Hi, Claubill. Here's a page that tells you how to take hardwood cuttings from your Physocarpus. I should mention that one or two sources warned against propagating patented varieties such as 'Diablo'. If you're caught and charged, please don't name me as an accomplice. :-) Good luck with your experiment.

Here is a link that might be useful: Propagating by Hardwood Cuttings

I've found that Ninebark is quite easy to propagate. I took cuttings in early spring, dipped the ends in rooting hormone and stuck them in a pot of potting soil mixed 50/50 with sand, covered. I then covered them with a dome (a large plastic bag tied up also works) to hold in humidity and then placed them in a warm spot out of direct sun. After they developed roots I potted them up and still kept them in a shadier spot, some morning sun. In the fall I sunk the pots into the ground and they survived winter.

A note about the plant protection act - you only need to worry about this if your propagating more plants to sell.