Answer: Full sun and partial sun are relative terms. Full sun is full sun all day long or for at least six hours including the hour of noon. Full sun from noon onward will be hotter than full sun all morning. Some plants will tolerate full sun all day long, some will tolerate it only all morning through noon, and some will be able to tolerate afternoon-only sun. In hot summer climates this can be an important distinction. A location with strong afternoon sun is considered a somewhat difficult, hot, full sun location.
Partial sun can mean a variety of situations from sun for the morning to dappled light all day long. If the plant would be a sun-lover but tolerates partial shade, then it does need some direct sun, the more the better. If the plant is a shade lover but tolerates part sun, then it probably will not tolerate any hot afternoon sun and would be happier with just some gentle morning sun or dappled light all day.
Soil moisture often has an effect on the a plant's ability to tolerate sun; a shade lover in a moister soil will tolerate more sun than it could in a drier soil. Heat also has an effect on plants, in that some tolerate a hotter summer climate better than others. (Summer in Oklahoma has higher temperatures than a summer in, say, England or Seattle.) You need to take all of these factors into account when looking at where to site a plant. In many instances, experience is the best indicator of what will do well in a given situation because microclimates can vary so much.
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