Material usually comes down to weight and frontage. Handmade terracotta (the Roma, Garda, Liscia and Antico troughs) is the most sympathetic on a Victorian or Georgian elevation, but it is heavy, and heavier once planted. Zinc troughs are light, weather to a soft grey, and suit a cleaner frontage. Fibreclay and polystone hold a convincing stone or terracotta look at a weight a bracket can take comfortably. Recycled plastic is the lightest, and sensible for a high sill or a rented flat.

Size the box to the window. A box close to the full width of the opening looks deliberate; a short one looks like an afterthought. Measure the sill depth, as many London sills are shallower than a standard box, in which case mount on brackets below the sill.

Fix for the filled weight, with brackets rated well above it, into masonry rather than render, and a restraining wire above ground floor. We would rather talk it through in the shop than sell you a box that does not suit the sill.

For planting: something upright in the centre, something mounding around it, something trailing over the front. Spring suits daffodils, crocus and pansies; summer geraniums, trailing verbena and ivy; autumn cyclamen, violas and heather; winter Skimmia, small evergreens and pansies. Water more often than a bed and refresh the compost each season.

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