The problem
Indoor plants accumulate dust on their leaves, mineral deposits on their soil and a general staleness that comes from living in the same spot, in poorly ventilated air, for months at a time.

The hack
Spending time in warm summer rain is said to give houseplants a spa day: soft water reaches their roots, dust is washed from their leaves, and they get a rare dose of the outdoor conditions they may be best suited to.

The method
Wait for a warm day, when the temperature outside is close to what your plants experience indoors. Move them to a spot protected from wind, and let them enjoy the rain shower for 20 minutes. Bring them back inside once the rain has stopped; don’t leave them out overnight or in a prolonged downpour.

The test
I put some tropical plants out during what looked like a light summer shower. The rain was colder than expected. One Calathea came back with darkened patches on the leaves. A Monstera fared better, but had clearly been knocked about by the rain, and the others were fine.

The verdict
British summer showers are not warm and are unpredictable in intensity. A cold downpour on plants acclimatised to a stable indoor temperature can cause chill damage and waterlogged soil if in a pot without drainage, which is a fast track to root rot. There is also the small matter of vine weevils, slugs and other outdoor pests hitching a ride back inside on your pots. I would stick to a lukewarm shower indoors, which provides the rinse without the risks.