Lemon balm

Light and exposure

Lemon balm is happy in partial shade, which makes it a precious ally for the somewhat shaded corners of the garden where many herbs sulk. A bright position but without scorching midday sun suits it perfectly: there the foliage stays tender, green and fragrant. In full sun and dry conditions, the leaves tend to yellow and dry out at the edges.

A spot that gets the morning sun, or the foot of a wall that shields it from the strongest part of the day, is ideal. In a pot too it accepts partial shade without flinching. Do not try to give it your best sunny spot: keep that for thyme or rosemary, and give the lemon balm a cool corner you did not know what to do with.

Watering

Lemon balm appreciates soil that stays cool. It bears prolonged drought badly, which makes its foliage wilt then brown. Water regularly in dry periods, especially the first year and for plants in pots, and mulch the base to hold moisture longer.

Once well established in the open ground, it becomes fairly self reliant and often makes do with the rain wherever summers stay wet. It is mainly during summer heat spells that you have to think of it. In a pot, the watch must be more regular because the compost dries fast; a morning watering at the base is generally enough.

Soil and repotting

Lemon balm is not fussy about the nature of the soil. Ordinary garden soil, cool and a little rich in humus, suits it very well. It grows in almost anything, as long as the soil does not stay waterlogged all winter, which could rot the rootstock.

An addition of compost in spring vigorously restarts the clump, but it is not essential, so resourceful is the plant. In a pot, good ordinary compost is enough; provide a fairly wide container, because the rootstock fills out year after year and ends up feeling cramped.

Temperature and humidity

This is where lemon balm shines: it is perfectly hardy and comes through winter without the slightest care. The aerial part disappears at the first frosts, but the rootstock stays well alive underground and starts again the following spring, often with even more vigour. No winter protection is needed in the open ground.

It starts early in the season as soon as the days lengthen and the soil warms. Late spring frosts do not worry an established clump; at most they scorch a few young shoots that start again at once. In a pot, a very harsh winter can freeze the rootball right through: then move the pot closer to a wall or mulch it to secure the rootstock.

Its hardiness goes far beyond what a temperate garden asks of it: horticultural references rate it robust from USDA zone 3 to 7, and it is commonly grown up to zone 9. A rootstock well anchored for at least a season takes cold well below -20 degrees. At the other end of the thermometer, it is dry heat that limits it more surely than cold: in full sun and dry soil the foliage yellows, dries at the edges and loses some of its scent.

Pruning and upkeep

Pruning is the real maintenance gesture for lemon balm, and it comes down to one word: cut back. After flowering, cut the clump fairly short, to about fifteen centimetres from the ground. It then starts again cleanly with new foliage, tender and fragrant, far more pleasant than the old stems that have run up and flowered. This cutting back also relaunches a second wave of harvest.

Above all, cut back before full seeding if you do not want it to self seed everywhere. That is the main point of vigilance for this plant: left to seed, it quickly becomes invasive and spreads throughout the bed. Cutting the flowers in time is the simplest way to keep the upper hand. A second cut at the end of summer keeps the clump low and neat before winter.

Pests and diseases

Lemon balm is robust and rarely diseased. Its main trouble is powdery mildew, that white powdery felt that appears on the leaves at the end of summer, especially when the clump is dense and the air stagnant. It is not serious: cut back the affected clump and the lemon balm starts again cleanly with healthy foliage.

To limit the problem, air the plants, avoid crowding them too much and do not wet the foliage in the evening. Rust can sometimes touch a few leaves without serious consequence. As for pests, the plant is rather peaceful and above all attracts bees, which are wild about its flowers. That, incidentally, is where its name comes from, from the Greek melissa, the bee.

Propagation

Lemon balm is very easily propagated by dividing the clump, the fastest and most reliable method. In spring, when the new shoots appear, dig up an established clump and separate it into pieces with a spade or by hand, each fitted with roots and a few shoots. Replant at once, water, and you have new plants guaranteed to match the parent.

It is also sown, on the surface in spring because the seeds need light to germinate, but emergence is sometimes slow and uneven. Stem cuttings in summer work too. In practice, you rarely need to propagate it deliberately: it self seeds so well on its own that each year you recover young spontaneous plants at the foot of the mother clump, ready to be transplanted elsewhere or given away.

Month-by-month care calendar

Green Hub tips

• Cut the clump back after flowering, before the seeds ripen: it is the key gesture to stop it self seeding everywhere and becoming invasive.
• Divide an old clump in spring to get new plants for free, all identical to the parent.
• If you are short of space, grow it in a pot: it stays just as vigorous but much easier to contain.
• In case of white powdery mildew at the end of summer, cut back without hesitation: the lemon balm starts again cleanly with healthy foliage.