I posted my problem with native plants, and many people offered great suggestions - which I fully appreciate - BUT - I've spent 8 years tackling this problem and want to do it absolutely right this time.
A section of my garden is framed by 2 tall arborvitae on the north side and a Black Hills spruce on the west side. For 8 years I've been unable to grow anything here successfully. The soil is sandy (I do keep amending it with compost). Do either of these species inhibit other plants somehow? Whenever I prepare the soil for something new, I dig up lots of shallow roots. Since it's at the back of the border I need something tall. Thank you!