Hello, plantlover. I suggest you just mulch them: put 3-4 inches (7.5-10 cm) of any type of acidic mulch past the drip line. H. petiolaris may take years to bloom or it may surprise you when you least expect it.

It appears that, once it reaches the top of whatever it is attached to, it then blooms. But there have been exceptions reported by some people so... your best bet may be to ignore it (but of course always maintain the soil moist and feed it with cottonseed meal or manure in June).