Crown Imperials - feed now?

My Crown Imperials "William rex" are just starting to flower, but the plants are all different height/sizes this year. They have been in position for 5 years and I get the feeling they are getting smaller and smaller each year. Should I feed? When and with what please ?

 

I would give them something with extra Potash like Tomato feed to bolster it up. John, and then a weekly feed of something like Miracle Gro,

 

Thanks @ARMANDII will do that.

 

@JWK, They really give value for money don't they John? My wife and I bought this one back in '93 at a Wednesday Market in Market Drayton, and it's still going strong!!

 

That has done well for a 27 year old plant! Must be your green fingers and well draining sandy soil.

That's a lovely memory to treasure of you both shopping together too.