Amaranthus palmeri

Amaranthus palmeri It is popularly known as Palmer's caruru, giant caruru, Palmer's amaranth or Palmer's pigweed. Its name honors the botanist Edward Palmer, who described it in the 19th century.

This species is a highly aggressive weed native to the southwestern United States and Mexico.

In South America, the presence of A. palmeri It was initially verified in Argentina. It was believed that, since it was found in that country, the southern region of Brazil would be its likely route of introduction into the country.

After detecting the presence of Amaranthus palmeri resistant to glyphosate in Argentina in 2013, research institutions in Brazil began to worry about the possibility of this harmful species entering the country.

Contrary to all expectations, the first detection of A. palmeri in Brazil it officially occurred in the state of Mato Grosso in 2015, a region far from the border with Argentina (doi.org/10.51694/AdvWeedSci/2023;41:00010).

Infested crops

Amaranthus palmeri It is a difficult pest to control, mainly affecting:

• Major crops: soybeans, cotton, corn and sorghum.

• Vegetables: tomatoes, peppers and watermelon.

• Fruit trees: citrus orchards and olive groves.

Its adaptation to diverse agricultural environments and rapid growth make it a threat to productivity in tropical and subtropical regions.

Biology

Amaranthus palmeri It is a short-cycle annual plant. It completes its cycle (germination, growth, flowering and seed production) in periods of four to six months, depending on environmental conditions.

The plant develops deep and extensive roots, allowing access to water and nutrients in deeper layers of the soil, which makes it highly resistant to moderate droughts.

A. palmeri presents C4 photosynthesis. It has an efficient carbon fixation mechanism, common in plants adapted to hot climates, which explains its accelerated growth even at high temperatures.

In terms of reproduction and genetics:

• Dioicia: Male and female plants are separated, promoting genetic crossing and variability. This contributes to rapid adaptation to herbicides and adverse conditions.

• Seed production: Depending on size and competition, each female plant produces 100 to 600 seeds. The seeds are small (1–2 mm) and light, making them easy to disperse by wind, water, animals, or agricultural equipment.

• Variable dormancy: some seeds germinate soon after dispersal, while others remain dormant in the soil for 10 years or more, forming a persistent "seed bank."

Its growth rate can reach 5 to 7 cm per day in ideal conditions, surpassing crops such as soybeans and corn in the competition for light.

A single plant can accumulate up to 1 kg of dry biomass, extracting large amounts of nitrogen, phosphorus and potassium from the soil.

Furthermore, it presents phenotypic plasticity. It adapts its morphology (height, leaf area) according to the availability of resources, becoming more robust in fertile soils or open spaces.

Amaranthus palmeri It develops best at temperatures between 28°C and 35°C, but tolerates temperatures from 15°C to 40°C. The plant grows vigorously in long days and with high light intensity, being common in tropical and subtropical regions.

In terms of soil and moisture, it grows in soils with a pH between 5,5 and 8, as long as they are well drained. It is common in sandy soils, but also thrives in clay soils if there is adequate moisture.

Its deep roots allow it to survive in dry periods, but for maximum growth it prefers regular humidity (500–800 mm of annual rainfall). On the other hand, it is sensitive to waterlogged soils.

Germination and establishment of A. palmeri:

• Emergence depth: seeds germinate up to 5 cm deep, but the ideal is between 0,5 cm and 2 cm.

• Triggers for germination: rain or irrigation after dry periods; soil disturbance (e.g. conventional tillage); soil temperatures above 18°C.

• Emergence period: germinates throughout the growing season, making continuous control a challenge.

Control

the management of Amaranthus palmeri requires integrated strategies.

• Prevention: monitor areas to identify infestations early; avoid introducing contaminated seeds into agricultural equipment.

• Cultural methods: crop rotation (using competitive species such as grasses); use of mulch or cover crops to suppress germination; hand or mechanical weeding before flowering; row cultivation to remove young seedlings.

• Chemical control: use registered products; populations resistant to multiple herbicides require rotation of active ingredients and modes of action. Click here to find out which herbicides are registered for use against Amaranthus palmeri

Herbicide resistance, already documented in several countries, makes Amaranthus palmeri one of the worst agricultural pests of the 90st century. Its inadequate management can cause losses of up to XNUMX% in productivity in crops such as soybeans.

The species has demonstrated documented resistance to several classes of active ingredients, including ALS inhibitors (such as imazethapyr and chlorimuron), PPO inhibitors (such as fomesafen and lactofen), dinitroanilines (such as pendimethalin), HPPD inhibitors (such as mesotrione and tembotrione), and auxinic herbicides (such as dicamba and 2,4-D).

By May 2025, Amaranthus palmeri resistant to glyphosate is officially present in the states of Mato Grosso and Mato Grosso do Sul (Naviraí and Aral Moreira). Other states are monitoring the pest, but without confirmation of new cases.