Answer: Air layering is a specialty form of propagation usually used on tropical plants in the greenhouse or sometimes for certain houseplants like rubber trees (Ficus elastica). It is a good method for these because it does not harm the parent the plant. The spagnum is used because it is relatively sterile and is fibrous rather than granular like potting mix -- so it is easier to keep securely in place. It is important to follow the directions.
Simple layering should work fine with a forsythia. Wound the bottom side of the branch where it will be under the soil, weight it down using a brick or something so it stays steady, and keep the soil evenly moist. Layering can take a few months so you do have to be patient, sometimes you get a better result by layering in the spring and separating it off a year later.
Forsythia is probably most often started with tip cuttings -- but at home where you have easy access to the plant, layering is a great choice.
Rooting hormone can make a big difference with certain hard to root plants but in other cases it is somewhat optional. It depends which plant you are trying to propagate.
I hope this answers your question.
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