Your mother in laws will bloom this year if they bloomed last year. Those roots just purchased will not bloom this year and possibly not next year. If your garden soil is well enough drained for gardening in general it will be OK for your peonies. I like to work in as much finished compost with your existing soil as I can, to about two feet deep. Every year I add compost to the area around the roots, but not ON the roots, as this would eventually cause them to be too deep in the soil and they will stop blooming.Al