Sunlight Calculator

What the labels on seed packets actually mean

Seed packets and plant labels use four terms, and they are worth learning exactly, because mixing them up is the most common reason a plant underperforms. Full sun means six or more hours of direct sun a day — the sun actually shining on the plant, not just bright daylight. Part sun means four to six hours of direct sun, preferably in the morning, with shade in the hottest part of the day. Part shade means two to four hours of direct sun, or dappled light for much of the day. Full shade means less than two hours of direct sun, though the spot may still be bright. Bright indirect light, the term used for most houseplants, means a bright room where the sun never shines directly on the leaves. The difference between direct sun and bright light is real: a plant in bright shade can be in a very light place while receiving almost no direct sun at all.

How to measure a spot honestly

The only reliable way to know how much sun a spot gets is to watch it. Check the spot in the morning, at midday and in the afternoon on a sunny day, and note when the sun arrives and when it leaves — this calculator’s time mode is built for exactly that. Do it in the season you plan to plant, because the sun’s path changes through the year: a bed that gets six hours of sun in June may get three in March, when spring crops are going in, and a south-facing wall that bakes in summer can be in shade in winter. Trees matter too — a spot under a bare tree in early spring can be full sun, and deep shade once the leaves open. South-facing spots (in the northern hemisphere) get the most sun, east-facing spots get gentle morning sun, west-facing spots get hot afternoon sun, and north-facing spots get the least. For houseplants, the same logic applies to windows: a south-facing window is the brightest, a north-facing window the dimmest.

Matching plants to the spot

Fruiting vegetables are the most demanding: tomatoes, peppers, cucumbers, squash, eggplant, corn and melons all need full sun to produce well, and in our crop data 115 of 120 crops are listed as full sun. Leafy greens are more forgiving — lettuce, spinach, kale and chard will produce in part shade, and in hot climates they actually prefer afternoon shade, which stops them bolting. Herbs split both ways: basil, rosemary and thyme want full sun, while mint, cilantro and parsley tolerate part shade. Root crops such as carrots and beets need full sun for the best roots but will grow smaller roots in part shade. If your spot gets less sun than the plant wants, the result is usually the same pattern: tall, pale, stretched growth, few flowers, and little or no fruit. If it gets more sun than the plant wants, leaves scorch and wilt, and the plant needs far more water.

What to do if the sun is not enough

If the spot is short of sun, you have three honest options. First, choose plants that suit the light you actually have — shade-tolerant greens, herbs and ornamentals will do far better than a sun-lover struggling in the shade. Second, improve the spot: prune overhanging branches, move containers to the sunniest part of the garden, or grow in pots you can move through the season. Third, add light: a grow light can make a windowsill or a dark corner productive, and the grow-light calculator works out how much you need. To check the rest of the spot’s conditions, use the humidity and temperature checker and the watering calculator, and for what to plant and when, the planting calendar and the houseplant care guides.