Square-stemmed willowherb
Epilobium tetragonum · Onagraceae
The gardener's calendar
Ideal time to plant square-stemmed willowherb: set it out when the soil is neither frozen nor waterlogged, and water generously as it establishes.
ID card
- Scientific name
- Epilobium tetragonum
- Family
- Onagraceae
- Type
- Perennial herb
- Foliage
- Deciduous
- Flowering
- July to September
- Adult size
- 30 to 80 cm in height
- Hardiness
- Very hardy (down to -20 °C)
- Origin
- Europe, temperate Asia
Care
Exposure
Square-stemmed willowherb prefers a sunny or semi-shaded position. Good light encourages generous flowering and a more compact form. However, it tolerates partially shaded areas, particularly in warm climates.
Soil and drainage
This plant adapts to most soils but prefers well-drained, slightly moist soil. It accepts poor and infertile soils, making it ideal for natural gardens and slopes. Avoid permanently waterlogged conditions.
Watering
Water regularly during the growing season, especially in dry periods, to keep the soil slightly moist. Once established, the plant is fairly drought-tolerant. In autumn and winter, reduce watering.
Fertilizing
Moderate feeding in spring is sufficient for this undemanding herb. A balanced or slightly phosphorus-rich fertilizer promotes good flowering. Avoid excess nitrogen, which would encourage foliage at the expense of flowers.
Pruning and maintenance
Deadhead regularly to extend the flowering period. In late winter or early spring, cut the clump back to 10-15 cm from the ground to rejuvenate the plant and encourage vigorous growth. This hard pruning is beneficial each year.
Pests and diseases
Willowherb is generally very robust and rarely troubled by pests. In excessively humid conditions, watch for powdery mildew or root rot. Good air circulation and adequate drainage prevent most problems.
Did you know?
Square-stemmed willowherb owes its scientific name 'tetragonum' to its characteristically square (or tetrahedral) stems, which clearly distinguish it from other species in the genus. Its small pink or purple flowers are magnificent when examined under a magnifying glass, and the plant has naturalized itself in many temperate regions, sometimes considered pioneering on disturbed ground, but never invasive.
Where to buy it
Plant in spring or early autumn, avoiding spells of intense heat. Pick a plant with healthy foliage and roots that are not exposed. Best time: March, April, May, September.
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Tips generated and checked by Viridia — adapt them to your local climate.