The best way to do so is to imitate the normal horizontal setting conditions of plants in your green wall.
This includes, once using peat grown plants keep cultivating them in peat. It is hard to find hydro grown plants anyway. Plants adapted to soil are forced to built new roots under hydro conditions.
Hydroroots which will be able to pick up minerals from the water. These roots are less in number and longer than normal roots and have less filaments. This process takes a considerably amount of energy and gives lots of moisture stress to all terrestrial plants. Very often the wished result is not achieved.
Perennials change into annuals.
Besides, in 1 cm3 of soil out of the rhizosphere ( that is the narrow region around plant roots) there are living over 30.000 bacteria , protecting the plant roots against fungi and infections and who also facilitate the absorption of hard to extricate elements.
In the northern regions specially take care of good protection of the roots in winter time. Be aware that water, frost and roots are difficult to combine. Once temperatures reach below zero, plant roots are dealing with low air humidity, so the substrate should neutralize this.
The answer to all of this is to take a biobased substrate which hold the water as well as distribute it regularly and which is sterile and shows also a more than normal cohesion. Besides the medium should be biodegradable for 100% and have no toxidity.
All photographs of vertical gardens on my website are showing plants grown in such a substrate.
