Why sun assessment matters
Plant tags and seed packets use terms like 'full sun,' 'part shade,' and 'full shade' — but most gardeners haven't actually measured the sun in the spots where they're planting. The result is plants placed in conditions they can't thrive in.
Assessing your yard's sun is simple and only needs to be done once for each area. It will change how you plant and dramatically improve your results.
The standard definitions
Full sun means six or more hours of direct sunlight per day. Most vegetables, many flowering perennials, and nearly all fruiting plants require full sun to perform well.
Part sun or part shade means four to six hours of direct sun per day, often with shade during the hottest part of the afternoon. Many flowering perennials and most edibles can manage in part sun.
Full shade means less than four hours of direct sun. Very few vegetables grow in full shade, but many ornamental plants — hostas, astilbe, ferns — are well-suited to it.
How to measure sun in your yard
On a clear day in the growing season, go outside once an hour from morning to evening and note which areas are in direct sun and which are in shade. Tally the hours of direct sun each area receives. This is your sun map.
Do this on a day when the sun is representative of your typical growing season — not in the middle of winter when the sun angle is very different. If you can only do it once, a midsummer day is the best reference.
Things that change throughout the season
The sun's angle changes significantly throughout the year. A spot that's in full sun in summer may be shaded by a house or fence in spring and fall when the sun is lower in the sky. Deciduous trees also create very different conditions in summer (shaded) versus spring and fall (sunnier).
If you're planting in spring, it can be worth observing how a spot changes as summer arrives before committing to permanent plants.
Matching plants to conditions
The simplest principle: match the plant to the light rather than trying to force a plant to adapt to the wrong conditions. A shade plant in full sun will burn and dry out. A sun plant in shade will grow weakly and fail to flower. Choosing the right plant for your specific conditions is the most important decision in the garden.