How much sun does your balcony get? Turn it and see

You buy a lavender for the balcony, it sulks, and the label said "full sun". The plant is rarely the problem. Most of us just don't know how many hours of sun our balcony really gets. It depends on the way it faces, the time of year, how far north you live and what stands in the way. This guide lets you turn all four and watch the hours change.

What "sun" means on a plant label

Plant labels count hours of direct sun, meaning light that falls on the leaves with nothing in between. Bright shade doesn't count. Guides vary a little, but the common rule of thumb is this:

• Full sun: about 6 hours or more of direct sun a day.
• Part sun or part shade: roughly 3 to 6 hours. Morning sun is gentler than the hot afternoon kind, so part-shade plants often prefer it.
• Shade: under about 3 hours, often much less.

Turn your balcony and see

Pick the direction your balcony faces, then move the sliders. The "blocked" slider starts at 10°, about what a gap between houses might hide. Use 30° or more for a tall building across a narrow street. The curve is the sun's height in the sky through the day. The orange part is when it shines onto your open side. The planner assumes a flat horizon and clear weather. It counts the sun as reaching you when it is within about 80° of straight-on to the open side, because low, sideways sun mostly hits the walls. Treat the answer as the best case.

What the planner is telling you

In the northern half of the world the sun sits in the south at midday, so a south-facing balcony gets the most hours, and it gets them all year. In midwinter the sun is low and the days are short, so a south balcony only gets those hours if nothing stands in front of it. Try December at 50° north with the "blocked" slider at 0°, then at 15°. The low winter sun is the first thing a building across the street steals.

A north-facing balcony gets almost none for most of the year. In high summer the sun rises and sets far enough north to touch it briefly, early and late. Slide to June and look at the orange slices at the edges of the day.

East balconies catch gentle morning sun. West balconies catch afternoon sun, which is hotter because the surfaces have been warming all day. Pots dry out faster there, so a west balcony is not automatically a "full sun" balcony for thirsty plants.

Notice what happens when you slide "how far north". Further north the summer days are longer but the sun stays lower, and the winter sun nearly vanishes. Lisbon is around 39° N, Rome 42°, Paris 49°, Berlin 52°, Stockholm 59° and Oslo 60°.

Matching plants to your hours

Use the result as a starting point, not a verdict. A few examples from each group:

• Full sun: rosemary, thyme, lavender, pelargoniums (the "geraniums" sold for balconies) and tomatoes.
• Part sun: parsley, chives, mint, fuchsias and hydrangeas. Mint in particular copes with less light than most herbs.
• Shade: ferns, hostas and ivy.

Always check the plant's own label too. Your balcony may sit between two groups, and the label knows the plant better than we do.

Checking it for real

The planner is a model, and your balcony is a place. Two things make it better:

• Find your direction. Stand in the open doorway looking out, hold your phone flat and read the compass. Keep it away from metal railings, which can throw it off.
• Look at it on a sunny day. Take a photo of the balcony floor at 9, 12, 15 and 18 o'clock. Where the light lands in each photo is your real map, including the shadow of the railing, the awning and the building next door.

When to break the rule

A light wall opposite your balcony can bounce a surprising amount of light back, which helps a shady spot. The same bounce on a hot west wall can scorch leaves. Pots on the floor often sit in shade even when the railing end is in full sun, so try moving them before you give up on a plant. Sun-shy plants can live under a table, sun-lovers can hang on the railing.

Moving, grouping and swapping pots costs nothing, and it is usually the first thing to try before buying a new plant. If you do hang planters on the outside of a railing, fix them firmly, remember that wet soil is heavy, and check your lease or building rules first.

What to look for

Before you buy a plant for your balcony, find the three things that matter: which way it faces, how many hours the planner and a few photos say it gets, and whether it gets them in the morning or the afternoon. Match those to the label, and the plant has a fair chance from day one.