Amaranth (Amaranthus sp)
Amaranth is a highly invasive Summer annual weed, that germinates in the Spring and then flowers from the late Summer onwards. There are two main varieties in Australia, which are A. viridis (Green Pigweed) and A. retroflexus (Redroot Pigweed).
It survives over Winter as seed, but in tropical or sub-tropical climates it tends to flower all the year round. In the tropics or sub-tropics Amaranth produces up to two generations of plants a year. Amaranth is a common cause of Summer allergies.
After you read this, you will be able to:
- Identify Amaranth or Redroot Pigweed.
- Know the habitat of Amaranth, Green or Red Pigwweed in Australia.
- Know the best options to control Amaranth.
Why is Amaranth such an Problem Weed?
Amaranth is a great competitor due to:
- Its early emergence.
- Ii is able to germinate in low light conditions.
- Has a C4 photosynthetic pathway. Amaranth is able to thrive under heat and drought conditions better than many other species.
- Allelopathic properties.
- Rapid growth.
- Its rapidly growing root system.
- Large seed production.
- Long-term seed viability.
- Grows all year-round growth in warmer climates.
- A lack of herbicide options for Amaranth in turf grass systems.
Redroot Pigweed.
- This is a hairy, annual herb that grows up to 1 m tall. The very hairy stems are red towards the base, and it has a red taproot.
- It contains toxic nitrates and oxalates in its roots and leaves. Cattle are at risk of nitrate poisoning if they eat large amounts of immature Red Pigweed stems. It also causes kidney damage.
- You often find this weed in wet areas.
- Amaranthus retroflexus produces at least twice the root length of most other weeds in the first month of growth. This fast-growing root system allows it to quickly access soil moisture in competition with other species.
- Redroot pigweed is most common in new turf areas where turf grass is not very dense or competitive.
Green Pigweed.
- Amaranthus viridis is a short-lived herbaceous plant that grows to 1 m tall. Its leaves are light green, have no hairs and often have notched tips.
- Green pigweed is often mistaken for Redroot Pigweed but does not have a red taproot.
The distribution map for Green Pigweed is courtesy of The Atlas of Living Australia.
How to Identify Amaranth.
Green Pigweed is found in all the mainland states of Australia, while you tend to only find Redroot Pigweed in the South-East of Australia.
Category: Amaranth is a Broadleaf (Dicot) weed.
Photosynthetic Pathway: Amaranth is a C4 Weed.
Flower: Amaranth flowers have a green colour, and there are male and female flowers on the same plant.
An easy way to identify Amaranth is the presence of slender, soft, green or brown spikes at ends of branches. The terminal spike tends to be 40 to 70 mm long, 5 mm in diameter and often has a red tinge.
Height: Amaranth is up to 70 cm tall.
Leaf length: The hairless leaves of Amaranth are alternate, have an egg shape, and have raised white veins on their underside. They are 3 to 9 cm in length, and are a darker green on their upperside.
Leaf width: The leaves are 2 to 6 cm wide.
Amaranth Reproduction.
- Amaranth reproduces only by seed, and Redroot Pigweed seeds prefer warm temperatures of 20ºC or more to germinate.
- Redroot Pigweed needs soil disturbance to expose the seed and germinate. It emerges mainly in the late Spring and early Summer, and then sporadically through the season. As the days become shorter it tends to flower, and then seed over a short period as the plants die.
- Estimates are that an Amaranth plant produces between 230,000 and 500,000 seeds. These seeds remain viable for up to 40 years but most are not viable after 6 to 10 years. This means that to successfully manage this weed you must reduce any seedbank.
- It takes 3 years to reduce the seedbank by 50%, and about 20 years to deplete the seedbank by 99%.
- As the depth of Amaranth seed burial increases above 1 cm, seed dormancy increases.
- Amaranth spreads by water, birds and livestock, and machinery. The seeds are relatively light in weight and wind easily transports them. However, without any wind the seeds only tend to fall within 2 m of the parent plant.
Comments:
- It is able to germinate in low levels of light, and grows all year-round in warm climates.
- Amaranth has a primary root, and deep secondary fibrous roots.
- Frost kills Amaranth, and it is moderately drought tolerant.
Habitat:
- Amaranth is a weed of disturbed areas.
- It grows in a wide range of soils, but prefers moist soils.
- It is less suited to acidic soils.
For more information on weeds check out our weed ID Chart.
How to Control Amaranth.
You can manage Amaranth by cultural and chemical means, but successful management of this weed is best if you adopt an integrated approach.
Cultural Control of Amaranth:
- If you maintain a healthy dense turf cover it reduces Amaranth seedling emergence, flowering and seed set. This is because the turf grass competes for light, soil nutrients and water. The poor tolerance of Amaranth to shade means that this strategy minimises the amount of light that reaches any seedlings.
- It is important that you make the right grass selection in order to out compete this weed. Whether this is turf seed or vegetative material is open to discussion, and depends on your climatic zone.
- Hand weeding is effective to remove Amaranth. However, it is only effective if the population is not too high or most of the plants are young. Once Amaranth matures control is much more difficult.
- Remove all flowering plants offsite, rather than leave them on the ground. This ensures that no viable seed remains to regrow.
- Pigweed does not persist in mowed turf. However, Amaranth does establish in cool-season lawns when mowing frequency is reduced due to high temperatures and minimal soil moisture.
Chemical Control of Amaranth.
In turf the only selective post emergent Pigweed herbicide in Australia is Pylex Herbicide. In vegetable crops Metalochlor, Ethofumesate and Propyzamide are registered.
Work in India on couch has shown that Metsulfuron at 5 g/Ha controls Green Amaranth.
The key to manage this weed is to reduce the seedbank. On this basis the pre-emergent herbicide BASF Freehand is worth a look. This is labelled to control Amaranth (Amaranthus cruentus).
Non Selective Control of Amaranth.
- Glufosinate-ammonium provides control for 4 to 6 weeks, but Amaranth regrows due to the limited movement of glufosinate.
- Glyphosate. You can use Glyphosate but if water quality is an issue then use ProForce Manta Ray.
The following are non-selective but also have a long term residual and stop re-growth of Amaranth.
- Renegade. Renegade stops seed germination for up to 12 months, and reduces the need for multiple herbicide applications.
- Numchuk Quad. This gives effective post and pre emergent Amaranth control for up to 12 months.
- Cortex Duo. Cortex Duo gives a rapid knockdown of Amarathus sp, and residual control for up to 3 months. It is also safe to use around trees.
