Sounds like you have got a fab garden, post some pics. and we may be able to give you some help.
Ah. you are back Verdun.
I thought you had gone off to be president of the US.
Oh. well, I usually plant perennials in 3's and it works for me, perhaps I am just a terrible gardener.
Verdun, dot planting is fine if you have large specimens, however many people plant very small plants, which does tend to look very dotty, until they have filled out.
This is why planting in 3s and 5s etc. is recommended.
Sausage sandwich, that is true love.
Possibly the greatest meal on the planet, brown sauce I trust.
Happy Valentines day.
Hope you all got something to express undying love.
I have foxes so I use Growmore in Spring and again in early Summer.
Dahlias, Cannas and other heavy feeders , get regular Tomato feed during the Summer
I think reliable moisture might be the key.
It is not the shade that is the problem, it is the space. An Acer would outgrow that space, within a few years.
I don't think you can make beds without any drainage. Essentially what you will end up with is like a pot without drainage holes.
You will need to find some way of breaking up the concrete base.