I have experimented with aerial root production on Ficus for several years. In Colorado they are difficult to start and grow, although not impossible. This is a note on what I am trying this year.
I built a mini 'greenhouse' in my basement. See the first picture. It is one of the seedling structures you can buy for hardening off seedlings outside in the spring. The bottom shelf holds a piece of plywood with a heat mat on top. And on top of that is a boot tray that holds the pots. Inside the boot tray is a lattice of plastic (used for florescent light fixtures) cut to fit to hold the pots out of any water that drains through. You can see the thermostat to the right to control the temperature. The prob is set in one of the pots and set to keep the substrate at a maximum of 75 degrees. At the top is an LED light fixture that is on 12 hours a day.
I set this up in early November.
The basement is heated but stays cool. The concrete floor stays at a pretty constant 55 degrees. The basement air temperature mostly stays between 60 and 65 degrees. Inside the cover the air temperature stays between 65 (night) and 75 (day) degrees. The substrate temperature stays between 70 and 75. The humidity stays pretty close to 100%.
The Ficus are all Macrocarpa. The picture and descriptions that follow are for cuttings originally taken in November 2016. They spent a year just growing. Then I transplanted them into different substrates and enclosed them in a large plastic bag for a few months. They did throw aerial roots but once I removed them from the bag almost all of the roots withered and died. I took them out in mid 2018 and since then they have just been growing indoors under T5s.
The first picture below is representative of the results after 2 months. There are aerial roots on the trunk and branches. More on the trunk. White where they are new growth and brown where they have lignified or died.
The picture below is one where there were aerial roots hanging form the top prior to November. What I am attempting here is to grow them down into the peat moss in the square pot. Prior to November they just hung in the air. The new extensions are white.
From what I have observed one of four things happen.
The aerial root starts and then just dies and withers.
The aerial root starts and at some point the end gets a cap of mold and the root dies.
The aerial root starts and then seems to halt growing while it lignifies, then it extends again with white growth.
The aerial root starts and at some point comes in contact with the trunk and attaches. From that point on it stays attached and seems to extend much faster than those in the air. The latter is based on only a couple of occurrences.
I treat these like my other ficus with the exception of watering. So far only minor problems with mold. I did treat today for gnats. I have had to prune twice so to keep the tops open and away from the light. These are not even pre pre bonsai. At some point I will have to decide if is worth putting one of my more developed Ficus into this environment. From previous attempts when I enclosed a Ficus I do know that the leaves produced while enclosed will wither and die once moved back into our arid Colorado environment. Based on a suggestion from Darell Havener I am going to defoliate these when I take them out and see how that works.