I read this on another forum today.
http://urbangardenmagazine.com/2010/11/qa-water-culture/ Really good read, Here's something pertaining to recent questions:
"Q: Can you run through your recommendations for propagating plants destined for DWC?
A: Establishing cuttings with an aeroponic cloner using 1/8th strength nutrients is ideal, preferably under mixed spectrum T-5 lighting. Propagating bare root plants suits DWC best. This offers an easier transition to water culture given there is no wicking substrate (rock wool, Sure To Grow) to hold excess moisture too close to the root crown.
Q: Is there an ideal water level to be maintained in the reservoir?
A: Start with the bare root submersed to the base of the rooted stalk, being sure to not submerse the stem or stalk tissue so as to avoid water logging. If using a wicking substrate, ensure the cube is approx. 1“ above water line; this may necessitate hand watering for a few days before the roots hit the water.
Q: What are the potentialities for plant steering using water level / amount of root zone exposed to the air?
A: With water as their growing media, growers can tailor nutrient solution parameters more specifically. Provoking plant responses such as essential oil production, fruiting and flowering are better manipulated when the substrate can be dialed in. For example, higher exposure of the root zone to atmospheric oxygen can help trigger a plant to increase oil production as a means to conserve water, and can also apply mild root stresses that are often interpreted by the plant as reproductive cues. While higher water levels can cause plants to focus more energy on vegetative production, particularly fan leaves, which in turn speeds transpiration and photosynthetic potential."