What compost should I use for lavender, rosemary and olives in pots?

These plants evolved on thin, sharply drained hillside soils. In the ground they cope with British rain because water moves away from the roots. In a container it cannot — the base of any pot holds a saturated layer regardless of what you put in it, and a fine peat-free compost keeps that layer high and wet for weeks. The mix is how you push it back down.

Step by step

1. Start with a loam-based compost, not multipurpose.John Innes No. 2 for smaller plants and No. 3 for anything permanent. Loam-based mixes have real mineral structure, so they hold their shape for years and drain in a way that peat-free multipurpose does not once it has slumped.
2. Add a third by volume of horticultural grit.Two parts compost to one part 4 to 6mm grit is the working ratio for lavender, rosemary, thyme, sage and olives. For agaves, aeoniums and true alpines go to half and half. Mix it right through rather than layering it.
3. Use grit rather than perlite for outdoor pots.Perlite improves aeration but it is very light, it floats to the surface in heavy rain and blows away, and it does nothing for stability. Grit adds weight low down, which matters for a tall rosemary or an olive standing in wind.
4. Skip the water-retaining gel entirely.Retention crystals are designed for hanging baskets. In a Mediterranean mix they hold water against exactly the roots you are trying to keep dry.
5. Mulch the surface with gravel or slate, not bark.A 25mm gravel mulch keeps the crown and lower stems dry, reflects light, stops rain splash and suppresses moss. Bark and compost mulches hold moisture at the collar and encourage rot on woody Mediterranean plants.
6. Get the pot and the position right too.Terracotta suits these plants because it breathes and loses moisture through the wall. Pot feet, no saucer through winter, and a spot in full sun with some shelter from persistent winter rain — under eaves or against a wall — finishes the job.
7. Feed sparingly.These are plants of poor soil. A single application of a balanced slow-release feed in spring is enough. Rich feeding produces soft, sappy growth that flops, does not flower well and is killed by the first hard frost.

Tips

• Horticultural grit is lime-free; builder's sharp sand and some path gravels are not, and can be alkaline. For lavender and rosemary that is harmless; for anything ericaceous it is not.
• Repot into the same size pot with fresh mix every three to four years rather than going bigger. These plants flower better slightly restricted.
• Sand alone is not a substitute for grit. Fine sand fills the gaps between compost particles and makes drainage worse, not better — the classic mistake with a bag of play sand.

Common mistakes

• A layer of gravel in the base instead of grit through the mix — A gravel layer raises the saturated zone rather than lowering it. Drainage is improved by changing the compost the roots are in, not by putting stones under it.
• Using multipurpose because the plant looked happy in it at the nursery — Nursery plants are grown under cover, watered precisely, and sold within months. The mix that works for one season under glass is the wrong mix for four years of British winter rain outside.
• Leaving a saucer under an olive or a lavender — It is the single commonest cause of death in these plants. Standing water at the base is fatal to drought-adapted roots even in cold weather when the plant is not growing.

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