DEAR RICKI: For hard, rejuvenative pruning, your Euonymus bushes should be cut back in late winter/early spring, before new growth begins. But if you just want to clean up their shape or reduce their size, you should do that in late spring or early summer.
How much can you cut back euonymus?
Euonymus – reducing size is an inside job
If your plants are overgrown by a lot, this will probably need to be done over two seasons but will then be needed much less frequently than with shearing. In her experience, a euonymus pruned correctly needs pruning no more than twice a year, and sometimes just once a year.
How do you prune a euonymus plant?
Six months or so after planting, normally April time, prune the top third of the shrub away. No other complicated methods, just take a pair of shears / secateurs and cut away. This will encourage new growth from the base of the plant which will make it bush out. After that your Euonymus will look after itself.