Window and railing boxes can add to your container garden space. They can be used to grow flowers, herbs, and even smaller vegetables and fruits. Because of their small size they can also be a challenge to keep adequately watered. By using container materials that reduce water loss and potting soils that manage moisture, these smaller containers can be both bountiful and beautiful.
Window and railing boxes should be at least six inches wide and deep – the wider and deeper the better. For a balanced appearance, your window boxes should be either a few inches longer or shorter than the width of your window, and one forth to one fifth of the height (20-25%).
Wood, particularly cedar and redwood because of their resistance to rot, are good materials for window and railing boxes. They are attractive, relatively lightweight, and can be painted to match your home. Wood containers also provide some insulation from heat, so they are excellent for use in a sunny location. To make them even more rot resistant and for ease of planting and seasonal cleanup, you can use them with plastic liners – planting the liners and then placing the liners inside the wooden box.
Plastic is good for window and railing boxes because it is inexpensive, lightweight, and holds water well. It is not as attractive as other materials, however, and will eventually become brittle and crack with sun and weather exposure. Some fiberglass containers are becoming available now which have all the advantages of plastic and are more attractive. They tend to be a bit expensive, however.
Terra cotta and ceramic are good for moisture retention and good looks, but they are heavy and can be difficult to mount.
Metal containers are lightweight, hold water well, and hold up to sun and weather. Avoid using metal and dark colored containers if your window or railing box will be in the sun all day. They will absorb the heat and cook your plants' roots.
Wire frames with a liner of sphagnum moss or coconut coir are attractive, lightweight, and inexpensive. You can even line them with heavy wool cloth for added color and appeal. Used alone they will dry out quickly, but if you line them with plastic when planting they make an excellent planter. Just be sure to poke a few holes in the liner for drainage.
Be sure your window or railing box will be securely mounted; remember that it will be much heavier once planted and watered. Mount the box brackets securely into wood railings or wall studs, then mount your box to the brackets with screws or straps before planting. Most hardware stores or garden shops that sell window boxes also sell mounting hardware. Be sure the hardware is strong and durable, and mount the brackets no further than eighteen inches apart, closer for deep or wide boxes.
Make sure your window or railing box will not block your view (unless you are using it to block an unpleasant view) and the plants, when grown, will not interfere with opening a window or door. Also consider the amount of sunlight and whether you will need to grow plants requiring full sun, partial shade, or full shade. Also choose a location protected from wind. Constant wind will dry out your container and could damage your plants.
All smaller containers have basically the same challenge – retaining enough water for the plants to thrive between waterings. Some of the new moisture managing potting mixes are excellent for window and railing boxes. They contain moisture retaining gel crystals which absorb moisture when you water and slowly release it as the soil drys. You can also make your own soil-less mix using
2 parts peat or coconut coir
1 part perlite
1 part vermiculite
You can also add ½ to 1 part good compost, and some slow-release pellet fertilizer to the mix. Moisten your mix thoroughly before filing your container for planting.
When you plant your box, fill your container to within one inch of the top and no more. This will give you room to soak the container well when watering. If you like you can top off with a layer of moss or shredded bark mulch to help hold in moisture. Never allow your container to completely dry out and water thoroughly each time - until water runs out the drain holes. If your box ever does completely dry out you may find it repels water when you try to wet it. If this happens place your box or box liner (if removable) in a bucket or tray of water overnight. If your box is not removable, poke several holes into the soil with your finger and fill with water. After the water is absorbed fill the holes again and again until the soil is re-moistened and water is absorbed.