Creating a corner bed under a cedar
I am in the west Midlands area, towards Wales. I have a corner in my garden that is south east facing and has an atlas cedar in it. The ground underneath was full of weeds, ivy etc which I've nearly cleared. There's a small red brick patio which used to house a summer house on it which I can keep to put small patio furniture on. The lower canopy of the tree is really high - 3 or 4 metres at a guess so it gets a lot of sunlight underneath still. But the ground is dry and the grass doesn't grow underneath it. I've been doing some homework before I want to plant a bed in that corner this spring.
1) I think I need to improve the soil a bit first. How?
2) The plants I've selected as suitable for these conditions are (white and purple preferably):
And maybe also:
Popcorn double fire viburnum
weigela ebony and ivory
Am I right at all?!
3) How do you go about choosing which plants you have (apart from position suitability and flowering time)? I have a feeling it's better to have more of the same plants instead of lots of different plants? Obviously tallest at the back, medium in the middle and low growing plants at the bottom I suppose? And in odd numbers? How would you plan that area?
Thanks so much in advance!!