I don't have personal experience with this plant, but you might find something of value at www.monrovia.com; I've given the specif link for Spiraea japonica 'Shirobana' in the "Sources" section. When you get to the page for your plant you'll see a list of sections. Both "Care Instructions" and "Growth Conditions" may have information of interest. Also, I've provided a link to a national directory of Extension Service offices. Though they deal primarlily with agriculture, it's worth asking the one closest to you if they have information on ornamentals, or if any of their reps have advise for you.
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