Discussion on rooting hormone strength because there is no information online about this.
So, RH ( rooting hormone) usually works with IBA / indole 3 butyric acid.

The most popular is the powder. It usually comes with the strength of 0.1% IBA. Now, the company called Hormex has different strength IBA homrones. They have a handful of strenghts.
#1 is 0.1 and #16 contains 1.60% that is a lot when you compare it to just 0.1%. Now, they advertise the 1.60 as " for difficult to root plants" Obviously, more iba means better and more chance of rooting. They also have a chart up stating which strength they recommend for different species ( weaker hormone for easy to root plants, stronger for harder).

But the question is, regarding difficulty, would the stronger 1.60# iba hormone produce roots faster? Its the highest strength so, you can probably root anything with it? Even hardwood cuttings?

Anyone has REAL experience with DIFFERENT STRENGTHS, or a chemist here?

thanks.

No real experience and not a chemist. But I am quite sure that rooting hormone dont have any effect on seadlings whatsoever. Rooting hormone is a signal substance that tells the wood to start producing roots. Seadlings allready have roots growing and not much area awailable to produce new roots from.

I do have experience from IBA on cuttings and airlayering. I know for sure that the hormone increase the success rate. Dont think it has anything to do with growth speed, for that you need sun, water and later some fertilizer.

It is NOT a matter of increasing the strength of the rooting hormone that will increase the changes of success or the speed of rooting. It has to be right for the species. If you have too high a concentration and in several species rooting may even be hampered.

Some academic work on it:

showing no effect of concentration:
www.sciencedirect.com/science/article/abs/pii/S0378112796039606

showing increasing rooting with increasing concentration:
www.sciencedirect.com/science/article/abs/pii/S0378112796039606

showing an optimal concentration:
link.springer.com/article/10.1023/A:1006752717350
link.springer.com/article/10.1007/s11676-012-0298-5

Substrate type and the state of the cutting material plays an equally important role in propagation by cuttings. And again, this varies from species to species.