and then how to best root them? Mine is not the invasive type.
I found several sources, one of which is linked below. It appears that the common sorts of Ruellia can be propagated by stem cuttings which root easily, or by root cuttings, as well as by seed. Mexican petunias are viewed as somewhat invasive in warm climates.
Here is a link that might be useful: Ruellia information
Cut off a piece of stem, remove the lower leaves, & stick it in the ground. You can also root them in water. If it is warm, they can wilt, so water rooting may be better. You can stick the entire cutting in water to refresh it, & when temperatures cool off, stick the roots only in water.
Bush & dwarf varieties root this way. Dwarf varieties also propagate easily by rhizome division.
You didn't mention what kind of Mexican Petunias you have- I have the tall purple mexican petunias. Basically, as long as your plant has flowered once, you can cut off about 6 inches from the top, cut the leaves all in half to reduce stress, then slice the cut end up about 1/2" to 1 " to allow more water to be absorbed. Stick the cutting in dirt in sun and just water a little daily for 1-2 weeks. By then you will have roots and the plant is on it's own. You can also easily propogate by dividing roots. I found that when rooting the Mexican Petunias I have in water- they drop leaves and sag.
I forgot to add that I add an airstone to my water, which may counteract wilting. You should wait until after they flower, but when mine break off they root this way. My variety is the tall purple Â the dwarf I divide the rhizome.