Light Conditions
Determining your light conditions is one of the most important factors for determining which plants will thrive in specific locations. This can be challenging since, unless you have a wide-open space, conditions change throughout the day. Sun angles also change substantially throughout the seasons and the shade from trees also changes as they grow and decline. The simplest approach is to spend time in your landscape observing how light conditions change throughout the days and seasons. You can make some rough estimates and plant accordingly to see what thrives and what may need transplanting to a more suitable location.
Shade Mapping
If you prefer a more precise and timely approach you can make a shade map for your yard. The article linked here from Treehugger.com has some helpful tips for this. Otherwise, there are some web-based tools that can help.
Photopills
We are particularly fond of a mobile app called Photopills which uses your phone's camera to create an augmented-reality view of the sun's path marked in hour increments so you can see exactly how much sun reaches any location at any time of year. www.photopills.com
Sunlight Terminology
The descriptions of light conditions vary somewhat depending on the source. We typically refer to the descriptions from the Missouri Wildflowers Nursery:
"When plants are in more intense light, they can capture larger amounts of energy and produce more flowers and seeds. Many plants that naturally grow in shade are damaged by mid-day, summer sun, yet need mid-day sun in spring (before trees leaf out) to flower and set seeds."
Sun:
An area that receives six hours or more of direct, mid-day sun in summer, including the south or west sides of buildings and trees.
Sun to light shade:
Includes all of the conditions listed for sun plus the east and north sides of buildings and trees where the plants are exposed to considerable open sky, particularly overhead.
Light to medium shade:
Includes dappled shade and bright areas slightly under trees (open sky not directly overhead).
Shade:
Dappled shade with a high or low canopy, and also dense shade. Areas that get morning and evening sun in summer, but not hot mid-day sun. Areas on the north sides of buildings and under trees where the sun doesn’t shine.
Expect better flowering and faster growth in brighter areas.