Answer: Hesperis is also known as Dame's Rocket or Sweet Rocket, for obvious reasons. They are woody upright perennials, growing 1'-4' tall. To keep them neat and trim, you can cut back the flowering stems when the blossoms fade. Hesperis is a short-lived perennial and usually needs to be replaced every few years, so your problem may be short-lived, as well. You can allow the plants to go to seed, then transplant the seedlings to a more suitable place, or you can wait until early fall and transplant the mature plants to a better location.
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