Most trees are happy growing in heavy clay soil, as long as it does not get waterlogged in Winter

Clay soil is fertile soil, the problem is that low-lying sites tend to trap water in winter, and exposed clay bakes hard in Summer.
Most trees grown in the UK will thrive in clay where:

• Drainage is decent due to slope or height
• The soil is mulched in Spring to protect it from drying sun & wind

So the real problem most people have is "which trees are suitable for clay soil with poor drainage?"

List of Trees that Thrive in Clay Soil Almost Anywhere in the UK

We’ve selected some of the best trees to grow in clay, although we still recommend improving clay soil prior by working in some organic matter. There's some beautiful flowering trees that cope admirably in clay soil, as well as trees with beautiful bark or evergreen foliage.

Many of the trees listed below have several varieties to choose from, where applicable we've also linked to the native British species.

Sun Loving Trees for Clay Soil:

• Alders: Alnus glutinosa is the native common Alder
• Blackthorn: Prunus spinosa AKA Sloe, native
• Cider Gum: Eucalyptus gunnii
• Crab Apples: Malus sylvestris is the native wild crab apple
• Dogwoods: (more large shrubs than trees) Cornus sanguinea is the native species.
• Eucalyptus: Eucalyptus gunnii
• Golden Rain Tree: Laburnum anagyroides
• Horse Chestnuts: Aesculus hippocastanum
• Poplar & Aspen: Populus species
• Rowans: Sorbus species
• Sweetgum: Liquidambar styraciflua
• Willows: Salix species

Shade Tolerant Trees for Clay Soil:

• Maples: Acer campestre is the native species.
• Hawthorn: All are native, but wild Crataegus monogyna is better for wildlife than double-flowered ornamental hawthorns.
• Holly: Ilex aquifolium
• Hornbeam: Carpinus betulus, also available in an upright / fastigiate form
• Horse Chestnuts: Aesculus hippocastanum
• Limes / Lindens: Most are native
• Oak: Quercus robur

Before I Plant Trees, Should I Try to Improve My Clay Soil's Drainage?

No, in the garden, we generally do not recommend trying to improve the clay soil itself.
It's too much work, and it's usually counterproductive to improve the soil only in one planting hole or trench. The hole itself makes a kind of bathtub, and the contents "improved" with organic matter will become a soggy sponge that most tree roots hate.
The exceptions would be:

• In major landscaping operations using machines to improve all the soil in the garden.
• In very small gardens where it's feasible to work over the whole area.
• In such cases, you can work in a mixture of organic matter, sharp sand, and perhaps things like charcoal, crushed bone, and bits of ceramic if you're feeling fancy.

However, it is often a good idea to improve your garden's overall drainage in other ways, by adding raised beds, drains, and sumps.

The key to planting in clay soil is to choose the right trees, and to mulch well every Spring.