Rose of Sharon is not likely to do well on the north side of your garage. The shrub needs full sun (at least 6-8 hours of direct sun a day). Instead of rose of Sharon, consider shade-tolerant shrubs, such as summersweet (Clethra), or a trellis with a shade-tolerant vine. You can buy inexpensive lengths of trellis that provide privacy. Perhaps you can find a sunnier spot in your yard for rose of Sharon. Although you can transplant it anytime the soil isn't frozen, spring and autumn are the best time to move it. Most rose of Sharon plants develop a spread of 6-10 feet. If you plant them 5-6 feet apart they should form a solid screen 8-12 feet tall.