Which houseplant fits your window?
Not the compass direction decides it — the building opposite does. Measure what really arrives at your window, then choose the plant that lives on it.
Every plant label says the same three words: full sun, bright, shade. Nobody says how to find out which one your window is. “South-facing” is the usual answer, and in a Vienna lane it is often wrong — a south window with a five-storey house eight metres away gets less in December than a north window on the top floor. The honest starting point is not the compass, it is the roofline opposite.
Measure your window →Three groups, one number
The number that sorts them is hours of direct sun on the window — the same number the light check calculates from the officially measured eaves heights next door, for the floor you live on. Take the summer figure for the upper limit and the December one for what the plant has to survive; a plant lives on the smaller of the two.
The two things every guide leaves out
First: the distance. Light falls off fast behind the glass — the sill and a spot two metres back are often a whole group apart. Before replacing a sulking plant, move it towards the window. Second: the season. Between June and December the direct hours at the same window can more than halve, because the low sun disappears behind the roofline opposite. That is why a plant that thrived all summer starts dropping leaves in November, and why the December figure is the one worth reading.
Frequently asked
How much light does my window actually get?
Far less than the compass direction suggests, if something tall stands opposite. The light check calculates the real obstruction horizon around one address — for every direction the height the sun has to clear — and holds the sun’s path of that day against it. The result is hours of direct sun, for your floor: the number this whole page is built on.
Does the floor matter for plants?
A lot. In a narrow lane the low winter sun never reaches the ground floor and still clears the neighbours’ roofs two floors up. The same flat, the same window direction, a different plant group. The check calculates both.
Why does the plant two metres from the window struggle?
Because light falls off fast with distance and even faster with the angle. Sitting on the sill is a different place from standing two metres back — often a whole group lower. Move it towards the glass before you replace it.
Does winter change the group?
Yes, and more than most people expect. Between June and December the direct hours at the same window can drop by more than half — the sun stands lower, so the roofline opposite takes more of it. The check calculates both dates, which is why a plant that thrived all summer starts sulking in November.
Is any of this a substitute for a light meter?
No. It calculates the direct beam onto the window — not how bright a room feels, and it knows nothing about your curtains, a balcony above the window or a light-coloured wall opposite that bounces a lot back. It answers the question a meter cannot: what will still arrive here in December.
The hours come from the light check: City of Vienna — generalised roof model LOD2.1 (CC BY 4.0) · OpenStreetMap contributors (ODbL) · sun position calculated astronomically. The plant groups are the ones every nursery uses; this page assigns them to a measured number, it does not replace advice on watering, humidity or soil.