How to Read Your Garden's Light Level (Without a Fancy Meter)
Ask most gardeners how much light a spot gets and you'll hear "pretty shady" or "gets some sun in the morning." That's not enough information to pick a plant that will actually thrive, and it's the single biggest reason shade-garden plantings fail in the first year. The plant wasn't wrong for shade in general — it was wrong for this shade, at this intensity, for this many hours.
Why "shade" isn't one thing
Nurseries and plant tags use a handful of standard categories — partial shade, full shade, deep shade — but a spot under a thin birch canopy and a spot in the tight gap between two garages can both get called "full shade" while receiving wildly different amounts of usable light. The birch canopy lets dappled light through most of the day; the gap between garages might get twenty minutes of direct sun and nothing else. Treating both as interchangeable is where trouble starts.
The old-fashioned method: track direct sun hours
Before smartphones, gardeners tracked light the slow way: checking a spot every hour or two across a full day and noting whether it was in direct sun. Do this once, ideally in late spring or early summer when deciduous trees are fully leafed out, and you'll get a rough hour count. As a working guide, partial shade means roughly three to six hours of direct sun, full shade means under three hours but still bright the rest of the day, and deep shade means essentially no direct sun at all.
The faster method: a light-meter app and a lux reading
Most smartphone camera apps, and a growing number of dedicated light-meter apps, can give you a lux reading — a measurement of light intensity. Hold the phone flat, at the height your plants will actually sit, and take readings at a few points across the day. A reading of a few thousand lux under tree cover is common for full shade; readings in the low hundreds suggest deep shade; readings above ten thousand lux for extended periods start to look like partial sun.
Intensity alone doesn't tell the whole story, though — duration matters just as much. That's what Daily Light Integral (DLI) captures: it multiplies intensity by how long it lasts, giving you one number that reflects the total light a spot receives over a full day. Our light level calculator takes a lux reading and an hour count and estimates both the DLI and which shade category the spot falls into, so you don't have to do the unit conversion by hand.
Where people go wrong
The most common mistake is measuring on an overcast day, or measuring once in early spring before deciduous trees leaf out and assuming that reading holds all season. A bed that looks sunny in April can be full shade by July once the canopy closes. If your garden sits under deciduous trees, take at least two readings — one in early spring and one once the leaves are fully out — and plan around the summer number, since that's when most perennials do their real growing.
The second mistake is measuring in one spot and assuming the whole bed matches. Light can shift dramatically over just a few feet, especially near a building corner or the edge of a tree's canopy. If a bed is more than a few feet across, take readings at both ends and the middle.
What to do with the number once you have it
Once you have a light category — partial, full, or deep shade — that becomes one of the three filters for choosing plants, alongside your USDA hardiness zone and your soil's moisture level. Our shade plant selector uses exactly those three inputs to shortlist real, well-known shade-garden plants, so the light reading you just took has an immediate, practical use rather than sitting as a curiosity.
It's worth repeating the measurement every few years, too, especially if nearby trees are growing, being pruned, or being removed. A bed's light level is not a fixed fact about the property — it changes as the landscape around it changes, and the plants that thrived under yesterday's canopy may need rethinking under tomorrow's.