Measure direct-sun intervals at the exact place a plant’s leaves will grow. A bright balcony can still spend much of the day in shade. A simple light map helps you choose crops before buying them or moving heavy pots.
Add up your direct-sun intervals
Enter periods of direct sun at one position on one day. Leave shaded gaps out. Overlapping entries count once. Start with your own times, or load the clearly labeled example.
Add the sunny intervals you observed at one position on one date.
Times are local clock times on the same day. This adds elapsed direct-sun time; it does not measure intensity, account for cloudy-day light, or estimate a crop’s yield. Inputs stay on this page and are not saved or sent to our server. Download the record before leaving.
Choose one measuring point
Mark a spot where you intend to put a container. Note the planned leaf height and whether a roof, wall, railing, tree, or neighboring building blocks it. A chair beside the door and a pot at the outer edge may receive different light even though they share a balcony.
Observe on a reasonably clear day to see the fixed shadows. If clouds prevent a useful reading, record that limitation and repeat. The aim is an estimate of your site’s exposure, not a claim to measure the amount of light available in every weather condition.
Record separate sunny intervals
Check through the day, more often around transitions between sun and shade. Write down when direct sunlight begins and ends instead of recording only “sunny at noon.” If you miss a transition, mark the time as an estimate. A phone photograph from the same position helps you compare the edge of a moving shadow.
| Illustrative interval | Direct-sun time |
|---|---|
| 9:00–11:00 a.m. | 2 hours |
| 11:00 a.m.–2:00 p.m., shaded | 0 hours |
| 2:00–4:30 p.m. | 2.5 hours |
| Total observed direct sun | 4.5 hours |
Use the calculator above or copy the table with your own times and date. The calculator sorts intervals and combines overlaps before adding their duration, so a repeated 10:00–11:00 entry cannot inflate the total. The download retains your original entries and the intervals counted once. Keep separate sheets for two candidate positions. If one gets four hours and another gets seven, you have a useful placement decision; averaging them into “the balcony gets five and a half” hides that difference.
Match the result to the crop and season
Maryland Extension gives a starting range of 6–8 direct-sun hours for warm-season fruiting vegetables and 3–5 for cool-season greens. Use those ranges to narrow choices. A leafy crop may be worth trying in a position that is a poor fit for a tomato, provided the season also suits it.
- For a sunny warm-season position, check the footprint and support needed by a compact tomato or pepper.
- For a position with fewer direct-sun hours, consider a small cool-season trial of lettuce or spinach.
- If the measured light falls below the guide’s range, start with a small experiment and modest expectations, or choose a better location before buying plants.
Hours are a useful first filter, not a complete measure of plant performance. Four cool morning hours and four hot afternoon hours are different environments. Note reflective paving, a hot wall, and drying wind alongside the light record. Afternoon shade can ease heat exposure for a cool-season crop; it cannot guarantee a summer lettuce harvest in extreme heat.
Recheck when the season or layout changes
Date the worksheet. The sun’s path, tree cover, and your own taller plants can change exposure as the season progresses. Repeat the observation before a fall sowing or after rearranging containers. An early-summer reading should not be treated as a permanent property of the balcony.
For an easy comparison, keep the same measuring point and camera angle. Record “tomato cage now shades the lettuce after lunch” rather than simply deciding the lettuce is doing badly. Then try a reachable position with better exposure and review its water needs after the move.
Make a placement decision you can use
Finish with one sentence: “This position has roughly four and a half hours of direct sun on the date observed; I will check a cool-season leaf crop first.” Add the crop’s local planting window and pot dimensions. Our crop finder can narrow the list, while the one-pot starter plan helps turn the choice into a complete setup.
Avoid solving a light problem by putting a heavy pot on an unsupported railing or blocking access. Keep plants in a stable, permitted position you can reach to water and inspect. If the available spot does not suit the crop you want, that information saves both effort and money before planting day.
Updated October 18, 2026: Added an interval calculator with overlap handling, a daily timeline and a downloadable observation record. The manual worksheet and original publication date are retained.
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Sources & further reading
Extension resources support the growing principles in this guide. Planning examples apply them to small container gardens. Local conditions and variety instructions take priority.
Prepared with AI assistance; this is research-based guidance, not a personal growing trial. Editorial policy · Suggest a correction