The easiest way to propagate Mexican Pinguicula is by leaf pullings. With about 40 species of Mexican Pinguicula, there is no best way to propagate all the species. However there are general principles that work with most species with minor modifications.
Many Mexican Pinguicula species have two leaf forms. The summer leaves are carnivorous and sticky. The winter leaves are succulent and non-carnivorous. The other species have carnivorous leaves year round but they may be smaller in the winter. Generally using the winter leaves late in the season works best for all species. However you can be successful with leaves taken at any time. It all depends on how thick or succulent the leaves are and the propagation technique you use.
Pinguicula tend to not have much in the way of roots when they are dormant. For pings like Pinguicula ehlersiae that have succulent leaves in winter, the best way to get leaf pullings is to carefully tease the leaves with your fingers and they will either fall off or come off very easily. If you are too rough the whole plant can fall apart. For dormant Pinguicula moranensis-type pings it is best to pull up the whole plant and peel off the outer leaves. You can take up to half the leaves without hurting the plant. This is also the best time of year to repot or refresh the potting medium for your plants. For Pinguicula gigantea and other species that don't go dormant, the leaves may pull of easily after wiggling back and forth any time. Again, be careful not to destroy the whole plant removing leaves unless that is what you want to do.
There are two common ways to start Mexican Pinguicula leaf pullings. The more traditional way is to put the leaf attachment part of leaf pullings with into a 1:2 mix of vermiculite and perlite. The way I prefer is to just put them in a plastic ziploc bag.
The timing in taking the pullings for the vermiculite/perlite method is very critical to success. I have had a lot of trouble with leaves rotting using this technique. The best time of year to take Mexican Pinguicula leaf pullings is in the spring just before or as the very first summer carnivorous leaves start emerge. In northern California this is late March and early April for greenhouse grown plants or from plants otherwise on a natural light cycle. If the leaves are taken too early they may act as if they think they should be dormant and not grow. If taken too late they don't do much either before they rot. The vermiculite/perlite mix is generally sterile so you don't have to worry about mold. For Pinguicula moranensis type pings, I sort of tuck the leaf into the medium. For Pinguicula ehlersiae type pings, I find it is best to just lay the leaves on the surface. I then put a piece of plastic wrap over the top of the pot with a rubber band and put the pots in a tray with a few mm of water under my fluorescent lights. If you want to keep the pots in a greenhouse, you might tuck the leaves deeper and not cover the pots since the plastic could "solarize" the leaves. The idea is to keep the leaves just moist but not wet.
To use the ziploc bag method, just put the clean leaves in a ziploc bag. You do have to watch them carefully. If the leaves are fairly dry the bag can be zipped shut. If they are wet the bag opening should be propped open with a clip. If you see a leaf start to rot, open the bag more. If the leaves shrivel, spray in some water and close the bag.
Generally with both methods, if you do nothing to the leaf pullings except keep them from drying out or rotting, they will sprout. Do keep them out of direct sunlight so they don't cook.
One year I did an experiment on P. ehlersiae leaves with rooting hormone, B1, and both. Neither alone were helpful and both was definitely not helpful. The leaves sent out roots quickly but it didn't help the new plantlets. I got more and larger plantlets from the untreated leaves. Some times it is best to give up and let nature takes its own course.
It takes 1 to 2 months to get small plants. They should be planted in your regular ping mix after the mother leaf deteriorates and you see roots. My preferred medium is equal parts perlite, sand, and peat with some calcium carbonate sand or dolomitic lime mixed into the top cm of medium in the pot. You should have mature plants in a year or two.
-- John Brittnacher
Fresh Pinguicula gigantea leaf pullings in a ziploc bag.
Pinguicula gigantea leaf pullings four months later.
Pinguicula moranensis leaf pullings two months after being taken during the summer.
Pinguicula 'Pirouette' before and after leaf pullings taken. Note the dried carnivorous leaves in the image on the left.
Leaf pullings in perlite/vermiculite mix covered with plastic under artificial lighting.
Pinguicula 'Titan' leaf pullings with sprouts. This picture was taken one month after the pullings were taken.