Why “Full Sun” and “Part Shade” Are Dangerously Vague
The label on a nursery pot reading full sun or part shade is dangerously vague, because the biological reality of a plant’s light requirement is highly localized and mathematically precise. A hosta planted on the north side of a building might thrive in July but completely fry in September once the sun’s angle drops and slips past the architectural shadows that were protecting it. In complex urban environments, where tall buildings, deep courtyards, and mature tree canopies constantly shift the light, relying on a single noon-time observation during a site visit guarantees high plant mortality and costly replacements.
Daily Light Integral and the Western Exposure Trap
Professionals calculate the exact path of the sun across a site’s specific latitude during the year’s two most extreme days, the summer solstice with its longest shadows and the winter solstice with its deepest ones. Daily light integral is the total photosynthetically active radiation a plant receives over a full 24-hour period, capturing not just how bright the sun is but how long it hits the leaf. Six hours of sun is not a uniform metric either: six hours of gentle morning sun on an eastern exposure suits delicate understory plants perfectly, while six hours of brutal afternoon sun on a western exposure, combined with high ambient temperature, will scorch those same plants to death.
Reflected Radiation in Urban Cores
When calculating solar loads in an urban courtyard, direct sunlight is only half the battle. Glass facades and light-colored concrete create intense reflected radiation, or albedo, meaning a shade-loving fern planted in the shadow of a glass building can still burn because the adjacent glass tower reflects concentrated ultraviolet rays directly into what looks like a shaded bed. Material reflectivity always needs to be accounted for in a solar map, not just direct exposure.
True Horticultural Light Categories
Deep shade means less than two hours of heavily filtered direct sun, suited to ferns, asarum, and woodland groundcovers. Part shade or part sun means three to five hours of direct sun, preferably morning, suited to hydrangeas, hostas, and astilbe. Full sun means six or more hours of unobstructed direct sunlight, suited to ornamental grasses, prairie perennials, and juniperus. Baked or reflected heat means eight or more hours plus heat radiation from adjacent masonry, suited only to sedums, yucca, and extreme drought-tolerant species.
Where the Calculation Work Goes
Tracking sun angles and calculating exact shadow lengths for multiple buildings across different seasons used to require tedious three-dimensional modeling. CalcType’s Shadow & Solar Analysis Calculators generate precise shadow projections for a site’s exact latitude and longitude and calculate total daily sun hours for specific planting beds directly.