Using P. ‘Purple Haze’ above as an example, a number of lengths have been cut from stems, trying to avoid those that have too long a gap between nodes. This is because if the plant is growing too quickly the stems are softer with more chances of rot. That said, as the plants focus on flowering and vegetative growth reduces, with shorter internodal lengths, the cuttings may take a lot longer to root. This is why I think the best time for cutting is spring rather than when the plants are in full flower. I know autumn is also favoured by some but under natural light conditions it does not work well for me. I have had some taken then take over 6 months to root. Some people however repeatedly take cuttings of hybrids only when they are in flower over the generations as they think that repeating this makes the plants more free flowering.
You will note the cuttings are also quite short & that the leaves are clipped back. I now tear the leaves by hand rather than clip them as this will reduce intercellular damage. The growing tips, tendrils & flower buds are also usually removed. This is because as the cutting has no roots to start with it cannot support much transpiration (pulling up of water through the leaves). This will lead to leaves wilting if the leaf area area is too large. Note that is very easy to put cuttings in upside down so start at the bottom end of any stem that you take for cuttings and then the growing tip at the end will help you keep your bearings.
It is very important to make the bottom cut at a right angle very tight to, but just below, the bottom node, which is the visible joint between sections of stem. I tend to strip a bit of the surface of the stem up to the bottom node and dip it generously in rooting powder. At present I use Strike 2 containing 1-naphthylacetic acid. After some experimentation I recommend using rooting powder rather than gel. The top cut should be half inch above the top node at a 45° angle. Charlie Pridham has advised that he has considerable success with single node cuttings with the bottom cut being mid node. As an alternative to the one cutting per peat pot method above, you can instead use a plastic pot & put a number of cuttings around the edge of the pot hard against the plastic sides.
Use vermiculite or perlite, with a little soil or peat added if you wish. Cutting are usually given bottom heat in a propagator (often not neccessary in the height of summer or they will be cooked) but a Mist Propagator will give even better results for difficult subjects. The lid should be left on for the first week or two until signs of leaf or root growth appear. Expose over a period of time to the ordinary atmosphere or they will wilt. Pot up when roots are growing out of pot or through peat pot.
If I have no room in propagators or it is hot I use plastic bags, 2 pots & saucers per bag, well watered, & tie them at the top. They are then put in good light for a few days followed by full sun, depending on species, in a south facing window. The most important tip for cuttings is…be patient, even if some leaves drop off or look mouldy (remove them) there is hope. Some cutting take in 2 weeks, others will take up to 6 months. Some will do better kept cool & shaded. The classic signs of success are when new top growth appears and/or the roots appear through the foot of the pot.
For many species it is worth simply putting short lengths of them into water. Transplant before the roots get too long as they are very fragile & they will find it hard to adapt to soil.