I'd like to share a brief summary and a few pictures of a demonstration I gave recently at a propagation workshop at Tondoon Botanic Gardens in Gladstone, Central Queensland. Ficus was selected as the demonstration plant for the Bonsai Group.
Soak the sphagnum in water and then thoroughly squeeze out excess water.
1. Select a branch on an established tree and decide where you want the base of your new plant to be.
2. Make a horizontal cut where the roots are going to form.
The cut should be through the bark, onto the wood and go all the way around the trunk. The intention is to sever the cambium and also the phloem which takes photosynthate from leaves to lower parts, but not the more centrally-placed xylem which transports sap upwards from the roots. Then make a second circular cut 24-30mm below the first one, followed by a vertical cut connecting the two horizontal cuts. Keep the blade of the knife in the vertical cut and start lifting the bark. Slowly continue around the trunk and remove the slice of bark. This will expose the soft shiny cambium layer.
3. With the knife, scrape off the cambium layer until there is no more gloss on the trunk. Clean the top cut to remove loose material to prevent a foothold for pathogens.
4. Use the Cling Wrap to form a container for the moist sphagnum. The roots are going to develop above and around the top cut, so ensure that there is enough room for the roots to grow down into the container.
5. Fill the container with moist sphagnum. The sphagnum should be firm to encourage roots to develop.
6. Secure the top and bottom of the container firmly with twine or some tape, sufficient to minimise moisture loss and any movement which could break tender roots.
7. Cover the container with aluminium foil.
a) Help keep the air layer cool by reflecting sunlight;
b) Exclude light and assist root formation;
c) Allow the container to be easily inspected.
8. After about a month, carefully open the foil, the Cling Wrap and the tape, and inspect the container. If it is dry, use a syringe to add some water. Inspect the container every week to check on root development.
9. When this is good, it's time to detach the top from the plant. Cut the branch under the root ball, then carefully remove the foil, the cling wrap and the tape.
Cut off the excess part of the branch below the roots. Cut off the leaves to minimise transpiration. Pot the plant up, being very careful with roots as they are so delicate at this stage. Stake the plant, water it and keep it protected from the afternoon sun until well-established. Wait for sufficient growth before fertilising your new plant.
I have found the highest rate of success when I did the air layer in the period when the sap flows, which is normally in spring or early summer when new leaves start to appear.
Choose branches from the periphery of the tree. The best branches to use vary in size according to the genus. Pomegranate shoots of pencil thickness are ideal. For plants with thick branches 15 to 20mm diameter ones will root satisfactorily. The branches should be healthy and free from any parasites or pathogens.
We did not use rooting hormones because Ficus spp. root so easily. For other species, rooting hormone is recommended.