Category: Plants, Borders & Planting Ideas Read time: 6 min read

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6 min read Easy Read Best read in Spring
Clay soilChalk soilSandy soilBuckinghamshireOxfordshireBerkshire

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Published

25 April 2026 · 6 min read

Last reviewed

19 July 2026

Reviewed by a senior CGM team member.

The short answer

Quick Answer

For clay and chalk mixed gardens in the Chilterns: choose plants that tolerate both alkaline and heavier soils, plant in raised beds on the chalk areas, and add organic matter throughout. Good plants for mixed soils include roses, viburnum, philadelphus, hardy geraniums, and hollies. Test the soil in different parts of the garden before planting.

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The Mixed Soil Challenge

Many gardens in the Chilterns have mixed soils: chalk in one part, clay in another. This is particularly common at the boundary between the chalk hills and the clay vale, around Princes Risborough, Wendover, and the edges of Aylesbury. The chalk is alkaline and free-draining; the clay is also alkaline but heavy and wet. Plants that suit one may not suit the other.

The good news is that both soils are alkaline, so the plant choice is wider than for acid soils. The challenge is the drainage: chalk drains fast, clay drains slowly. This guide explains how to garden on mixed clay and chalk soils in the Chilterns.

Step 1: Test the Soil

Test the soil in several places in the garden. Dig small holes 30cm deep and check:

  • Colour: pale soil is chalky, dark soil is clay
  • Texture: crumbly soil is chalky, sticky soil is clay
  • Drainage: fill the hole with water and see how long it takes to drain
  • pH: both chalk and clay in the Chilterns are alkaline (pH 7.5 to 8.5)

Map the garden: which parts are chalky, which are clay, which are mixed.

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Plants That Tolerate Both Chalk and Clay

These plants grow well on both chalk and clay, making them ideal for mixed gardens:

Trees

Shrubs

Perennials

Bulbs

Plants for Chalk Areas

On the chalky parts of the garden, plant:

See our chalk soil guide.

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Plants for Clay Areas

On the clay parts of the garden, plant:

How to Manage Mixed Soils

  1. Test the soil in several places and map the chalk and clay areas.
  2. Choose versatile plants that tolerate both soils.
  3. Add organic matter to both soils. This improves drainage in clay and water retention in chalk.
  4. Use raised beds on the chalk areas for plants that need deeper soil.
  5. Improve drainage on the clay areas with grit and land drains.
  6. Plant in the right place: chalk-tolerant plants on chalk, clay-tolerant plants on clay.

Cost of Planting a Mixed Soil Garden

ItemCost
Soil testing kit10-20 pounds
Compost (1 cubic metre)80-120 pounds
Grit (for clay areas)50-100 pounds
Raised bed materials200-500 pounds
Plants (20 versatile plants)300-600 pounds
Total640-1,340 pounds

Common Mistakes

  • Not testing the soil. Mixed soils need different management in different parts of the garden. Test first.
  • Planting the wrong plant in the wrong place. Match plants to the soil in each part of the garden.
  • Not adding organic matter. Both chalk and clay benefit from added organic matter.
  • Not improving drainage on clay. Clay needs grit and possibly land drains to prevent waterlogging.
  • Not using raised beds on chalk. Shallow chalk needs raised beds for deeper-rooted plants.

Frequently Asked Questions

How do I know if my soil is chalk or clay?

Dig a small hole 30cm deep. Pale, crumbly soil is chalk. Dark, sticky soil is clay. Both are alkaline in the Chilterns (pH 7.5 to 8.5).

Can I grow the same plants on chalk and clay?

Some plants tolerate both (roses, viburnum, geraniums). Others are more specific (lavender on chalk, hydrangea on clay). Test the soil and match plants accordingly.

Should I improve the soil or use raised beds?

Both. Add organic matter to improve the existing soil, and use raised beds on shallow chalk or poorly-draining clay.

Get Help with Your Mixed Soil Garden

If you have a mixed soil garden in Oxfordshire, Buckinghamshire, Berkshire or Bedfordshire and want help with planting, send us a WhatsApp with photos and we will aim to respond within 12 to 36 hours, Monday to Sunday.

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Further Reading

Estate Note

In mature and established gardens, always assess the existing ecosystem before making changes. Small adjustments to timing and technique can preserve decades of growth and wildlife value.

Relevant local areas

This advice is particularly relevant to gardens in these areas.

Useful next steps

Put this advice into practice

Use the relevant CGM tool or follow a related guide, then send us a few photos if you would like a tailored plan.

Open the plant planning toolOpen guide Low-maintenance plants for Chiltern gardensOpen guide Best plants for chalk soil in the ChilternsOpen guide
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Sources and citations

This article draws on the following sources. Specific claims are cited inline where relevant.

  1. CGM field records (2024-10. Site observations from gardens across Oxfordshire, Buckinghamshire, Berkshire and Bedfordshire.
  2. Royal Horticultural Society (2024). RHS Gardening Advice and Plant Finder. Available at rhs.org.uk.
  3. Met Office (2024). Regional climate averages, South East England. Available at metoffice.gov.uk.
  4. British Geological Survey (2024). Soil parent material data for the Chilterns and Thames Valley. Available at bgs.ac.uk.