How do I plant a window box that still looks good in August?

A window box holds very little compost, sits in a wind stream against a warm wall, and is usually sheltered from rain by the building itself. Everything that makes a container difficult is concentrated in it. The compensation is that it is at eye level, so five minutes of attention a week is easy to give.

What you'll need

• A box at least 20cm deep and 20cm front to back, true
• Loam-based compost and multipurpose compost, true
• Controlled-release fertiliser, true
• Gravel or mesh for the drainage holes
• Watering can with a long spout

Step by step

1. Get the depth right first.20cm of compost is the working minimum and 25cm is much better. A shallow trough of 12cm will need watering twice a day in July and nothing will thrive in it. Depth front to back matters as much as length.
2. Check the aspect honestly.South and west: pelargoniums, osteospermum, gazania, verbena, trailing rosemary, thyme, sedum, petunias. North and east: fuchsia, begonia, busy lizzie, heuchera, ivy, ferns, violas. Very few plants do both, and the commonest reason a box fails is the wrong list.
3. Mix the compost for weight and water holding.Roughly half loam-based, half peat-free multipurpose. The loam gives it structure, weight and stability; the multipurpose keeps it light enough to fill and open enough to root into. Mix a controlled-release feed through it as you go.
4. Cover the drainage holes and check the fall.Mesh or a crock over each hole so compost does not wash out onto whoever is below. Set the box level or a fraction back-tilted so the water does not run down the front of the wall.
5. Plant in three layers.Something upright at the back — a small grass, a salvia, a dwarf shrub. Something bushy in the middle for volume. Something trailing at the front: ivy, bacopa, lobelia, trailing verbena, helichrysum. It is the trailers that make a window box look like a window box.
6. Plant densely, but leave room.Closer than in the ground, because you want it full quickly and the display is for one season, but not so tight that nothing can grow. Roughly one plant per 10 to 12cm of length in a standard box.
7. Water at the base, thoroughly, daily in summer.A long-spouted can reaching under the foliage. Water until it drains from the base, then stop. Little and often only wets the top and encourages surface rooting.
8. Feed weekly from about six weeks after planting.The controlled-release feed carries the first weeks; after that a balanced liquid feed weekly, switching to a high-potash tomato feed once flowering is established, is what keeps it going into September.
9. Deadhead constantly.Every faded flower removed is energy redirected into the next one. At eye level this takes seconds. Petunias, pelargoniums, violas and osteospermum all stop flowering if allowed to set seed.

Tips

• Plant into a plastic liner that drops into the decorative box. You can then swap the whole planting for an autumn or winter one in two minutes without disturbing the fixing.
• In a hot sunny position, add a mulch of fine gravel and choose plants with small or grey leaves. Grey foliage is nature's marker for drought tolerance.
• Winter and spring boxes are much less work than summer ones: violas, cyclamen, small ivies, skimmia, heather and dwarf bulbs will run from October to April with almost no watering.

Common mistakes

• Using a shallow decorative trough because it fits — It cannot hold enough compost or water for a summer display. If the box is shallow, plant it with succulents, herbs or spring bulbs rather than fighting it.
• Relying on rain — The building shelters the box from most rain, especially under a deep sill or an overhang. A window box needs watering whether or not it has rained.
• Planting sun and shade plants in the same box — The two ends of a long box on a corner really do get different light, but mixing incompatible plants across a single aspect just means half of them look wrong all summer.

Keeping on top of it

• Water thoroughly at the base (Daily in summer, less in spring and autumn)
• Liquid feed (Weekly from six weeks after planting)
• Deadhead and remove yellow leaves (Every few days)
• Replant for the season (Twice a year, spring and autumn)

Questions people ask