How do I garden on chalky, alkaline soil?

Chalk gardens are often described as difficult, which is misleading. They are difficult for a specific and fairly short list of plants — rhododendron, camellia, pieris, most heathers, blueberries — and excellent for a great many others.

Step by step

1. Confirm you are on chalk.Dig a hole. White flecks and lumps, thin topsoil over solid pale rock, and a soil that fizzes when a little vinegar is dropped on it all indicate chalk or limestone.
2. Measure the topsoil depth.This matters more than the pH. 300mm of decent topsoil over chalk grows almost anything; 100mm over solid rock limits you to shallow-rooted and drought-tolerant species.
3. Add organic matter annually.It increases water holding, buffers the pH slightly at the surface, and deepens the effective root zone. On chalk this is the single most valuable job in the gardening year.
4. Mulch thickly.Chalk soils dry out fast in summer despite feeling cool. A 50 to 75mm mulch after rain in spring makes a large difference.
5. Expect iron and manganese deficiency in the wrong plants.Yellow leaves with green veins on lime-haters is chlorosis. Chelated iron treats the symptom; the real answer is different plants or a container.
6. Plant the things that love it.Clematis, lilac, philadelphus, buddleia, viburnum, ceanothus, hellebore, pulsatilla, dianthus, scabious, verbascum, bearded iris and most herbs are all at home on chalk.
7. Grow lime-haters in pots.Ericaceous compost, a container large enough to matter, and rainwater rather than hard tap water. Tap water on chalk is itself alkaline and will slowly raise the pot's pH.
8. Plant in autumn where you can.Autumn planting gives roots months to establish before the first dry summer, which on thin chalk is what decides survival.
9. Water new plants for two seasons.The reservoir is small. New shrubs and trees on chalk need watering through their first two summers, not just their first.

Tips

• The plants that struggle on chalk are a short list. The plants that thrive on it are a very long one.
• Topsoil depth matters more than the pH reading.
• Hard tap water slowly raises the pH of ericaceous compost in pots — use rainwater.

Common mistakes

• Digging in ericaceous compost or sulphur to grow acid-lovers in the ground — The chalk beneath buffers the pH straight back up. The plant survives a few years in decline and then dies, having cost a great deal of effort.
• Assuming chalk soil never needs watering because it looks pale and cool — It drains fast and holds little. New planting on thin chalk dies of drought more often than of anything else.

If it doesn't work

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