Hot weather survival guide for your plants and gardens

Blackfly on clematis
Blackfly on clematis

The Royal Horticultural Society offers these practical tips to help your plot stay healthy in the fierce summer heat

Plants wilting? Grass looking forlorn? Containers dried out? Don’t despair. The Royal Horticultural Society (RHS) is asking gardeners to take a few simple steps to ensure their plants don’t suffer in the heat.

RHS chief horticulturist Guy Barter has pulled together a plant survival guide to help gardeners of every level.

Move some pots and

hanging baskets into shade

Although houseplants and conservatory plants - bougainvillea and coleus, for example - like to spend summer on the patio, in this weather they might have to go back inside.

Hanging baskets can be re-hung in shade if they begin to flag. If the compost has completely dried out, dip the basket into a big container of water for an hour to re-wet them.

Place plants in pots beneath hanging baskets; when you water the hanging basket, both are wetted. RHS research has shown a mere mugful of water will sustain hanging baskets well enough, especially if they are shaded.

Don’t worry about

perennials in borders

Plants grown in the soil often wilt at midday but then recover overnight. Many herbaceous perennials and some vegetables, such as beetroot, do this. They usually carry on well enough until the rain or cooler weather return. Some stress reduces the need for staking penstemons and phlox, for example, while beetroot will attain a more concentrated flavour.

Give watering priority to leafy salads

Peas and broad beans in flower and leafy salads nearing maturity are the priority for watering in the vegetable garden. The set of pods and succulence of leaves are hugely enhanced by a drink, but other crops can soldier on using soil moisture for a while yet.

There is still ample soil moisture, but from next month, it will begin to run low. Carrots, French beans and sweet corn are notably water-efficient. Once runner beans start flowering, they too become a priority for watering.

Sow seeds at cooler times of day

If sowing seeds, water the seedbed in the evening and sow the following day. Always sow lettuce in late afternoon so they don’t get hot and go into ‘thermodormancy’. Other seeds being sown now - hollyhocks, sweet williams, wallflowers and vegetables such as Chinese greens and Florence fennel - don’t suffer from thermodormancy and will emerge quickly. But they will be vulnerable to pests, such as flea beetle and cabbage root fly, unless covered by horticultural fleece or insect-proof mesh.

Puddle in new plants

Place plants in a hole and fill it with water repeatedly until the hole is full of wet soil and then bed the plants in well before mulching.

Although planting from October to March is advised, if you ‘puddle in’ your summer purchases, they will get off to a good start. Cauliflowers, broccoli and leeks are being planted now and, if you puddle in, they can often go two weeks after planting without needing further watering.

Conversely, plants showing signs of post-planting stress - wilting, going grey and losing leaves - will benefit from being temporarily covered by a sheet of newspaper, held down with some stones, along with watering, until conditions become less fierce.

Give greenhouses plenty of ventilation

Open greenhouses and conservatory doors and windows wide, removing panes if necessary, and consider increasing shade. A min-max thermometer is essential; anything over 30C is damaging to plants, although cacti and succulents can withstand intense heat.

Wetting paths and staging is often recommended, but not that helpful compared with increasing ventilation and shade.

Give lawns a break

Raise the mower cutting height (to about 5cm/2in) and leave lawns a little longer. Watering is not essential as lawns recover once cooler, wetter weather returns.

Collect seeds to increase your stock

Hot dry weather is ideal for collecting seeds from early flowering plants, such as hellebores.

However, cuttings of shrubs, such as cornus, deutzia and philadephus, or of clematis, are best collected in the morning while fully charged with moisture and ‘struck’ (roots formed), and placed under a plastic bag or in a propagator in the shade.

Keep on top of pests and diseases

Even a light touch with the hoe in hot weather causes weeds serious harm. Keep attacking annual weeds, such as chickweed, groundsel and fat hen, and perennial ones, including couch grass and bindweed.

Hot dry weather inhibits slugs, but watch out for aphids, including greenfly and blackfly, which can be rampant in early summer. Their natural enemies, including hoverflies, lacewings and ladybirds, should help keep numbers down.

Most plant diseases are also inhibited by heat and dryness, but expect powdery mildew on delphiniums, courgettes, roses and sweet peas, and be ready to water affected plants well, as drought-stressed plants are especially susceptible.