How do plants survive in walls or between paving slabs

A. Many old style boundary walls are basically banks of earth clad with stones, so if plants can get established between the stone they can root in the soil. The same applies to paving slabs that have been laid on top of the ground, rather than on a concrete base. Being contained or topped by stone the earth in walls or underneath slabs is far less susceptible to evaporation and will stay moist, making it ideal territory for many plants.

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A. Many modern walls and paths are filled with cement, which is much more difficult to breach than the traditional lime mortar. Walls made of bricks are unlikely to have enough earth (unless introduced) to support plant life and also have the disadvantage of heating up quickly, baking the roots of anything trying to grow between them. Under these circumstances it is probably best to try and plant around the margins, i.e. try lifting the edge of paving slabs with a pick to find some soil, break off a corner that will be disguised when covered by plants or plant a self clinging climber at the base of walls.

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A.� Certain herbs are brilliant for the cracks in paving. Low growing thyme and marjoram will spread across slabs and are as tough as any grass. Walking on them releases enticing culinary smells, and, of course, they can be picked and used for the pot. If you can't lift a slab to plant a pot plant, just sow a few seeds in the gaps.

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In walls, valerian (Centranthus) is often regarded as a weed, but both the white and pink version will thrive in the most starved conditions and will not undermine the fabric of the wall if cut back before it can self-seed each year. Wallflowers, as their name suggests, can do well, but are surprisingly hard to establish. Try wrapping their roots in soaked tissue paper before pushing them into gaps.

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Much easier is Erigeron karvinskianus, the Spanish daisy, which will grow just about anywhere and, if chopped right back in spring, will produce white through pink flowers from June to October.

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