We find that is easy enough to water the cuttings with a one third concentration every time you water. The plants love the continuous availability of nutrients and they grow very evenly. After two weeks or so, replant to four inch pots using a good soil mix . This step is necessary to keep the plants from getting too root bound. Also it is difficult to keep a large plant watered in small pot.
Plants grown in gallon pots can get very large in the green house.
I have had some of them bloom there. Dahlias are like tomatoes in that they will produce roots from any stem that is buried. You may get more tubers if they form on the buried stem. Also the plant will be less likely to fall over in the wind. Be sure to water in the plants very well.