Most houseplant trouble comes down to one thing: the plant is getting a different amount of light than it evolved for. A pothos that stretches out pale and leggy is hungry for light. A cactus that goes soft and floppy on a dim shelf is the same story. Water and fancy pots matter far less.
The good news is that you do not need a gadget to work out what your room offers. You need to know which way the window faces and how far from it the plant will sit. The second one surprises almost everyone.
Light is measured in lux. A plant on a bright windowsill can get around 10,000 lux, while the same plant about a metre back may get closer to 2,000, and about two metres back closer to 500. That is a fall of roughly twenty times over the width of a sofa. So a spot that feels bright to your eyes, because your eyes adjust so well, can still be dim for a plant.
Rough bands that plant guides tend to use: low light is about 50â500 lux, medium about 500â2,500, bright indirect about 2,500â10,000, and direct sun above that. Treat them as fuzzy edges, not hard lines. Plenty of plants cope with two neighbouring bands.
In most of Europe the sun sits in the southern half of the sky, so the direction of your window sets the character of its light.
Anything that blocks the sky counts too: a balcony above, a tall building opposite, a tree in leaf. That is what the checkbox in the planner is for.
Instead of buying a plant you like the look of and hoping, work backwards from the light you have. These are typical, widely recommended matches:
Many of these plants are toxic to cats and dogs if chewed, including pothos, monstera, peace lily and snake plant. If you share your home with pets, check each plant before it comes home, or put it somewhere they cannot reach.
Too little light usually looks like long gaps between leaves, small new leaves, pale colour, and leaning towards the window. Too much looks like bleached or crispy patches, especially on the side facing the glass. Both are fixed by moving the plant, often only 50 cm, long before you need to think about anything else.
You can often get more light without buying anything. Wipe the glass, since dust takes a surprising bite out of daylight. Put a pale wall or a mirror opposite the window to bounce some of it back. Rotate the plant a quarter turn every week or two so it grows evenly. Take a cutting from a friend's pothos or spider plant and root it in a glass of water, and reuse pots you already own or find second-hand, as long as they have a drainage hole or sit inside a pot that does.
Measure before you place anything: the depth of your windowsill, the distance from the glass to the spot you have in mind, and the width of any shelf or plant stand. A deep planter can overhang a narrow sill. Heavy pots on a high shelf or a balcony railing should be fixed securely so they cannot fall.
Outdoors the light is much stronger, even in shade, so the same rules apply but shifted up a band. A balcony facing north can still suit plants that like shade outdoors, while a south-facing one in summer can be too hot for plants that live happily indoors. Wind dries pots quickly, so a bigger pot holds moisture better than a small one, and weight matters on a railing. Watch the first few weeks and move things if leaves scorch or flop.
Check which way the window faces, measure how far the plant will sit from it, and pick plants for that light. If a plant looks unhappy, try a different spot before blaming yourself or the plant. There is nothing here you have to buy.