any of the phlox sublata would look well as would the dwarf campanula, dianthus, aubretia, arabis, iris reticulata. then lots of spring bulbs etc especially small species like narcisus tete et tete.

what is behind the bed? can you get all round it? does it get full sun etc? if shade then small ferns/hostas etc.
there are lots of suitable plants. many online gc display plants according to their height.

I would only put in drainage if the border has puddles of water after it rains. Digging all over the border normally helps with that as it is normally soil compaction that causes puddles. Aspect as SBG has said is important. Perennials that I like are Delphiniums, Hollyhock, Red Hot Pokers/Kniphofia, Lupins, Erygium (pair with Campion Rose as greys and pink/blue look great) at back of border, smaller perennials like Echinacea and Campanula towards the front. It depends on what you like. I am planing a border with Crocosmia all along the back (250 of them) and spring bulbs along the bottom, to be supplemented with annuals.

I like the sound of the crocosmia, Kildermorie. Your question covers a lot of the same information that I need, too, Bainliam, so I hope you don't mind if I join in? How do you think crocosmia would do in a slightly raised bed in full, hot sunshine, K.?

Moist, loose soil and they are anyone's - so your border sounds perfect. The need the loose soil for their corms to grow. They also self seed well where they are happy. Crocosmia prefer areas without strong midday sun, but they come from the Cape in South Africa so Italian sun is like home.

Another one for the shopping list then. Thank you, K.

i have a block raised planter about 30" tall and i put some crocks in and grow acers and a climber in it . montanna i think and there all fine . obviously if you say had bog plants you wouldnt or if you had cactie youd have maybe more and just gravel .

Basically what you have is a very long pot. Drainage is always a good thing and/but the depth of your plot soil will determine what you wish to grow. I believe caithness is somewhere in scotland, so the weather will be particularly involved?! Where exactly are you?

i agree x .