Lemon verbena

Light and exposure

Lemon verbena wants full sun, no compromise. It is in the sun that its leaves load up with essential oils and develop that so distinct lemon scent. Give it the warmest and brightest spot in the garden or on the terrace, against a wall that holds the heat if possible.

A corner that is too shady gives a weak plant, with soft stems and a disappointing scent. In a pot, turn the container from time to time so the shrub develops evenly, and choose from the outset the sunniest position you can offer it over the growing season.

Watering

In full growth, from spring to autumn, verbena likes regular watering but never waterlogged soil. Let the top few centimetres of the substrate dry between waterings, then water generously. In a pot and in strong heat, that can mean water every two or three days, more during a real hot spell.

In winter, everything changes. Once the leaves have fallen and the plant is at rest under shelter, cut watering back very sharply: the substrate must stay just barely moist, almost dry. A rootball soaked with water on a leafless, cool plant means guaranteed root rot. It is the most common mistake of overwintering.

Soil and repotting

Lemon verbena needs a light and above all very well-drained soil. A mix of quality potting soil lightened with coarse sand or perlite suits it well in a pot. It dreads stagnant moisture around the collar and roots, even more under our rainy climate.

In open ground, in the rare cases where the gamble is taken, a heavy, clayey soil as often found in the region is frankly unfavourable to it. You then have to lighten the planting hole generously with sand and compost, and play a raised position so the water runs off. Drainage comes before the richness of the soil.

Temperature and humidity

This is the point that governs its whole cultivation here. Lemon verbena is frost-tender: it takes a few short, light frosts, of the order of minus 5 degrees on a well-established, mulched specimen, but a classic Belgian winter, damp and prolonged, gets the better of it in open ground in the Herve area. The combination of cold plus damp is far more deadly than dry cold.

The solution that works is overwintering frost-free, between 2 and 10 degrees: an unheated conservatory, a bright garage, a cool stairwell, a cold greenhouse. It spends the winter leafless, dormant, and comes back from its stems or from the stump in spring. No need to heat it, it is the rest in the cool that puts it in reserve for the following season.

Pruning and upkeep

The main pruning is done in spring, and not in autumn. Wait until the new shoots come away clearly, often in April or May here, to see which wood has restarted and which is dead. You then prune firmly above the young shoots, removing all the dry, brittle wood, to restore a compact shape and force branching.

Resist the urge to prune in autumn. A plant cut back before winter faces the cold weakened, and you do not yet know which stems will have held. A cleaning cut at the right moment, as growth restarts, gives a dense shrub, generous in fragrant leaves. The fresh trimmings can, moreover, be kept to flavour an infusion.

Pests and diseases

Well grown, in the sun and in drained soil, lemon verbena is little prone to disease. Its number one enemy is not a fungus but excess winter water: root and collar rot, caused by a soaked substrate on a resting plant. Prevention lies in watering almost cut off during dormancy and a pot that never sits in its saucer.

On the pest side, watch for red spider mites during an overwintering in a dry, warm atmosphere, they sting the foliage and grey it, as well as aphids and sometimes scale insects on the plants brought in. Good airing of the overwintering room and a regular inspection of the undersides of the leaves are generally enough to keep them in check.

Propagation

Cuttings are the prime method, and they are quite easy in summer. In June or July, take cuttings of semi-ripe stems ten to fifteen centimetres long, remove the lower leaves, and plant them in a light, moist mix, under cover beneath a bag or a mini-greenhouse to keep the air moist. Rooting takes a few weeks in the warmth.

You can also try the cutting in a glass of water, less reliable but fun to follow. Sowing exists but stays slow and uncertain, leave it to the nurserymen. A cutting that succeeds in summer ideally spends its first winter well protected from frost, in a pot, because a young plant is even more fragile than an established one.

Month-by-month care calendar

Green Hub tips

• In winter, a bare, leafless verbena is not dead: it is sleeping. Scratch a stem, if it is green underneath, it is alive and will restart in spring.
• During overwintering, the enemy is water, not the moderate cold. Water barely, a soaked substrate on a resting plant rots the roots for sure.
• Prune only in spring, once the young shoots are away. You will then know which wood is alive and which to cut, and you will not weaken the plant before winter.
• Pick the leaves in the morning, in the sun, when their lemon scent is most concentrated. Dried in the shade, they keep their aroma for winter teas.
• Do not confuse it with common vervain or with the annual bedding verbenas: only this South American shrub carries that true lemon scent.