When the frost comes it's important to have a game plan for harvest and storage
Learning the right time to harvest your plants can help you successfully shepherd your tubers through winter. Typically, you'll wait for a hard frost before you start digging. However, this isn't a requirement- it's just a good rule of thumb. According to experts at the American Dahlia Society, the most important factor in getting quality tubers that will store well for winter is make sure the tubers have at least 120 days in the ground (it counts if you start them in pots as well). There are some locations that rarely get a hard frost, or years when you may have a very mild fall and won't get a hard frost until the ground is covered in snow. Just be sure to leave yourself plenty of time to dig all your plants and safely get them into storage before the weather makes it too difficult.
Remember that the most important thing is to dig your dahlias out before the ground freezes hard. The air temperature may be freezing over night- and that's fine, but once the cold comes and daytime temps aren't getting much above freezing, you need to get those babies up and someplace safe.
Before your plants die back, be sure you have everything labeled! A good method is to write the name with permanent marker on flagging tape and tie it to the bottom of the stem just above the ground. Clean the lower leaves off the stem and tie the flagging tape just above the soil so it will still be attached after you cut the plant back.
Whether it's frost or the calendar guiding you, you'll want to cut back your plants almost all the way to the ground. Use good loppers and cut the whole plant down, leaving just a few inches of thick stalk above the ground. Many people recommend letting your tubers "cure" in the ground for a week after cutting them back and before digging them. I am going to make a bold statement and tell you that from our experience this isn't necessary. If you want to do this, that's just fine! But from our experience I would agree with the folks at the Dahlia Society- making sure the plants have had the longest time possible growing in the ground is what seems to "cure" the tubers best. One reason to wait after cutting back the plant is so that the eyes will begin to show. If you're planning to divide in the fall, then this is very helpful! It's much easier to divide a tuber correctly when you can see the eyes.
Use a garden fork to loosen the soil all around the plant- about 12 inches away from the stem so you don't puncture the tubers. Then carefully lift the tubers from the ground. Go slowly and use caution. Then, gently shake or comb away the loose dirt from the clump, being careful not to break the tubers from the neck.
Then you should have a nice clump of tubers attached to the stem!
Summing it up:
- Cut back the whole plant with good loppers, leaving a few inches of thick stem for a handle, after the first hard frost- or in the late fall.
- Carefully loosen the soil around your plant and lift them with a garden fork. Don't just grab them by the "handle" and pull- you can break a lot of tubers that way
- Gently shake or brush (a paint brush works great) the dirt off from around the plan
Now you're ready for the next step!
Now you have some decisions to make. To wash, or not to wash? Leave them whole or divide them? Should you divide them now or in the spring? Gah! So many decisions!
If you have enough space, you can simply store your dahlias in whole clumps. If you don't have heavy soil you could simply brush off the loose dirt, cut the stem back, allow them to dry and then store them away in you favorite medium just like that. Easy. You just need to make sure you have the proper storage conditions- but more on that later.
Many of us don't have sufficient storage space to keep all our dahlias whole and need to break them down into smaller chunks for winter. In this case, you may want to wash them first. If you’re not blessed with light soil that easily brushes off, washing them will make it easier to see what's going on and saves your clippers from getting too dull from trying to cut through dirt. Washing is simple- just use a gentle direct spray from your hose. If you're too aggressive you can damage the skin and even break tubers from the neck of the stem (which you'll learn later isn't a good thing). Wash them as best as you can, especially in the middle of the clump where you'll want to be able to see where they all attach to the stem.