How Much of Your Bed Is Actually Under Canopy? Measuring Real Coverage

"My bed is under a tree, so it's shady" is where a lot of planting plans start, and it's not wrong, exactly — it's just not specific enough to be useful. Shade is not one thing. Full shade under a high, open canopy, part shade at a bed's sunnier edge, dappled shade filtering through a thin tree, and deep, dry shade under a dense conifer or a mature beech are four genuinely different growing conditions that all get flattened into the same word. Measuring how much of a bed actually sits under canopy — and understanding what that measurement can and can't tell you — is the first step to telling them apart.

The geometry, and what it actually computes

A tree's canopy, viewed from above, is reasonably well approximated as a circle, with an area of π×(diameter÷2)². Divide that by your bed's area and you get a coverage percentage, capped at 100% once the canopy fully overlaps the bed. Our canopy cover calculator does exactly this arithmetic, taking a tree's crown diameter and a bed's length and width, and translating the resulting percentage into a shade category: over 80% coverage reads as deep shade, over 50% as full shade, over 20% as partial shade, and under that as dappled or light shade.

Worked example: a small ornamental tree over a modest bed

Take a small ornamental tree with a 10-foot crown diameter, over a 14×8 ft bed. The canopy area works out to 78.54 sq ft against a bed area of 112 sq ft, giving 70.13% coverage — landing in the full shade category. That's a genuinely useful number for planning: it tells you most, but not quite all, of the bed sits under the crown, with roughly 30% still getting whatever light reaches past the canopy's edge.

Worked example: a thin birch and a dense beech on the same bed

Here's where the geometry starts to reveal its own limits. Take a considerably larger bed — 25×20 ft, 500 sq ft — and run two different trees over it. An 18-foot-crown birch produces a canopy area of 254.47 sq ft, for 50.89% coverage: just over the full shade threshold. A 35-foot-crown beech over the identical bed produces a canopy area of 962.11 sq ft — the coverage caps at 100%, landing squarely in deep shade.

Numerically, that looks like a clean, well-behaved result: bigger canopy, more coverage, deeper shade category. But anyone who has actually stood under a mature birch and a mature beech knows the light doesn't behave quite that tidily. A birch canopy is thin and open — narrow leaves, gaps between branches, a dappled, moving pattern of light that shifts across the ground all day. A beech canopy is notoriously dense — broad leaves packed tightly, very little sky visible looking straight up, genuinely dark shade underneath by midsummer. The calculator's 50.89% and 100% numbers capture geometric coverage correctly. They say nothing about crown density, and density is a big part of what actually determines how dark a shaded spot feels and how much usable light reaches a plant beneath it.

Why this matters for choosing a plant

This is exactly why canopy coverage and a direct light reading are two different, complementary measurements rather than substitutes for each other. A bed reading 100% canopy coverage under a thin, high birch might still register a perfectly workable full-shade DLI on a light meter, supporting many of the same perennials that a partial-shade bed would. The identical 100% coverage under a dense, low beech can register as genuine deep shade by DLI, ruling out anything that isn't in the shortest, toughest list of deep-shade-tolerant plants. See our companion piece on lux, foot-candles, and Daily Light Integral for how to take that direct reading, and don't skip it just because you already know the canopy percentage — they're measuring different things.

The harder case: dry shade under a dense canopy

A dense, high-coverage canopy — the beech example above, or an evergreen conifer — usually brings a second problem along with the light problem: dry soil. The same dense canopy that blocks light also intercepts more rainfall before it reaches the ground, and the tree behind a dense, mature canopy typically has an equally extensive root system competing hard for whatever moisture does get through. This combination — low light plus dry, root-competed soil — is what gardeners mean by "dry shade," and it's the single hardest condition to plant into, because it eliminates most of the moisture-loving perennials that would otherwise tolerate the low light just fine. Our companion guide on dry shade root competition goes into the mechanics of why this happens and what actually survives it. A high canopy coverage percentage, especially paired with a dense species like beech or a mature conifer, is a strong hint to check soil moisture directly before assuming any full- or deep-shade-tolerant plant will do.

The other side of canopy geometry: cast shadow

Coverage percentage isn't the only thing a canopy's geometry can tell you. Our calculator also estimates shadow length from a tree's height and the sun's angle above the horizon, using simple right-triangle trigonometry: shadow length equals tree height divided by the tangent of the sun angle. Run a 28-foot tree at a high summer sun angle of 65° and the shadow stretches only 13.06 ft. Run the same tree at a low winter angle of 20° and the shadow stretches to 76.93 ft — nearly six times longer, from the exact same tree. That's a genuinely large swing, and it means a bed sitting well clear of a tree's shadow in July can find itself fully shaded by that same tree in January, simply because the sun sits so much lower in the sky. Our piece on how canopy and leaf-out change shade through the year works through this seasonal swing in more depth, combining it with how deciduous leaf-out changes the coverage percentage itself.

Three ways to measure "shade," and why none of them alone is complete

Across this site you'll run into three related but distinct ways of describing a spot's shade: canopy coverage percentage (geometric, from a tree's crown size relative to your bed), Daily Light Integral category (from an actual light-meter reading, capturing both intensity and duration), and the three-tier partial/full/deep classification the shade plant selector uses to filter plants. They usually agree in broad strokes, but they're not interchangeable, and each answers a slightly different practical question: coverage percentage is useful for planning root-safe zones and mulch coverage under a tree; DLI is the most honest measure of how much light a plant actually gets; and the three-tier system is what most published plant hardiness data was written against. Our shade level reference ties the three together in one table, driven by the same functions each individual calculator uses.

Using the number practically

Beyond plant choice, canopy coverage has a couple of other direct uses worth knowing. It's a reasonable starting estimate for how much ground a mulch ring under a tree needs to cover, and it's a useful sanity check when planning where a new bed's edge should sit relative to an existing tree — a bed that only captures 20% canopy coverage is mostly living in the open, sunnier conditions past the tree's real influence, no matter how "under the tree" it might look on a rough sketch of the yard.

Measuring your own tree's crown diameter

Getting a usable crown diameter doesn't require anything fancier than a tape measure or a rolling measuring wheel. Stand at the tree's trunk and walk out to the canopy's drip line — the point on the ground roughly beneath the outermost leaves — in two or three different directions, since most real canopies aren't perfectly circular, and average the results. A lopsided canopy, common on a tree growing close to a building or another large tree, is worth measuring in its longest and shortest directions separately rather than forcing a single average, since the calculator's circular-area assumption will understate coverage on the long axis and overstate it on the short one if the shape is meaningfully oval rather than round.

What a young tree's canopy tells you about the future

If you're planning a bed around a young or recently planted tree, it's worth running the coverage numbers twice: once for the tree's current crown diameter, and once for a reasonable estimate of its mature spread, available from most nursery listings or a regional extension service's tree database. A newly planted shade tree with a 6-foot crown might barely register on a modest bed today, while the same species at a mature 30- or 40-foot spread could eventually cover the entire yard. Planning the bed's plant choices around where the shade is heading, not just where it is now, avoids having to redo a planting in five or ten years once a young tree's canopy has finally caught up to the bed underneath it. It's a small bit of extra homework that saves a full bed redesign later.