Not necessarily. You did not identify what "not growing well" means, how far apart they all are, how old and what type/variety of hydrangea and rhododendron you have so, there could be many other reasons. Just an example... consider sunlight: Rose of Sharon (RoS) may be in full sun; some hydrangeas/rhododendrons may not like the full sun growing conditions.

But, I do hear you. I would not try to grow plants too close to RoS because the RoS may eventually encroach on them: RoS reproduces with new shoots coming up from the ground right next to itself so, if left unchecked, the plant can quickly get out of control in your garden. RoS can also reseed abundantly so, be ready to remove additional plants appearing in unwanted areas near or far away. A metal barrier in between them might minimize the underground roots problem. Pruning of the new shoots and deadheading of spent flowers can also help. There are seedless varieties of RoS out there like Orchid Satin.

luis,

Thank you for the detail explanation. The hydrangea and the rhododendron are about 12 to 16 inches from the Shallot, I have a very small space. Actually both plants are in shade because the shallot block a lot of direct sun. I think I will get a metal sheet to block the root and see what happen.

thanks
elsa

Oh, that is close. All three plants produce shallow roots that may compete for resources/nutrients/water although the RoS develops a tap root that the others do not have.