There are a number of factors to consider dealing with compact soil (medium that’s difficult to penetrate and work over). It can often be waterlogged. The plant might find sourcing food difficult, the roots to develop too. The surface “crust” may make it difficult to allow air in.
These problems can be alleviated by working over repeatedly, improving the movement of air; encouraging worms to condition the quality.
The soil particles ideally need to be finer, sand can be added to make it more free draining (since not many plants cope in excessive moisture). However, under this surface “crust” it can also be thin and lacking lustre, nutrients and bulk.
In this instance, the soil can be nourished with manure, organic matter, any rotted compost that might be ready. There will be worms in this certainly, these will populate the designated area and treat. It will still be on the heavy side but much better. It may need adding to periodically with humus rich matter, since all soil becomes tired.
There are plants, without these steps, that will tolerate clay conditions despite no action being taken. This is not a comprehensive list, there are many others.
Fatsia japonica is a resilient evergreen with very attractive foliage. It prefers a fertile soil and perhaps some shade if anything. As long as it is relatively sheltered it will thrive. This shrub will bear black berries aswell as flowers but its really recognised for its leaves. It has an anti-pollutant quality too.
Garrya elliptica (Silk tassel) prefers a sheltered spot but generally is forgiving of most conditions. It doesn’t require any maintenance – to do would cause more harm. It flowers very early in the year, provides a food source for birds and bears attractive catkins in spring.
Sarcococca confusa (Christmas box) is an evergreen shrub which like a few flower in wintertime. It will provide delicate scented flowers from winter to spring. It keeps it shape well by habit. It is very resilient to drought, needs little support. A shrub good in a shaded, neglected spot. After it flowers, it will bear black fruit.
Lavatera (Mallow) an annual, a perennial, and warrants the term “shrub” aswell. A large specimen that flowers mid summer, it’s very resilient and can cope with an exposed site with salt laden winds. Keep spent growth on until spring and then remove. It protects it for the future year.
Magnolia grandiflora is an evergreen with glossy leaves. It prefers a well drained soil with humus matter. It bears fragrant flowers throughout summer. Its roots system is relatively shallow so can cope with partially compact soil. Having said this, be mindful that any roots protruding from the surface are coveted to avoid unecessary drying out.
Magnolia stellata, a very different variety (starry flowers) but equally impressive. It offers spring flowers that are scented but not heavily. It does prefer a more sheltered spot although roots are very so good close to the surface. It does not need planting deeply albeit protected though. It prefers a heavy humus rich medium. If it’s in a sheltered position, it is protected from both the wind and sun.
Weigelia is decidious, so over the dormant is non descrpt. However, W. florida “variegata” does provide you with the most wonderful cream edged leaves and fragrant flowers from late spring to summer. A resilient shrub, it can cope in most aspects, exposed or sheltered. It also attracts beneficial insects too which always a bonus.
Rhododendrons/Azaleas are together since they are so closely related. Both flower from spring to summer. A variety of colours, some scented and others not. R. occidentale offers orange/yellowish flowers which are fragrant in early summer. This is decidious so will lose its leaves. R kaempferi (which is an Azaelea really) flowers late spring to summer but is evergreen, so will provide you with foliage all year round. What is certain, however, of this family is the soil must be acidic soil to perform well. This is something entirely for a different post.
It can also be said that an Azalea is a Rhododendron but a Rhodendron is not an Azalea. This is something else entirely too.