Plant tags compress a complicated light pattern into three labels: full sun, part sun, and shade. Those labels are useful only when you know how many hours of direct light actually reach the bed—and when.
The one-day light map
- Choose a clear day after nearby deciduous trees have leafed out.
- Sketch the bed and mark north.
- At 9 a.m., noon, 3 p.m., and 6 p.m., shade the part of your sketch receiving direct sunlight.
- Add the intervals to estimate direct-light hours for each section.
Translate the result
For this site, “full sun” means roughly six or more hours of direct sun during the growing season. “Part sun” means about three to six hours. “Shade” still has usable ambient light; it does not mean a windowless darkness.
Repeat where shadows change
A narrow bed can contain several exposures. Measure the front edge, the wall side, and the area beneath any tree canopy separately. Recheck after major pruning, construction, or removal of a nearby tree.
What this method cannot tell you
It does not measure soil moisture, reflected heat, wind, root competition, or seasonal changes in tree canopy. Use it as one layer of a site profile, not as the entire recommendation.
Editorial method: direct observation framework reviewed against common extension definitions; see Sources & Methodology.