Brian Whyer brian.whyer@btinternet.com
Fri, 11 Mar 2011 06:08:36 PST
It is incorrect to say the cuttings have to form tubers first. Dahlias are
flowering without tubers from cutings. They only make tubers around
september when days getting shorter.
You are correct of course, I over simplyfied to keep it short, but given a limited amount of nutrients a tuber that has formed roots and shoots may be several weeks ahead of a newly rooted cutting taken from it, that has to form the top growth and a new root and tuber system. It is just the limited nutrients and too many shoots that gives smaller flowers.

I also suggest to take basal cuttings
from the dahlias as clos as you can cut to the roots. This is necessary
because stems are hollow, but at the base not.
This also applies to many other herbaceous plants such as named forms of Lupins and Delphiniums, where you need to avoid the hollow stems. It amazed me last summer how quickly a Lupin formed an initial root, in just a few days, although they were just as brittle and difficult to handle as seed grown Lupin roots.
Brian Whyer, Buckinghamshire, England, zone ~8

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