Answer: It is in fact possible to divide a spirea, although it is a huge job to dig the entire thing up. Instead, you might try taking sections of younger and newer sprouted stems (along with their roots) from the outer edges of the shrub. To do this, water the shrub thoroughly the day before and use a sharp flat spade. Then identify a likely clump of stems, cut down straight next to the clump on the side toward the shrub, but take a nice root ball from the outer side. Leave the main part of the shrub intact and fill in the resulting hole with compost or topsoil. Transplant the rooted section immediately into a prepared hole, water it in well, mulch it and care for it as you would any new plant. You might trim it back by about half to compensate for the root loss when digging. This would be best done in very early spring.
Another simple method would be to take tip cuttings or to layer some branches, but these both take much longer to achieve a large plant. Enjoy that spirea!
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