It's not something that I had ever tried with any plant, but a friend in Orange County recently had great success with his P. parritae. I won't tell the details--it's his story--but after a period of 4 months this late Winter/Spring he was able to get tremendous rooting and isolate I believe about a dozen plants.

So I started trying this on my small P. parritae, which is overpotted in a 5 gallon pot. I took a vine and removed three non-adjacent leaves and buried those nodes, leaving the nodes (and leaves) in between above the soil line. I managed to do this in a one gallon pot containing soil, which I kept moist. I didn't damage the stem or in any other way prepare it--I just removed the leaves on the three nodes. I checked a few days ago, after 3 weeks, and there were roots already on one of the below ground segments. The vine had grown, so I buried yet another node. I'll check on everything at the two month point.

I also want to try this with my P. membranacea, which sometimes roots for me, sometimes not.

I'm guessing that layering would be most useful for those plants that are difficult to propagate from cuttings.

That's a good idea trying the same method with membranacea, which can sometimes be slow and difficult with cuttings. I've got a monster membranacea in my garden, so there's plenty of material to work with.

I started with a whip about 7 feet long. The upper part of the whip had a number of side shoots going but there was no branching on the base of the whip. I serpentine layered the branchless bottom part and put the branching upper part in a trench and pulled the side shoots up above ground.

The serpentine layered lower part which had no side shoots did root, but I got very little bud break or growth on the above ground nodes. Rooting was all along the buried parts of the whip, not just at the buried nodes. A couple of the above ground "shootless" nodes from the serpentine layered part of the whip did break into growth, but weakly. No shoots formed nor came up from buried nodes.

The side shoots that were attached to the buried main whip were the ones that did really well. The ones that got the most light were the ones that did the best. Not every node that was buried had a branch on it, and no buried node generated a new shoot. Only existing shoots that were pulled above ground ended up being new plants. Rooting was from all along the buried whip, not from the bases of the side shoots.

After I lifted, potted, and waited a few weeks for them to reestablish in their pots, I ended up with 9 good layers from the side shoots off the whip, and two weak layers from the serpentine layered part of the whip. I've set up two more layers to see if injuring the buried whip and using a rooting hormone will help the rooting process. Of particular note was that no buried node produced a shoot.