Plants indoors, and the claims that do not hold up

Treat the sections below as a sequence. With indoor plants and the air they sit in, getting the early decisions right makes the later ones much easier.

Where the air claim came from

The widely repeated purification claim originates in experiments conducted in small sealed chambers, designed for spacecraft applications. Scaling those results to a ventilated room implies an implausible number of plants per square metre. Subsequent analyses have been clear that ordinary indoor planting has negligible effect on air pollutant concentrations.

It survives because it is repeated by everyone selling plants and because it is the kind of claim that sounds as though it ought to be true.

Ventilation is what works

Opening a window exchanges room air far faster than any quantity of foliage. For specific pollutants, source removal and extraction are the effective interventions.

Where indoor air quality is a genuine concern, a monitor and ventilation are the answer rather than plants. Cooking, candles, wood burners and some cleaning products are the usual indoor sources, and an extractor fan that actually gets switched on is worth more than any purifier bought instead of one.

The real benefits are psychological

Studies on indoor plants and views of nature report improvements in mood, attention restoration and perceived comfort. These effects are modest and reasonably consistent, and they are the honest case for houseplants.

Where it helps most, caring for something and watching it change is also a small ongoing source of satisfaction. Much of that research uses small samples and short exposures, so the honest summary is a consistent small effect rather than anything dramatic.

Humidity, marginally

Transpiration adds some moisture to a room, which is noticeable with many plants in a small space and negligible with two on a shelf. In very dry heated rooms a group of plants can help slightly. Over-watering to chase this creates fungus and gnats rather than comfort.

A bowl of water near a radiator or a dish of wet pebbles under a pot does the same job far more directly and without drowning anything.

Keeping them alive

Light is the constraint people misjudge most; most failures are a plant in too little light rather than too little water. Matching the plant to the actual light in the position, and watering less than instinct suggests, resolves most problems.

Some common houseplants are toxic to cats and dogs, which is worth checking before buying. Plants also need far less water in winter, and carrying on with the summer schedule is the single most common way one gets killed.

Some of this will suit you and some will not, and that is the point.

What they actually do to a room is spatial

A large plant breaks up a blank wall, softens a hard corner and gives the eye somewhere to settle, which is the work of furniture rather than of biology. Height matters more than quantity: one tall plant changes a room more than six small ones lined up on a windowsill. They also mark a room as inhabited and looked after, which is a good part of why they do so much for rentals and newly occupied flats.

Permanently damp compost grows mould and attracts fungus gnats, so anyone with allergies or a respiratory condition is better served by a free-draining mix and letting the surface dry between waterings.

The takeaway

Have plants because they are pleasant. Open a window for the air.

Pick the one that costs you least, and let the rest wait.