Lemon Verbena

Check the undersides of the leaves near the veins every week or two for tiny white flecks, then graying or bronzing foliage, and webbing in bad cases. Also look for whitefly nymphs, which sit still and translucent on the leaf undersides while the gnat-like adults fly up when disturbed. Raise humidity, rinse the plant periodically with jets of water, and hang yellow sticky traps if whitefly adults are about. Hot, dry indoor sites are exactly what spider mites want, and heated winter air next to a bright window turns an overwintering lemon verbena into ideal mite habitat; mites and whiteflies both feed hidden on leaf undersides, so an infestation is usually well established by the time the top of the leaf looks wrong.

If rinsing has not cleared them, spray insecticidal soap or horticultural oil, covering the top of the leaf and especially the underside where the pests actually sit. Repeat every four to seven days until they are gone, since both are contact-only materials with no residual activity. For spider mites, reach for the horticultural oil first: Colorado State Extension rates oil 'probably the most effective spray' and insecticidal soap 'marginally effective' against mites. Note that sources differ here — Clemson lists spider mites among the soft-bodied pests most susceptible to soap — so if you already have soap on hand it is worth trying, but oil is the better bet against a real mite population. Soap works well on whitefly nymphs. Soaps kill by disrupting the pest's cell membranes on contact, so anything the spray does not physically wet survives, and the eggs left behind hatch into a new generation within days, which is why one thorough application is never enough.

Wait until the plant actually begins to leaf out, then cut out the branches that stayed bare and shorten the rest to encourage a bushier form. Thin crowded branches while you are in there. Lemon verbena leafs out late and cold weather kills scattered branches over winter, so bare wood in early spring is not reliably dead; letting the buds break first is the only way to tell live wood from dead, and cutting back the survivors forces the branching that carries the season's leaves.

Lemon verbena in a pot is a heavy feeder; use a general purpose liquid fertilizer about every two weeks through the growing season, then taper off as you bring it indoors for winter. Plants in the ground in well-prepared, fertile soil generally do not need this. A heavy feeder in a container exhausts the small volume of media it has, and frequent leaf harvest keeps removing the nitrogen the plant needs to push the next flush; in-ground plants reach far more soil and rarely run short.

Pinch tips and leaves as needed all season. For a big batch to dry, cut branches once the flower buds appear but before they open, harvesting in the morning after the dew has dried but before the day heats up. You can take up to 75 percent of the current season's growth at once as long as plenty of foliage remains. Volatile oil in the leaves peaks just before bloom and drops once the plant shifts its energy into flowering, and picking after the dew dries but ahead of the day's heat avoids both mildew in the drying bundle and the sun volatilizing off the oils that carry the lemon scent.

Below zone 8, bring the pot inside before frost arrives and set it in the brightest window you have. Keep the soil uniformly moist but never soggy, and cut back on fertilizer for the winter. Expect it to drop most of its leaves indoors; that is normal and not a death sentence, so keep watering lightly and wait for spring. Lemon verbena is a frost-sensitive tender perennial that is only hardy outdoors in zone 8 and warmer, and the leaf drop indoors is its deciduous response to short days and lower light rather than a sign the plant is dying.