Amaranth (Amaranthus sp)
Amaranth is usually a Summer annual weed, that germinates in the Spring and flowers from late Summer onwards. It survives over-Winter as seed. In tropical or sub-tropical climates it is capable of flowering all year, and produces two generations of plants.
There are two main varieties in Australia, and these are A. viridis (Green Amaranth) and A. retroflexus (Redroot Amaranth).
Amaranth has an allelopathic effect on certain plants.
The distribution map for Green Amaranth is courtesy of The Atlas of Living Australia.
After you finish reading this, you will be able to:
- Identify Amaranth.
- Know the habitat of Amaranth.
- Know the best cultural and chemical options to control Amaranth.
How to Identify Amaranth.
In Australia, this is usually a Summer annual weed. Green Amaranth is found in all mainland states, while Redroot Amaranth is usually present in Australia’s south-east.
Category: Broadleaf (Dicot).
Photosynthetic Pathway: C4 Weed.
Flower: The flowers are greenish, and have separate male and female flowers on the same plant. There are more female flowers than the male flowers.
A way to identify Amaranth is the presence of slender, soft, green or brown spikes at ends of branches. The terminal spike is usually 40-70 mm long and 5 mm diameter and often has a red tinge.
Height: Up to 70 cm tall.
Leaf length: The hairless leaves are alternate, egg shaped to oblong, with prominent white veins on the underside. They are 3-9 cm in length, and darker green on the upperside.
Leaf width: The leaves are 2-6 cm wide
Reproduction: Amaranth reproduces only by seed. Estimates are that each plant produces between 230,000 and 500,000 seeds. The seeds can remain viable for up to 40 years but most are not viable after 6-10 years.
It spreads by water, birds and livestock, and machinery movement. Amaranth seeds are relatively light in weight and can be transported by the wind , but seeds largely fall within 2 m of the parent plant.
Comments: Amaranth is a great competitor due to its early emergence, rapid growth, large seed production and long-term seed viability. It is able to germinate in low light conditions, and grows all year-round in warmer climates. Amaranth has a primary root with deeper spreading secondary fibrous root structures.
Habitat: A weed of disturbed areas. It can grow in a wide range of soils, but prefers moist soils and is less suited to acidic soil types.
For more information on weeds check out our weed ID Chart.
How to control Amaranth.
You can control Amaranth by cultural and chemical means, but successful management of this weed is best if you adopt an integrated approach.
Cultural control:
A well established and maintained turf cover will reduce amaranthus seedling emergence, flowering and seed set. That’s because the turfgrass competes for light, soil nutrients and water. The poor tolerance to shade means that this strategy will minimise the amount of light that reaches seedling amaranth plants.
Hand weeding is effective for the removal of Amaranth if the population is not too high. Aim to remove all plants that are flowering offsite, rather than leave them on the ground. This ensures that no viable seed remains.
Chemical control of Amaranth:
In turf the only selective turf specific herbicide registered for Amaranth control in Australia are Pylex Herbicide. In vegetable crops, Chlorthal-dimethyl, Metalochlor, Ethofumesate and Propyzamide are registered.
Work in India on couch has shown that Metsulfuron at 5 g/Ha was effective to control Green Amaranth.
The aim of control with this weed is to reduce the seedbank and keep it at a low level. On this basis the pre-emergent herbicide BASF Freehand is worth looking at. This is registered to control Amaranth (Amaranthus cruentus).
You can use Glyphosate as a non selective option to control this weed, although herbicide resistance has been reported overseas. If you use Glyphosate and water quality is an issue then we recommend the use of ProForce Manta Ray.
