Care & maintenance
Whatever type of window box you use, make sure excess water will easily drain away. A 90-centimetre-long box needs two or three drainage holes. Place squares of fibreglass screening (the kind sold in screen door and window repair kits) or some other porous material over the holes before filling the box with soil. Do not use pot shards or gravel, which take up valuable space and do nothing to improve the drainage.
- Fill container with lightweight potting soil or soilless mix.
- Plant closely so your box looks lush and generous right away. Don't be afraid to gently squeeze a root ball to make it fit.
- Avoid placing trailing plants close to the front edge. Instead, give them more stability by siting them midway back to let their long stems thread through plants at the front and over the edge.
- If you want to encourage a soft-stemmed, upright plant, such as coleus, to spill over the front edge, tip its root ball at an angle when planting so the stems lean forward.
- Apply transplanter solution according to manufacturer's directions to get plants off to a fast start. Once you notice new growth, begin fertilizing lightly and frequently: once a week with a water-soluble product designed for flowering plants, diluted to at least half the recommended concentration. (For example, if the label calls for 15 millilitres of fertilizer per two litres of water, use 15 millilitres per four litres or more.) A balanced formula or one with a slightly higher amount of phosphorus (the middle number) is fine. If you're using potting soil that includes time-released fertilizer, you probably won't need to start feeding until midsummer.
- Deadhead regularly and ruthlessly. This keeps your display looking tidy and encourages plants to produce more flowers.
- If one plant starts to take over (such as English ivy, coleus, New Wave petunias or sweet potato vines), don't be afraid to cut it back.
- Check boxes daily to ensure soil is kept consistently moist from top to bottom.
Mounting window boxes
Most window ledges dip down ever so slightly in front to prevent water from standing on the surface. Before positioning a planted box on a window ledge, you may want to put a couple of stoppers on its bottom front edge to prevent slippage.
If you're mounting a window box on a wall, use L-shaped brackets beneath the box rather than trying to attach its back side directly to the wall. Brackets underneath provide more stability and can be decorative. Use wood screws for wooden siding, or drill holes and use a masonry anchor to hold screws in mortar or brick.