Answer: Rhus lancea, or African sumac, is an evergreen tree with open, spreading habit, often chosen for landscapes because of the interesting branch pattern. This is generally a slow growing tree so I suspect yours is getting too much water and fertilizer which is encouraging rank growth. If the trees are in or near the lawn, you may want to consider moving them to a drier site. Sumacs can be staked and trained when young to achieve the affect you want. If you want a single trunk, cut away all other stems coming from ground level. If you want a multi-trunked specimen, leave those stems that you want to develop into trunks. Rub out any sprouts along the trunk(s) that grow below where you want the branching to start, and cut away any extra branches (making your cut even with the trunk). African sumac is often grown as a hedge, so you should be able to prune your sumacs back quite a bit without fear of cutting back too far.
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