Leafcutter bee Colony?

For the third year running I have colony? of leafcutter bees visiting my patio. They have taken up residence in an interesting old pottery "Patented seltzer" which has a weakly fushia in it, struggling to survive in very shallow soil as the tall container is mostly full of filter and leaves! The first year it was just one bee, taking circles of leaf down a hole at the edge of the pot, excavating earth out the very round hole. We put the pot in the garage to overwinter and was fascinated when 5 holes appeared with busy bees. This year we have 9 holes andt hough not all are full,I love to watch several return in the sunshine, so industrious. My question is where do I go from here as at this rate we will need an extention! and how best can I care for my friends in ther best interests? Also how rare are these as I can find little at my library about them XX