Despite the shrub's fresh citrus scent, with verbena (Aloysia triphylla), also called lemon verbena, sometimes more is less. When the herbaceous shrub, which thrives in U.S. Department of Agriculture plant hardiness zones 8 through 10 and as an annual elsewhere, shoots up beyond its expected 12 to 36 inches -- it regularly tops 6 feet above USDA zone 8 -- the plant splays open unattractively and you don't get as many of the lemony leaves as with a properly pruned verbena. Cut your lemon verbena back regularly to keep its size in check, and the plant bushy and fragrant.
Spray the blades of your bypass pruners or loppers with household antiseptic cleaner, and wipe them dry with paper towels to decrease the chance of spreading any disease or fungal spores they may carry from plants previously pruned with the tools.
Cut an excessively tall verbena back to a few buds from the base of each stem in mid spring, after all chances of frost have passed and buds are starting to swell or new leaves push out. Make the cuts at a 45-degree angle, about 1/4 inch above a live bud.
Remove any dead stems at the base. Lemon verbena stems can die back to the ground over winter.
Pinch off the tips of new shoots once they have put on a few inches of growth to encourage bushy, rather than lanky growth.
Reach inside the plant at midsummer, removing congested and crossing stems at the base, concentrating on the longest stems.
Cut the branch tips off one more time -- cutting back to a lateral stem -- if the plant is still too tall and you don't mind missing the panicles of small light purple flowers the plant produces in late summer.