Lemongrass

Identification and origin

Cymbopogon citratus belongs to the family Poaceae, the grass family. It is considered native to southern India and Sri Lanka, but no wild form is known: it is above all a cultivated plant, grown throughout the tropics and widely farmed in Southeast Asia for Thai, Vietnamese, Laotian, and Cambodian cooking.

In warm climates it is an evergreen perennial that forms a large clump from a short rhizome. Stiff stalks rise from swollen, bulb-like bases and carry long, narrow, arching leaves, light green to blue-green, that droop at the tips. The leaves reach up to 3 ft long and 1 in wide and release a strong lemon scent as soon as they are bruised. In fall and winter they can take on red tones.

In a container in a temperate climate, the clump usually grows 2–4 ft tall and 2–3 ft wide. In ideal tropical conditions it can reach 5–6 ft. Outside the tropics it very rarely flowers.

The genus name comes from the Greek kymbe, “boat,” and pogon, “beard,” for the shape of the flower clusters; the epithet citratus refers to the citrus scent. Do not confuse it with East Indian lemongrass, Cymbopogon flexuosus, which is slightly more compact and sometimes sold under the same name.

Light

Full sun is the key to vigorous growth: at least six hours of direct sun a day during the warm season. It tolerates light shade but grows less there. In summer, set the pot outdoors in a warm, sunny spot sheltered from wind.

Indoors in winter, place it in the brightest room you have, next to a sunny window. The low light of the season slows its growth: that is normal, and a plant brought inside produces fewer stalks than one grown outdoors.

Watering

As a plant of humid tropical climates, it likes steady moisture. In a container, check the mix often, especially in hot weather, and water as soon as it starts to dry so that it stays evenly moist. Never let it dry out, but avoid soggy soil just as much: water must drain freely.

Water at the base rather than overhead to limit foliage diseases. In a dry climate, misting the leaves helps the plant.

Indoors in winter, growth slows: water sparingly, letting the surface dry between waterings, but never let the root ball dry out completely.

Potting mix

It likes rich, well-drained, moist soil with plenty of organic matter. In a container, a peat-free multipurpose mix works well. It tolerates poor soil as long as water and drainage are provided, and a wide pH range, from acidic to alkaline.

Like other grasses, it uses a lot of nitrogen in summer. For a potted plant, mix a slow-release fertilizer into the mix when growth resumes in spring, or apply a balanced liquid fertilizer every two to three weeks until late summer.

Temperature

It grows best between 77 °F and 86 °F and survives between 50 °F and 91 °F. It needs at least 55 °F to grow well. Move it outdoors only in early summer, after the last frost, getting it used to outdoor conditions gradually.

Bring it inside in fall, before nights drop below 45 °F, and keep it in a bright place that stays above that mark: an unheated greenhouse is usually too cold. The top growth stays evergreen in USDA zones 10 and 11; the roots can survive down to zone 8b.

Repotting

In early summer, choose a pot slightly wider and deeper than the current one, with drainage holes. Set the root ball in the center at the same level as before, fill in with mix, firm lightly, and water well.

An older clump also benefits from being divided: it then produces more new stalks. In the ground, space plants at least 12 in apart; in good conditions, a clump can spread 3 ft wide.

Pruning

Remove damaged or faded leaves as they appear. Then cut all the leaves back to 2–4 in from the base, either in fall if indoor space is short or in early spring when growth resumes.

To harvest, choose stalks with well-swollen bases at the edge of the clump, at least 1 in thick. Cut or pull them off at ground level, starting with the oldest, which encourages new stalks to form. Keep the bottom 4 in, the most flavorful part, for cooking; the tougher leaves can be used for tea.

Propagation

Division is the simplest method. In spring, slide the plant out of its pot and cut the root ball with a knife or spade into two or more pieces, each with good roots and several strong stalks. Pot each piece separately and water well. Before frost, you can also save sections of swollen stalk bases with roots attached and pot them up for the winter.

Seed is possible, although it is hard to find. Sow in early spring on the surface of seed-starting mix, cover with a thin layer, and keep at 68–77 °F, slightly moist and in good light. Move the seedlings on once roots show through the bottom of the cell.

Fresh stalks bought at a grocery store can root in a glass of water in about two weeks, but this practice risks introducing diseases: nursery plants are a better choice.

Common problems

• Small yellow spots, brown streaks, dark brown pustules on the leaf undersides: rust, encouraged by water splashing on the foliage; remove affected parts and water at the base.
• Reddish-brown spots on leaf tips and margins: leaf blight.
• Foliage finely stippled with yellow or white: spider mites, especially indoors; hose off the foliage or use an insecticidal soap.
• Tiny white flies under the leaves: whiteflies, common in greenhouses.
• Slow growth in winter: low light, normal for the season.

The same symptom can have several causes: also check light, watering, and temperature before drawing conclusions.

Toxicity

Toxic to dogs, cats, and horses. The plant contains essential oils and cyanogenic glycosides: in dogs and cats, eating it causes stomach upset; in horses, it can cause difficulty breathing, weakness, and, rarely, death. For people it is a common culinary herb; just watch out for the mature leaves, which are sharp: wear gloves to handle them. Keep pots out of reach of pets and, if a pet eats some, contact a veterinarian.