Answer: Hemlocks can sometimes be maintained as a clipped hedge, so they may survive for quite some time if trimmed regularly to maintain a steady height. However, if the sudden topping left raw cuts across heavy branches and broad trunk wounds, these will be an open invitation to pests and diseases, and as a result they may not live terribly long. (Topping in general is stressful and also destroys the shape/natural strength and natural defenses of any tree.) Another consideration with hemlocks is that they are being attacked by a pest called the hemlock wooly adelgid over a broad area including the mid-Atlantic region. When these appear, they require frequent spraying to control or else they will kill the hemlock pretty quickly. All in all, I think you will probably be forced to replace them sooner or later. When you do, keep in mind the mature height of the replacement plant so it will fit comfortably under the wires when it reaches its mature height. I'm sorry I can't be more encouraging about the hemlocks for you.
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