Answer: Autumn olive is a large, spreading plant. According to Michael Dirr's "Manual of Woody Landscape Plants" it can reach 12 to 18 feet tall and wide but sometimes runs as much as 20 or 30 feet across, so you will need to prune it regularly to keep it at a manageable size. To encourage dense branching at the base, you need to tip it back frequently while it is still quite short. I am assuming you are looking at an informal type of hedge, so once it has reached the size you need, an occasional moderate to heavy pruning will keep it in bounds. Since it is spiny I doubt you will want to attack it too often!
Rosa rugosa is also thorny and also naturally has a rounded shape. Pruning for this plant is limited to removal of dead wood and occasional thinning (from the base) in order to encourage vigorous new growth. This plants tend to grow about 4 to 6 feet high and wide. Since you will be growing these next to each other, you will need to allow space to get in between them to prune; the actual space will depend on how large you want the autumn olive to be.
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