Choosing Drought-Tolerant Plants for Summer Window Boxes

Why Window Boxes Struggle When the Sun Comes Out

A window box is a wonderfully artificial growing environment. Compost volume is tiny, the sides are exposed to sun and wind, and the whole thing sits at head height where drying breezes do their worst. A south-facing box in July can lose its moisture in a single afternoon, and plants that sulk in dry compost will stop flowering, drop buds and turn crisp at the edges.

The good news is that a hot, sunny ledge is also a wonderful place to garden, provided you choose plants that evolved somewhere dry. Mediterranean herbs, South African succulents and silver-leaved shrubs all cope with a bit of neglect. Combine them with a few water-saving tricks at planting time and you can cut your watering from twice a day to once every few days, even during a proper British heatwave.

Choose Flowering Plants That Shrug Off Dry Spells

Start with the reliable performers that have earned their place in sunny boxes for decades.

• Pelargoniums – both zonal types for upright flower and ivy-leaved varieties for trailing. They are succulent-stemmed, store water in their leaves and simply stop flowering rather than collapsing when compost dries out. They will forgive a missed watering far more readily than petunias.
• Trailing verbena – excellent for spilling over the front of a box, flowering for months in full sun. Look for compact or naturally trailing forms rather than tall bedding types.
• Osteospermum – daisy-like flowers that close in the evening and keep going in hot, exposed positions.
• Gazania – another sun-worshipper with silver-backed foliage. It needs full sun to open and detests wet roots, so always mix plenty of grit into the compost.
• Portulaca – low-growing, fleshy-leaved and almost cactus-like in its tolerance of drought. Perfect for the sunniest inch of a box.

Add one or two filler plants with finer texture, such as diascia or a compact euphorbia, to knit the display together without demanding extra water.

Silver Foliage Is Your Secret Weapon

Leaves that are grey, silver or woolly are covered in tiny hairs or a waxy coating that reflects light and reduces moisture loss. They also look superb against terracotta and pale stone, giving a border or window box a genuinely Mediterranean feel.

• Helichrysum – the classic trailing silver, wonderful spilling over the edge of a box.
• Lavender – choose a compact variety for containers and clip lightly after flowering to keep it bushy.
• Common thyme and sage – culinary, drought-proof and useful in the kitchen.
• Curry plant – silver, aromatic and reliably tough in a hot spot.

Silver foliage also means you can reduce the number of flowers in the box. A third foliage to two-thirds flowers looks better and drinks less.

Compost, Granules and Mulch

What you plant into matters as much as what you plant. Use a good-quality multipurpose compost and mix in about one part horticultural grit to four parts compost for drainage. Pelargoniums, gazanias and lavender all resent sitting in soggy roots during a wet spell.

Water-retaining granules are the single most useful addition for a window box. Soak them in a bucket of water for at least half an hour until they swell into clear jelly, then mix them through the lower two-thirds of the compost at the rate given on the packet. Do not put them only around the roots of the plant, and never use them dry, as they will steal moisture from the compost before they help.

Finish with a mulch of about two centimetres of gravel, decorative grit or slate. This slows evaporation from the surface, keeps compost off the leaves during watering and stops splashing that can spread disease. Cocoa shell is another option, though keep it away from inquisitive dogs.

Watering and Feeding Without the Faff

Water thoroughly in the early morning, before the sun reaches the box, filling the compost until water runs from the drainage holes. Little and often is worse than useless, because it encourages shallow roots that dry out instantly.

Feed with a high-potassium liquid fertiliser every fortnight from June onward, diluted to the strength recommended for containers. Alternatively, mix a slow-release fertiliser into the compost at planting time and top it up once midsummer arrives. Never feed dry compost; water first, then feed.

If you are away often, consider a self-watering window box with a built-in reservoir. They are worth the investment and can double the time between waterings.

Keeping the Display Going Through Late Summer

Deadhead pelargoniums and verbena regularly, pinching or snipping just above the next set of leaves. This keeps energy going into fresh flowers rather than seed. If plants look tired in August, give them a light trim, water well and start feeding again; most will reward you with a second flush into September.

Should something collapse completely, don't nurse it. Empty that section, refresh the compost and drop in a replacement from a garden centre or a rooted cutting from elsewhere in the garden. Window boxes are meant to be edited constantly, and a little ruthless swapping keeps the whole display looking full and fresh right through to the first cool nights.