’t go below freezing. Easy and effective!