Plants grown outside their native territories often need special care, and the South American native lemon verbena (Aloysia triphylla) is an excellent example. North American home gardeners who try to grow this tender, wonderfully fragrant perennial herb may succeed more often than they fail, but a large number who try year after year eventually develop a deep sense of frustration and guilt when they repeatedly “commit herbicide.”
Although our research doesn’t claim to have solved the mystery or to offer a sure-fire formula for success, it has uncovered a few misconceptions about the plant and a lot of solid advice. Those who have succeeded with lemon verbena agree that the plant is worth a bit of effort, and we hope that discouraged verbenaphiles will give it another try, armed with solid information and a positive outlook.
Profile of the victim
Lemon verbena is one of more than 30 species of aromatic shrubs in the genus Aloysia (family Verbenaceae), all native to the warmer parts of North and South America. Its botanical name has undergone a cycle of change in the two centuries since it was introduced to England as Verbena triphylla. Its lemon scent was the source of an alternative name, V. citriodora (verveine citronelle in France). A Spanish researcher assigned it to the genus Aloysia (named for Maria Louisa, wife of King Charles IV of Spain) because its fruit separates into two nutlets, whereas the fruits of Verbena species separate into four. The plant was known as A. citriodora until it was moved again in the early nineteenth century, this time to the genus Lippia (named for Lippi, an Italian botanist). Though lemon verbena is sometimes still offered as L. citriodora, it has long been reassigned to the genus Aloysia, this time as A. triphylla. The species name describes the characteristic whorls of three leaves that form along the stems. Occasionally, however, the whorls consist of four leaves, sometimes on the entire plant and sometimes just on certain stems.
Lemon verbena grows best in loose, well-drained soil that’s rich in organic matter, and drainage is the more important of those two characteristics. Neither clay nor very acidic soils are hospitable to lemon verbena; a lot of sand and a little lime, respectively, seem to be the best remedies. Though moisture-retentive soil is often recommended, lemon verbena will rot if its roots are constantly wet.
Given adequate drainage, lemon verbena can tolerate a wide range of watering regimes. To err on the dry side seems to be most advisable, but observation and familiarity are the best tools for determining water needs. If you live in a climate colder than Zone 9 and plan to winter your lemon verbena outdoors, you should withhold water as freezing weather approaches so the plant can harden off and so the roots will not be wet when they freeze. The plant will need little to no water while it is dormant, whether it’s kept indoors or out.
Fertilize lemon verbena as you would any other herb plant: as often as every two weeks indoors or every four weeks in the garden when the plant is growing vigorously, less during periods of slower growth and not at all during dormancy. In spring, following winter dormancy, some gardeners apply fish emulsion or other fertilizer to encourage growth to begin, but others question whether it’s the fertilizer or just the water that stimulates the process.
In more northern regions, lemon verbena thrives in full sun; even better is a site in the reflected light of a white fence or greenhouse wall. Closer to its native latitude near the equator, it grows better with at least some shade during part of the day.
Cultivated lemon verbena flowers and sets its two-seeded fruit most dependably in southern zones where the growing season is long, or in the more northern zones under lights. Flowering depends not only on the length of the growing season, but also on stem length, and gardeners who tend to prune lemon verbena fairly hard probably will not see many flowers. The blossoms are small, numerous and white to pale purple, clustered along the last few inches of the main stem and on short stems in the leaf axils.
For many herbs, pruning stimulates the emergence of new growth at several points along the remaining stem, but lemon verbena responds mainly at the whorl of leaves immediately below the cut. This habit gives the topiary gardener quite a bit of control, but it also means that frequent, severe pruning is required to keep the plant from becoming inordinately leggy and to increase foliage production (not to mention keeping the drying racks full).
Leafless, but not lifeless
Almost without exception, gardeners growing lemon verbena for the first time are dismayed when the plant drops all of its leaves, which this herb does with the slightest
provocation. The leafless sticks look so pitiful that many gardeners, thinking the plant has died, toss it onto the compost pile. Some later have discovered the “dead” plant growing vigorously where it was thrown.
Although lemon verbena often loses its leaves and becomes dormant when the days become short, it grows year round in its native locations, where day length is virtually unchanging, and dormancy does not seem to be a requirement for its health and longevity. When temperatures fall significantly below freezing, the leaves often are damaged and eventually fall off. Some sources indicate that freezing temperatures alone can trigger dormancy, but Thomas DeBaggio of Arlington, Virginia, has found that a frozen plant that is brought indoors and placed under lights to simulate summer day length will continue to produce new growth after the damaged leaves have dropped off. His experience has convinced him that day length is the main factor that triggers the metabolic slowdown of dormancy.
In plants that are wintered indoors, sudden leaf loss frequently appears to be a reaction to a rapid temperature change or root disturbance. It can be triggered by bringing a potted plant indoors in late summer or after the first frost, by digging up a plant from its summer garden spot and potting it for indoor winter growing, by transplanting a small plant into a larger pot or simply in response to a strong, cold draft.
Many gardeners grow lemon verbena in pots so that it will be easy to move indoors and out as the weather dictates. This is convenient and avoids annual transplant shock, but a pot differs from the ground in that the soil inside the pot changes temperature much more quickly—another source of shock. Choose a pot at least 12 inches in diameter to allow the roots ample growing room and to limit the effect of short-term air-temperature changes on soil temperature. This effect can be decreased further during the plant’s outdoor sojourn by burying the pot in the garden. The risk in this strategy, however, is that the roots may grow out of the drainage holes in the pot and be broken when the plant is exhumed in early fall. Such root breakage can retard growth significantly and will probably cause leaf loss. It may be helpful to cut such plants back a little to accommodate the loss of roots.
