I read this in "Raising Beautiful Dahlias the Easy Way" edited by Richard W. Peters, M.D. He is an advanced master gardener and has grown dahlias for 20 years and holds many seats on various local and national dahlia groups. Another contributor was Kevin Larkin, co-owner of Corrolito's Gardens. I have talked with Kevin at some length about propagating dahlias as well as growing them in general. I highly recommend this little soft cover book. It has great explanations about growing, storing, propagating dahlias, plus a whole lot more.
"Plastic Wrap Method: After the tubers are cut from the clump, they can sit for a couple of hours so they are damp, but not dripping wet. They can also be allowed to dry in about 12-24 hours. Use a heavy-duty plastic bag and put in one and one half cuts of dry vermiculite. To this add 1/4 cup powdered sulfur. Take 3-7 dahlia tubers and put them in the bag, shake for a few seconds, and remove from the bag using a gloved hand. The tubers will be lightly coated with the powdered sulphur. If the tubers are too wet or if the sulfur is excessive in amount, as messy, thick coating will occur.
Cut off a piece of plastic wrap about 2' long and place it on a table. Completely wrap a tuber, then add another if the warp allows, assuring the tubers don't touch. Make sure the ends are tightly closed also. You can store them all piled in a box. No need for air circulation in this method as the packages are almost air tight."
He goes on much more so I would recommend you get this booklet. Lots of good color pictures.
Sprouting: In a nutshell, I wait until I get a sprout "about 3 inches tall with one or two pairs of good leaves". Then I use a sharp knife to cut the shoot about 1-16" above the crown. Actually I just cut it close to the tuber.You will find a pair of rubimentary leaves in this area and the cut is usually made right in the lower part of the stem from where these leaves emergy. Trim off the rudimentary leaves (not the ones at the top.) Dip in rooting hormone that includes a fungicide. Don't let it cake or use the gel type. They recommend the styrofoam cups (4 oz) size for the 'pot' and 12 ounce clear plastic drinking cups for tops. Put 4-6 holes in the sides of the 4oz cup. Fill almost to the top with sterile potting soil. I use a pencil to poke a hole in the dirt into which I put the cut plant. Pour enough water in the cup to totally wet it then push on the top of the soils so the excess water exits the holes at the bottom. Put on the clear top, and voila, a tiny greenhouse. Roots form in 16-20 days and the plant will begin to grow. I have them under shop lights about 6" away from the plant. Very important! When the plant looks viable, tilt the top slightly to allow a little of the drier air inside the cup. If you take the top off all at once the plant will wilt and die within hours. Each day tilt it a little more, then take it off to test the plant. Watch it carefully to assure it doesn't wilt. At that point you have a new dahlia. Don't forget to label the cup.
You can take 12-14 cuttings from a tuber if you can, but 6 is more the norm.
I sometimes put smaller dahlias (12-18") IN their pots directly into the ground. They will send roots out the pots to the soil. Makes it easier to get them up in the fall. These are full size pots of course, 1-3 gallon.
Guess I best quit taking up space here. I will let others kick in with soil, light, fertilizer etc. I am not that good at those things.
