The term "full sunlight" is defined as six or more hours of direct sunlight per day. In the case of drought-tolerant spirea shrubs, however, a general rule to follow is: The more sunlight, the better. A spirea may grow in shade, but it will not grow well; it will develop a loose, leggy shape, and its blooming will be greatly reduced.
Other Cultural Requirements
Although full sunlight is important for spireas, so are other environmental factors. The shrubs grow best in rich, moist soil, according to a Fine Gardening website article. They tolerate poorer soil conditions, however. Spireas grow very quickly and need to be pruned, which they tolerate very well. Like many other flowering shrubs, some Spirea species bloom on the previous season's new wood. Those species should be cut back soon after their flowers fade. Shrubs that bloom on the current year's new wood should be pruned as their buds begin to develop in spring.
Spireas range greatly in size, but they all can be distinguished from other kinds of shrubs by their fine, small leaves and slender branches. Spireas grow in one of two forms: an upright shrub with gracefully arching branches tipped with white flowers or a low-growing, spreading shrub. The species and cultivars that grow in the former form feature white flowers and often are called "bridal wreath" spireas. Low-growing, spreading spireas have pink, red or white flowers.
Varieties of spireas with yellow leaves are the best performers in shady conditions, according to the Bachman's website. They include Japanese spirea (Spiraea japonica "Goldflame"), which features pink flowers and golden foliage that deepens in fall. This colorful spirea is hardy in USDA zones 4 through 8.