Planter advice please

Hi all, I’m new and also new to gardening. I’m just wondering if anyone could give me advice.

I planted from seed Some stock, sweet William and 1 other flower in my planter and all is going swimmingly, however the stock (planted at the back) seems to be taking over the other 2 rows and this is without it flowering yet, would I be best to replant in another planter to give all of the plants more space when they flower?

many thanks In advance