|How do I propagate and/or prune the Arabian jasmine plant I've been growing.|
Jasminum sambac varieties such as Grand Duke ultimately develop into a shrub, so it can be pruned either to reduce size or to encourage lateral branching thereby increasing the number of blooms. If you need to control the size, prune it back quite hard in mid winter while it is out of bloom. You could also selectively trim it back to encourage branching at that time. It can also be tip pruned but more lightly once in midsummer to increase branching and encourage a second flush of flowers.
Propagation is a lengthy process using cuttings since they can take up to six months to root. Typical procedure includes taking a tip cutting from mature wood, using a rooting hormone, plunging it into a fast draining soil mix, and in a commercial setting, using mist to keep the humidity level up. At home, covering the pot with plastic wrap or a clear plastic dome works to maintain humidity. Place the cutting in a very bright location but keep it out of direct sun to prevent overheating and stress. Open the cover periodically to allow for some air exchange and to check the soil moisture level.
In my experience, it seems easier to propagate the plant by layering. To do this, bend a branch downward in a "U" shape. Lightly wound the bottom of the U, remove any leaves there, and cover the bottom of the U with soil, being sure to bury at least one node. Leave the growing end or branch tip, several inches long, exposed. Weight it or pin it down if needed to keep it steady and under the soil level. Keep the soil evenly moist but not sopping wet and wait for roots to form. Once rooted, the new plant can be separated from the parent.
Good luck with your jasmines!