Different sources list different temperatures—from 14 degrees to 22 degrees—below which lemon verbena is not likely to survive. However, Andy Van Hevelingen of Newberg, Oregon, had an uncovered lemon verbena that survived a single night at 3 degrees. Conditions were optimal: the stems were protected from wind, the soil was completely dry around the roots, and the decrease in temperature was gradual over a few weeks so that the plant had time to harden off and become fully dormant. Protection from wind seems to be critical near the edge of the plant’s hardiness range; try wrapping the dormant top with weatherproof plastic foam or burlap or covering it with mulch. Kae Snow-Stephens of Shreveport, Louisiana, covers the small but actively growing lemon verbenas in her garden with plastic garbage bags when the cold becomes threatening, and the plants have not only survived one-night temperatures as low as 22 degrees, but have done so without losing leaves or slowing their growth.
For some gardeners, lemon verbena that winters outdoors is one of the first plants to emerge from dormancy, but others in similar climates report that growth resumes later in lemon verbena than in other perennials. If spring has sprung and you’re wondering whether your lemon verbena will ever come back, you can test for signs of life by bending or clipping off the ends of the dormant woody stems. Dry, brittle wood is dead, but you may find that the stems are alive closer to the base of the plant. One experienced gardener recommends that you resist the temptation to perform such a test because the dead wood protects that which is alive; if your curiosity can survive the wait, the answer will come eventually in the form of new growth (or its continued absence).
Dealing with leaf loss and dormancy
Like other plants, lemon verbena transpires or gives off water mostly through its leaves, and it stands to reason that a leafless plant uses far less water than one covered with leaves. One of the common ways gardeners kill lemon verbena is by overwatering during leafless periods; this is especially easy to do if you’ve been watering on a time schedule. Whatever the immediate cause of leaf loss, watering must be cut back drastically, preferably by reducing the frequency of watering rather than by applying smaller amounts on the same time schedule.
In the northern United States, where lemon verbena is sure to succumb to winter cold outdoors, many gardeners allow the plant to endure early frosts, then bring the leafless plant inside and put it in a cool, dark place to rest for the winter. During this period of indoor dormancy, it receives little or no water. In early spring, the plant is watered, occasionally fertilized and placed in a warm, sunny spot; growth should begin within a couple of weeks.
If your lemon verbena does flower, the chances of obtaining viable seed are marginal, so lemon verbena is usually propagated vegetatively. Those who grow this plant successfully advise taking basal cuttings of the current year’s growth in summer when the plant is growing vigorously. (Such cuttings root fairly easily). If taken in early fall or later, growth slows as the days shorten, and cuttings will take longer to root (which increases the chance of failure) and are less likely to survive transplanting. If you do take cuttings late in the growing season, root them in 2 1/2- or 3-inch pots to postpone the need to disturb the new root systems, and use supplemental lighting, if possible.
Lemon verbena is a favored delicacy of whiteflies and spider mites; many experienced gardeners and commercial growers refuse to have this herb around because they believe it attracts those pests. However, an equal number—many in the same climates—either experience no such problems or find the pests easy to deal with. Home gardeners with just a few plants can combat an infestation of whiteflies or spider mites by spraying the leaves, top and bottom, with insecticidal soap or a solution of dishwashing liquid (1 teaspoon), vegetable oil (1 tablespoon) and water (1 quart) three times at 10-day intervals, rinsing about three hours after application. Indoor plants should be isolated from other houseplants during treatment. Misting the plants thoroughly at least twice a week after treatment is said to discourage mites from recolonizing.
Why we keep trying
The charm of lemon verbena is apparent to anyone who encounters it. It can be a neat bush if kept tightly groomed, or its stems can extend into quite a sprawl. The leaves are a cheery shade of light green. But the great joy of lemon verbena is the sweet, lemony scent that leaps from the leaves at the slightest touch. This fragrance is especially strong at the peak of growth in mid- to late summer.
The most common home use of the herb is in potpourri; the dried leaves can retain their scent for years, and they are commonly available, whole or powdered, from suppliers of potpourri ingredients. Tea made from the dried leaves is a delicious lemony beverage, thought by some to be the best of the lemon-herb teas. In cooking, however, lemon verbena is deceptive; the smell is lemony but the taste is bitter and hot, more like citrus zest than fruit.
The essential oil retains the delightful lemon scent that’s characteristic of the leaves, and the oil is sometimes used commercially in flavoring liqueurs. However, it is difficult to distinguish between lemon verbena and lemongrass essential oils. Lemon oil (pressed from the lemon zest) and the essential oils of lemongrass and lemon balm seem to have cornered the retail market for lemon scent and flavor; most suppliers of herbal products do not offer lemon verbena essential oil.
The herb is rarely mentioned in literature on medicinal plants, perhaps because its medicinal effects are quite mild and can be obtained more easily with other herbs. The pleasant, fragrant tea is said to act as a gentle sedative and has been used in reducing fever, settling stomach upset and intestinal spasms and soothing bronchial and nasal congestion.