Some very attractive window boxes are now available in plastic and glass fiber but being mass produced they come in standard sizes – usually between 600 and 750mm long and 225mm wide. Before investing in one, check not only that the window sill is long enough for but also that it is wide enough. Check that the windows will not knock the plants when they are opened. Casement windows do not rule out window boxes altogether, but the latter will almost certainly have to be fixed on stout brackets below sill level.
Do not place a window box on too narrow a sill, and make sure it is in a sheltered position. For safety you should screw the box onto the window frame or put a permanent guard rail in front. Remember that a window cleaner, for instance, might well grab hold of the box to steady himself, and that strong windows can move quite heavy things. You need to think carefully, too, before positioning a box over a doorway without a porch: it may drip onto callers awaiting entry.
Think of your garden decor when adding window boxes on the sides of the house that face the garden. Attractive wooden window boxes, for example, will add a different type of visual impact than resin boxes or metal ones. Also, wooden boxes are quite easy to make. Old floor boards can sometimes be found for the job, but any hard wood that is at least 25mm thick can be used.
Excellent window boxes can be made from furniture such as old tables, wardrobes or drawers. Softwood can be used, but it will not last for more than three or four years, even with great care. Never make a window box without treating the timber well in advance against woodworm, and dosing it with a preservative. Creosote should never be used, as it is poisonous and may damage the plants. Think of garden mirrors when positioning your boxes if the window sill is wide enough. Two at each side of the box will give the illusion of a never-ending green space.