It looks as if that one in the photo is in sunlight, but it is critical to know whether they're all in full sun, half sun, or shade. I note also you live somewhere considered cold, but if you're not far from a body of water or the sea, that's another important factor - means it'll be warmer than inland, and that's a consideration regarding the compost in the pots possibly freezing during winter.

On the subject of long flowering, the only plants that fit the bill are summer bedding plants, which need replacing every year. They do, though, keep on flowering all summer. Otherwise, if you have permanent planting, even if whatever it is flowers, they'll only be in flower for 2-6 weeks at most, depending on the plant, which means they'd be chosen for their general appearance and foliage year round rather than what the flowers are like, so not sure which of these options is one you like most!

Permanent Bamboo or a mixture of the two! main plant being a permanent fixture, they are quite heavy containers so once in situ they are staying put, all three are in full sun, we get lots of rain here I mean lots! I don't live near the sea but it can get quite cold here but I've never had compost that has frozen, as for 2/6 weeks that's ok obvious the longer the better, two of the planters are in opposite border so symmetry is important to these two hope that helps more.

Another question or two - how deep are the containers on the inside from top to bottom, and is there a drainage hole in the bottom? And I'm assuming your 16 inch measurement is from outer edge to outer edge at the top...

13 inch deep pre drilled hole about the size of pound coin the hole itself is an inch deep

Unfortunately, at the size your pots are, if you want permanent plants, each will need a pot to itself - you could probably get in some trailing lobelia or something similar for summer though, specially in the first year or two.

Evergreen shrubs:

Pieris varieties (P. Katsura and P. Little Heath are good ones) Needs ericaceous/acid compost.
Camellia varieties - smaller ones, check height and spread before buying, this can vary by 4 or 5 feet between varieties. Needs ericaceous/acid compost.
Yucca flaccida 'Golden Sword' - the pot may need lagging in a hard winter to protect the roots a bit, but this plant is very hardy otherwise.
Osmanthus heterophyllus 'Goshiki'
Viburnum davidii

I'd like to suggest Phormium - these come in a range of leaf colours, green, yellow and green, striped red and pink, black, and everything in between, but they can be vulnerable in a hard winter.

Deciduous shrubs:

Buddleia Blue Chip
Weigela follis purpureus or Weigela florida variegata - there are newer varieties available now with darker leaves or multi coloured leaves too, so worth having a look first, but don't choose one that gets 9 feet in the ground.
Prunus cistena
Syringa meyeri 'Palibin' (small lilac). Really needs a pot that's 16 inches deep, but you could try it.

I'm expecting you to google all these! And if you do use shrubs, use John Innes No 2 or 3 compost for those that don't need ericaceous, and a word about watering - no matter how much rain you get in summer, most of it will not get in the pots because the foliage of the plants blocks it, so you will need to keep checking and watering.

Thankyou for your help Bamboo I'll be checking out all the above plants, I very much appreciate you taking the time and effort to help me out.

Not a very exciting selection, to be honest, but you're limited by size of pot and local climate.... fuchsia would have been good, they flower from about June through to first frosts, but they're sensitive to cold through their roots... and they do not like to dry out completely, so need regular watering.

And I forgot to say, when looking at Pieris varieties, don't choose the one called 'Flaming Silver' - its only just frost hardy.

ok thanks will keep that in mind, climate here is a nightmare everything is trial and error

Just thought of another one that's evergreen - Euonymus pierrolino. Doesn't really flower, but the new leaf growth is creamy white, turns green later, so it almost looks like its flowering, good in a pot. You don't need to respond, I'll add things in here if any more come to mind.

Personally, I would look to see what my local nursery is growing - depending on where you are in the county you could try somewhere like McLaren's at Barrhead. I'd avoid the chains of garden centres as they will probably just have the plants that are provided up and down the country (ie what sells to soft southerners)

I stay 7 miles from McLarens Urbanite went last year for the first time what a brilliant place! although I'll be leaving it until June before going or I'll spend to much to early lol, the quality of there plants is brilliant and such a good price for big healthy specimens much better than any chains or garden centres near me