Dry, clay, ‘rooty’ border

Good morning. More advice please. A friend of mine is planning to move house during the summer so doesn’t want to spend too much money on the current garden but obviously wants it to look cared for. She wants some perennials for her border to give her a splash of colour and to see her through the coming months. However, her relatively small garden is dominated by a very large (very beautiful) silver birch which saps the ground of any moisture and the roots have spread through much of the border making it pretty impenetrable. The border gets late afternoon and evening sun and the soil, such as it is, is best described as dry clay. Last year I dug in a load of compost (and hurt my back in the process) which improved it for last year but it’s now back to as it was before. My question is, are there any perennials which will be happy in this type of border, bearing in mind it’ll be me planting them and I ain’t that strong! Those that only require shallow planting would be useful!! Apologies for long post but wanted you to get a picture of what it’s like. Thanks in advance everyone.
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