Need some more info first please - what part of the country are you in? Is the area very exposed, and which way does it face? Sun or shade? How big are the planters, height and depth?

Hi, we're in Kent, the area is very exposed, SW facing and gets the sun most of the day. The planters are 20 inches deep, with a 7 inch square opening at the top. Thanks.

Hmm, Kent can be pretty cold in the winter. I'm thinking that if we have winters like the last one every year, anything permanent in those pots is susceptible to being frozen solid and being killed off. The other problem is water - out in the open in full sun, they'll need frequent watering, so hopefully you'll be able to do that.
Your options are to fill with bedding for the summer, then replace with winter bedding. Or for permanent planting, if you like the tropical look, Phormiums would do well in that size pot. If you prefer formal, standard Bay trees or ordinary Box (Buxus) trimmed into balls or squares or whatever shape you fancy. You could also use Senecio (now known as Brachyglottis) instead of Box - being grey leaved, it tolerates dry conditions much better and full sun is no problem. Smaller varieties of Holly would work too, but you'd have to choose one male and one female to get berries on of them - both male and female plants flower, but only females produce berries. Otherwise, Viburnum davidii has large, interesting leaves. There are flowering shrubs which will go in the pots too, but these are largely deciduous, and the vast majority of flowering shrubs are only in flower for 2/4 weeks at most, which means you're just looking at what might be boring leaves the rest of the year, or a skeleton of branches in the winter.

Thank you so much I really appreciate the time you've taken to answer my question. I've only this year started taking an interest in our very large garden, and I'm finding it quite daunting although very enjoyable, I will probably be using this site quite a lot in the future. Thank you again. Claire

You're welcome, Claire.

If you did decide on box, i noticed today there are some deals on the crocus website...(not sure if i can mention websites, sorry if i shouldn't)

No, that's fine Bill, specially Crocus - there's a link from here to there anyway, lol!

Thank you too Bill. Claire

Your welcome Claire. Oh phew Bamboo i've only been a member for a week so not sure on the rules yet

Welcome to you too then Bill - its okay to share with other members if you've seen a good deal at Lidl's or somewhere - what's not okay is if you offer to sell stuff to people yourself, cheap or otherwise...