Hard to Root

We're seeing so many different ways to root brugs. Some are harder to root than others. Maybe it'd be a good idea to list the difficult ones. I've had problems with variegata - for every one I root, I lose one. Anyone else know which are the difficult ones that need special treatment?

"Ecador Pink" is a hard-to-root species. I don`t know, what it takes to make it root faster or better ... maybe the way, that Monika showed us for "RothKirch"?

HG is a hard to root one.

I used the first experiment on P&C and though I only had two tiny cuttings less than an inch long, they are growing and have 4 or 5 leaves now!!(they are rooted) HG, I had great success with the burried stem method from Eric. EP I've never had a problem getting it to root, but I do use the tea tree oil.
The one I had trouble with(I killed it) was Rosabelle.