Only some perennials can be propagated by cuttings. Annuals and biennials cannot be propagated from cuttings, they have to be started by seed. Only perennials in certain plant families can successfully be propagated by cuttings, an example would be the Mint family which includes thyme, bergamot, lavender, hyssop, and rosemary. The best time to take cuttings is in the spring with fresh sturdy new growth.
Remove the cuttings from the mother plant when the shoots are fully turgid, that is, in the morning, before the temperature warms up. Take 2-4in-long, non-flowering shoots, from healthy plants. Never take cuttings from a diseased or distressed plant.
Place them in a plastic bag to prevent evaporation and keep them cool or in the shade until you are able to pot them up, which should be done as soon as possible.
Cuttings are very vulnerable, since, if they lose moisture, they will shrivel before they can form roots. This means that they should be kept in an atmosphere of high humidity, so that they do not transpire much. Once they have made roots they can draw moisture from the potting soil. On the other hand, the stem ends should not be waterlogged, as they may rot. I usually add a bit of sand or perlite, to increase drainage and air circulation, to a sterile potting mix as a media to root my cuttings.
Using a sharp knife remove the bottom leaves close to the stem and cut each cutting just below a leaf node. The cutting should be 2 inches long, roughly. If the remaining leaves are big, remove them as well, as this will prevent wilting.
Dip the very ends of each cutting in hormone rooting powder, making sure that any excess is tapped off. If you don't have any hormone rooting powder, don't worry, since softwood cuttings are the easiest cuttings to root. You can also use willow or poplar water to root your cuttings. To make ‘willow water’ soak fresh twigs in water overnight and then dip your cuttings in this. You can also use the willow water to water in your transplants and encourage rooting.
Then take a dibber or pencil and make a hole in the soil deep enough to take the cutting up to the bottom leaves but no deeper. Insert the cutting, pressing the soil against the shoot so that it is secure. There should be no air hole; if there is, the cutting won't "strike".
Make sure to label your cuttings with the name of your plant and the date. Water them in gently so that the cuttings are moist, and then place trays in a warm spot with ample light (you don’t want your cuttings to go leggy). The use of bottom heat will greatly speed up the rooting process, and increase your success rate.
Check the pot of cuttings weekly, by giving the leaves a very gentle tug; if the shoot remains firm, rooting has begun. The process can take as little as a two weeks. Once the cuttings start visibly to grow, or when they have formed a plug of roots, you can pot them up into pots.