Forsythia are fun to propagate, and you can easily grow several more plants from an existing mature hedge.
The best time to propagate forsythia is June and July, when the plant produces vigorous new shoots. Forsythia can be propagate from soft wood cuttings. Look for a new flush of growth among the branches, and choose your cuttings carefully, seeking healthy, vigorous branches.
Clean your pruning shears with alcohol and take several cuttings from among this new growth. Each branch should be at least four to six inches long. Strip off the leaves or pick off the leaves on the lower half of the cutting. Dip the end into rooting hormone, then place the end with the rooting hormone into a pot of rooting medium. You can also use perlite. Several cuttings can be rooted at once in a flat or tray, or use small pots for your propagation experiment.
After inserting the cut end into the pot, firm the soil with your fingers and water it well. Use a clear plastic bag, such as a bag from the produce department of the supermarket, to cover the top of the tray or pot like a dome. This creates a sort of miniature greenhouse to keep the cuttings moist and the humidity high.
Place the covered forsythia cuttings in a bright, sunny spot, but not in direct sunlight. Within six to eight weeks, the cuttings should root, and you’ll see new leaves begin to emerge. At this stage, transfer them to larger containers with sterile potting soil, and continue to nurture them along. Plant them in the fall so that they get a head start on root development for the following spring.
Propagating via Layering
Another popular method of propagating forsythia is through a process called layering. Many plants such as lavender and forsythia develop new roots along the woody stems when the stems touch the ground. You can help this natural habit along and grow a forsythia hedge by using simple layering techniques.
To grow a new forsythia plant by layering, follow these steps:
Use a mature, healthy forsythia plant as the parent plant. It should have at least one long cane on the side where you wish to grow the new shrub. The cane should naturally reach the ground.
Layering can be done in the early spring while the plant is dormant or during the late fall.
Bend the brand to the ground. A sharp bend tends to help the plant root more easily.
Use a U-shaped landscape staple, the kind use to affix landscape fabric to the ground, or make a U-shaped bracket by cutting a piece of wire from a coat hanger and bending it into shape. Use the bracket to affix the middle of the flexible branch to the ground.
Cover the portion of the branch touching the ground with soil. You may also wish to use a clean, sterilize knife to make a little wound or two under the side of the branch touching the ground.
That’s it. Just water it well, and look for new growth near the bend. New growth usually indicates a separate shrub has grown at the bend.
You can leave the new plant where it is, or transplant it once it has grown several sets of leaves.
Above info gathered from this link.
https://davesgarden.com/guides/articles/pruning-and-propagating-forsythia