How to Measure Hours of Direct Sun in Your Garden, and What Counts
"Full sun" on a plant tag means six or more hours of direct sunlight a day. Most gardeners guess how much their beds get, and most guess high: a spot that feels sunny because it is bright at lunchtime may be in shade by two o'clock. The fix is simple and costs nothing but a day of glances out of the window. Count the hours, then enter the total in the Sunlight Requirements Guide to see which sun category the spot falls in and which plants suit it.
One point before starting: the guide does not measure anything. It sorts the number you type into one of four categories. The quality of the answer depends entirely on the quality of your count.
What counts as direct sun
Direct sun means the sun itself is visible from the spot, at plant height, with nothing in between. A practical test: if a hand held flat over the bed casts a crisp, dark shadow, that is direct sun. If the shadow is faint and fuzzy, the light is filtered or reflected. Bright shade under a high canopy can feel sunny but does not count toward the hours a fruiting crop needs.
Never look at the sun to check. Look at the ground and the shadows instead. Staring at the sun, even briefly or through sunglasses, can damage your eyes, and children helping with the log should be told the same.
Measure at the height the plants will grow. A low lettuce bed can be shaded by a fence that a tomato plant would grow above by midsummer.
The one-day sun log
- Pick a clear day in the growing season, ideally close to when the plants will be in the ground.
- Mark the spots you want to measure with a stake or a stone, and give each a name.
- Check each spot every hour from shortly after sunrise to near sunset. Note "sun", "shade" or "dappled" against the time. A phone reminder every hour makes this easy.
- Add up only the hours marked "sun". Count dappled light through a thin tree as half at most, or leave it out if you want a conservative number.
- Enter the total for each spot in the guide.
If you cannot be home all day, a few photos from the same place on a timer, or a neighbor's help, fills the gaps. Checking every hour is enough; plants do not care about the minutes.
Turning the log into a category
The guide uses four bands: full sun is 6 hours or more, partial sun is 4 up to 6, partial shade is 2 up to 4, and full shade is under 2. Here is how several real-looking logs come out:
| Log for the spot | Hours entered | Category in the guide |
|---|---|---|
| Sun from 9 until 1, then shade from the house | 4 | Partial Sun |
| Sun 8 until 2, shade from a tree, sun again 4 until 5 | 7 | Full Sun |
| Sun only from 10:30 to noon through a gap in the fence | 1.5 | Full Shade |
| Sun 11 until 1 | 2 | Partial Shade |
| Sun from about 7:30 until 1:24 | 5.9 | Partial Sun |
| Sun from 7 until 1 | 6 | Full Sun |
The boundaries are sharp. A spot with 5.9 hours lands in Partial Sun and one with 6 lands in Full Sun, although the plants would barely notice the difference. If your total sits within half an hour of a boundary, treat it as the lower category for crops that need a lot of light, such as tomatoes, peppers and squash, and do not be surprised if they produce less than in a sunnier bed.
The guide accepts numbers up to 24 in the box but caps the hours at 14 before sorting, since no garden gets more direct sun than that in a day, and it now says so on the result. Type 16 and it returns Full Sun with a note that it has counted 14 hours, plus a reminder to count only hours when the sun itself reaches the spot. If you see that note, recheck your log: you have probably counted bright daylight as sun.
Morning sun and afternoon sun are not equal
The guide counts hours, not which hours. Four hours of morning sun and four hours of late-afternoon sun both read as Partial Sun. To a plant they are different. Morning sun is cooler and dries dew off the leaves early, which helps keep fungal disease down. Afternoon sun in summer is hotter and harder on lettuce, spinach and other cool-season crops, which can scorch or bolt to seed. Note in your log which part of the day the sun arrives, and use it as a tiebreaker: cool-season greens and shade-tolerant ornamentals prefer the east-facing, morning-sun spots; heat lovers prefer the west and south.
The sun moves through the season
A single day is a snapshot. In the Northern Hemisphere the sun rides higher and stays up longer in early summer, and sits lower in spring and fall, so shadows from buildings, fences and evergreens are longer early and late in the season. Deciduous trees add another twist: a bed that gets full sun in April before the leaves open can be in deep shade by June.
For a bed you will plant in spring and harvest in late summer, a log in late spring and another in midsummer give a much better picture than one reading. If you can only do one, do it during the main growing months rather than in early spring.
From hours to a plant list
Once the guide has placed a spot, it shows a short list of plants for that category. For full sun the list includes tomato, pepper, squash, cucumber, bean, corn and herbs like rosemary and lavender. Partial sun suits lettuce, carrot, beet, pea, broccoli, cabbage, spinach and parsley. Partial shade suits kale, chard, arugula, mint, cilantro, radish and leaf lettuce. Full shade is mainly for ornamentals such as hosta, fern, astilbe, coleus, begonia and impatiens.
For the edible side of that question, including what to expect from greens in two to four hours of sun, see what vegetables grow in partial shade. If you are planning a brand new bed, combine the sun log with a soil check and a spacing plan, as in before you buy plants: measure sun, space and soil.
A word on the shade list: some popular shade plants are toxic. Lily of the valley is poisonous to people and pets if eaten, and hostas can upset dogs and cats that chew them. In a garden where children or pets play, choose and place these with that in mind, and never eat any plant you have not positively identified.
What the guide does not cover
- Measuring. It does not detect light. It sorts the hours you enter.
- Light intensity. An hour of sun in a hot, high-altitude summer is stronger than an hour in a cool, cloudy coastal one. The guide treats them the same.
- Time of day. Morning and afternoon hours count equally.
- Seasonal change. One number stands for the whole season. Re-measure if trees leaf out or the sun's path changes the shadows.
- Indoor light. Window glass and distance from the window cut light considerably. For windowsills, see growing herbs on a windowsill.
Sun also changes water use: a full-sun bed dries faster than a shaded one with the same soil. Once you know a spot's hours, the Watering Schedule Planner helps set its weekly water.
Checklist
- Mark the spots and log sun, shade or dappled every hour on a clear day.
- Count only crisp-shadow direct sun, at plant height.
- Note whether the sun comes in the morning or the afternoon.
- Enter each total in the Sunlight Requirements Guide.
- Repeat in midsummer if trees or buildings shade the bed.