Planting shrubs in clay soil can be difficult, but on the plus side, it can be very fertile. If you choose the right shrubs and remove the weeds, then all you have to do is prepare the soil and decide on the right planting time.
It is really important to dig as large of a planting hole as you can and to mix plenty of compost with the garden soil before you replace it around the plant, to help the roots to acclimatize to their new home.
Clay soils can dry out and bake hard in the summer, so early fall is the best time to plant in order to give the shrub longer to establish. Planting in spring may be difficult if the ground is wet and heavy.
Once established, shrubs grown on the clay have great stability: the weight of the soil keeps the roots firm and the plants rarely suffer from wind damage. Provided planting is well established and good ground cover is achieved, little further maintenance or cultivation is needed.
Adapted from the Horticulture Gardener's Guide to Shrubs by Andrew McIndoe. Learn more about this book.