"umvoti" opined:

Does anyone know of a growth retardant for hanging baskets,
particularly Petunias? Needed to stop the plant bolting
but to flower profusely...



There are several chemicals that are used to control growth: e.g.
Florel, Bonzi, B-Nine and Sumagic (I believe these are all trade names).
Florel is one of the oldest ones and it works by maintaining the plant
in a vegetative state (non-flowering) while the plant is being treated.
This encourages branching and helps to keep the internodes short for a
bushier plant. I haven't used the others, but there are different modes
of operation, particularly in the newer regulators.

However, Florel won't really control petunias once you stop treating
them so they can flower. Your best bet is to change the variety of
petunia you are growing to one with a more compact habit.