Answer: Depending upon variety, bougainvilleas will reach 15'-30' in height. Most will mature to 6'-8' in width. The rule of thumb in spacing plants is to provide each with enough elbow room to reach it's full potential, but close enough that you don't have to wait several years for them to fill in. With bougainvilleas, most growth is up, not out, so the closer you space your plants, the faster coverage your wall will receive. I'd plant them 6' apart and fill the voids between them with a temporary shrub or mass of flowering perennials. As your vines grow outward you can remove the shrubs or perennials to make more room for the bougainvilleas.
Bougainvillea hates to have its roots disturbed. For stress-free transplanting, dig a hole as large as the container, then cut away the bottom of the container, place the container in the hole and carefully pull it up over the top of the plant while a helper places soil in the hole around the roots. You may damage some of the vining stems with this method of planting, but they'll be replaced within a short period of time, and without trauma to the roots, the plant will barely know it's been transplanted.
Good luck with your bougainvillea!
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