Houseplants like good light but can be scorched by the sun on south-facing windowsills. Cacti and some succulents tolerate more sun than most plants, as long as ventilation is good and heat can escape, but avoid conservatories at all costs – they can be ill-ventilated and lack shade, especially if south facing, making them lethal to plants when temperatures rise.
A good option is to move houseplants to a shady spot outdoors – once nights are warmer and they are not inhibited by the chilly nights that can persist deep into June. For tropical plants, wait until night temperatures exceed 12C, but tougher plants, citrus and monstera for example, will tough it out at above 7C in the summer.
Indoor plants are well adapted to and accustomed to shady indoor conditions. They need careful weaning into brighter conditions, place them first under strong shade, then light-dappled shade. That should suit them well for the rest of the summer. The base of north-facing walls and shady porches are also favourable.
Although houseplants should be kept reasonably dust free, summer showers will wash away any remaining dust. However, a summer downpours may drown roots if plants are stood in saucers or in decorative outer pots that lack drainage holes. Plants must rest on a surface that allows drainage.
On the other hand, rain is rarely sufficient to keep potted outdoor plants adequately watered, particularly in very hot weather. Houseplants remaining indoors will also need more water in summer, although not as much as those placed outdoors, unless rain falls.
With good light and warmth houseplants placed outdoors and, even those remaining inside, respond to feeding with a liquid fertiliser every week, such as seaweed.
Slugs and vine weevil are potential problems. Be ready to pick off the former if leaf damage and slime trails are seen, while the latter cause u-shaped nips on leaf edges. Trap adults with sticky barriers placed around pots and apply nematodes in September before the plants return indoors.
Many houseplants get top heavy as they grow and it is not always convenient to put them in larger pots. To keep them upright they can be weighted with bricks or supported with steel-fencing pins that are sturdy, easy to drive into the soil and fairly inexpensive – even for the smarter ones.
Where potting into larger pots is feasible, do it as soon as possible so the roots permeate the potting compost, which will help to protect them against inadvertent over-watering in winter.
However, there is ample time to prune and trim houseplants, with plenty of summer left for them to recover and start growing again.
Trimmings and pruning can make good material for cuttings. Traditionally houseplants are simply propagated by standing severed shoots in a jar of water and being potted up once roots arise. Root formation can take some weeks, so an early start is advised.
Houseplants that naturally make offsets or baby plantlets will be especially prolific in summer.
As summer winds down and nights begin to get chilly, usually from mid-September, return plants indoors. Pick off any unwelcome passengers en route. Some indoor plants flower better if they experience some chill, Cymbidium and Clivia for example, and these can remain outdoors until frosts threaten.
Finally, summer is a good time to buy more houseplants, so they have time to adapt to harsher conditions than the glasshouses where they were raised, before autumn.