Fathen (Chenopodium album)
Fathen (pronounced fat-hen) is a fast-growing summer annual weed that appears across New Zealand in gardens, bare ground, and occasionally in neglected lawns. While it’s one of the country’s most competitive cropping weeds, it can also invade thin or recently renovated lawns, especially where the soil is disturbed and exposed to light.
Fathen germinates in spring and early summer, grows rapidly through the warm months, and dies back with frost in late autumn. It thrives in nutrient-rich, moist soil and can grow from a few centimetres tall to over 2 metres in ideal conditions. Its quick growth and prolific seeding make it a problem wherever grass cover is thin or the soil has been recently cultivated.
In agricultural settings, some populations have developed herbicide resistance to triazine and dicamba-based products, but in lawns, fathen is still relatively easy to control with the right herbicide mix and good turf density.
Identification
Fathen is an upright, leafy plant with pale green, soft foliage that often looks as if it has been dusted with flour. The stems are smooth and branching, and the upper parts of the plant may also appear whitish or powdery.
The leaves are variable in shape, ranging from oval and smooth-edged to jagged and toothed, depending on growing conditions and species type. The closely related nettle-leaved fathen (Chenopodium murale) has more sharply serrated leaves and tolerates cooler conditions.
Fathen produces dense clusters of small, greenish flowers near the top of the stems. The seeds that form later in summer are tiny, black, and round, allowing the plant to spread easily when the soil is disturbed.
Why It’s a Problem in Lawns
Fathen competes aggressively with grass for light, space, and nutrients. Its rapid growth allows it to outpace turf seedlings and dominate freshly sown lawns or open soil patches. Because it germinates from spring through late summer, it often establishes after renovation or repair work, especially where irrigation or fertiliser encourages quick growth.
Although short-lived, each plant can produce thousands of seeds, which remain viable in the soil for years. Once established, it can give lawns a patchy, uneven appearance, and its tall, coarse growth stands out sharply against fine grass.
Control in Home Lawns
Fathen is easy to control if treated early, before it flowers and sets seed. In established lawns, selective herbicides are usually effective. For new lawns or heavy infestations, both chemical and cultural control may be needed.
Best control methods:
- Physical removal: Hand-pull young plants before they seed. The roots are shallow and come away easily when the soil is moist.
- Chemical control: Use selective turf herbicides containing dicamba, MCPA, or triclopyr/picloram (NZLA Gold) for effective control. These actives target broadleaf weeds without harming most turf species.
- Prevention during renovation: When reseeding, use a starter fertiliser but avoid over-cultivation of the soil. Keep new grass well-watered to help it outcompete germinating weeds.
- Cultural management: Maintain a dense, healthy lawn to shade the soil surface and prevent new seedlings from establishing.
Prevention Tips
- Fertilise regularly to promote thick turf and reduce open soil space.
- Mow regularly to prevent any surviving plants from flowering.
- Avoid over-tilling or disturbing bare patches, as this stimulates seed germination.
- Reseed thin or bare areas in autumn to improve ground cover before the next spring.
- Keep mower blades clean to avoid spreading weed seeds between areas.