It’s one thing to want to plant a few plants here and there in a tree’s root zone, but quite another to want to install a mass planting, like a ground cover or a flower bed. Digging 250 holes to receive 250 plants in a mass of roots that won’t give way easily is a monster task that goes far beyond the limits of the laidback gardener.
Instead, use the “planting with newspaper” tip to make things easy.
Cover the entire sector with 7 to 10 sheets of newspaper (or cardboard, if you have no newspaper available) and cover that with 8 inches (20 cm) of good topsoil. Now do your planting. It will be easy enough in such light soil and the newspaper or cardboard will prevent tree roots from growing back too quickly, giving your plants the time it takes to settle in. In addition, 8 inches (20 cm) of soil poured over a tree’s root system is not enough to damage it. So everyone will be happy!