Ideally a window box should match up with the length and depth of your window sill and not get in the way of the window itself. But if the box is too shallow, the plants will suffer from not having enough soil and will need watering more frequently.
Aim at a minimum soil depth and width of 15cm (6in) which means that the inside of the box should be 18cm (7in) depth to allow a space at the top for watering.
There’s a wide choice of boxes to be found now in the shops. There are, for instance, convincing imitation lead boxes, made from glass fibre, which is light, strong and rot-proof but expensive. At the other end of the price scale are thin plastic boxes which are also lightweight and rot-proof but they do not last forever as constant sunlight makes the plastic brittle and they crack.
Extra long plastic boxes can distort and be difficult to handle when they are filled with soil, so they are better used with lighter soil-less composts. Both fibreglass and plastic boxes retain moisture better than wooden ones and some come with there own drip trays and automatic watering.
Wooden window boxes go with every type of house style and seem to compliment the plants better than any other material but those made of softwood will rot quickly. Avoid them if you can and go for hardwood- elm, oak, ash or teak which should last for many years without needing any preservative paint or varnish.
If you cannot find a box to sit or balcony, it is a relatively simple matter to make your own boxes. Wood is the ideal material- hardwood of course. Cut two pieces the requisite length and two box ends and join them with brass screws.
For the bottom of the box use pieces of 5 by 2cm (2 by 1in) battens, screwed to the endpieces but with a small gap between each to allow for drainage. It is a good idea to put ‘legs’ on each corner of the box to allow the water to drain away; sawn off pieces of cotton reel are ideal for this.
Apart from wooden and plastic boxes, it is possible to find some attractive versions in terra cotta and reconstituted stone. But these should only be used in a permanent site, and are rarely suitable for window sills. They also have a tendency to be cracked by frost.
Whatever kind of box you use, make sure that it is fastened securely and is level if it is on a ledge. Window boxes are best backed up by brackets or chains. Balcony boxes can usually be supported by iron brackets and there are some attractive wrought iron versions to be found. Remember watering problems when you site window boxes overlooking the street; it is all too easy to drench passers-by.
If for some reason the box is not suitable, try a row of flower pots fronted by a plank of wood fixed on the edge instead. Or you can place flower pots in a window box on a bed of pebbles and moist peat so that you can ring the changes more often. You can also achieve a framing effect by hanging baskets on either side of the window.