The genus Amaranthus encompasses a group of roughly 70 species distributed globally, presenting a unique biological paradox. This group includes both valuable ancient food crops and some of the most problematic agricultural weeds worldwide. Certain species are intentionally cultivated for their superior nutritional properties, while others are aggressive, invasive pests. This article clarifies this biological duality, outlining how to differentiate between the beneficial and detrimental members of the amaranth family.
The Dual Identity of Amaranth
The term “amaranth” applies to species serving two opposite roles in agriculture. A select number of species are purposefully grown for human consumption, yielding a nutritious grain, a leafy vegetable, or sometimes for ornamental purposes. These cultivated varieties have been a dietary staple for cultures in the Americas for thousands of years, earning them the classification of a pseudo-cereal.
Conversely, many other species within the same genus are commonly known as “pigweeds,” responsible for substantial crop losses across the globe. These wild amaranths aggressively compete with commercial row crops like corn, soy, and cotton. The distinction between a beneficial crop and a noxious weed is often unclear, as some species used for food in one region are considered pests in another.
Key Differences for Identification
Differentiating between cultivated amaranth and weed species requires observing specific physical traits, particularly in the stem and leaf structure. A field-level distinction for many noxious “pigweed” types, such as Palmer amaranth, is the length of the petiole (the stalk attaching the leaf blade to the stem). In Palmer amaranth, the petiole is noticeably longer than the leaf blade itself, a trait uncommon in other amaranth weeds or cultivated varieties.
Another identifier is the presence of hairs on the stem and leaves. Species like redroot pigweed have a fine coating of curly hairs (pubescent), especially on the upper stems. In contrast, aggressive weeds like Palmer amaranth and waterhemp have smooth, hairless stems and leaves (glabrous). The inflorescence, or flower cluster, also varies; cultivated types often have dense, drooping seed heads, while many weed species feature stiff, spiky, and upright flower clusters. Furthermore, some weed species, like spiny amaranth, possess sharp spines at the leaf axils.
Amaranth as a Highly Nutritious Crop
The cultivated varieties of amaranth are recognized as a valuable pseudo-cereal—a non-grass plant that produces starchy seeds used like true grains. Amaranth grain stands out for its exceptional protein quality and content, ranging from 14% to over 15% in some varieties. This level is nearly double the protein found in corn or rice.
The grain is notable because it contains high levels of the amino acid lysine, which is often deficient in true cereal grains. This makes amaranth a more complete protein source, offering a balanced profile of essential amino acids. Since it is a pseudo-cereal, amaranth is also naturally gluten-free, making it a suitable option for individuals with celiac disease or gluten sensitivities.
Beyond the grain, amaranth leaves are a source of micronutrients, often eaten as a leafy green vegetable. Dried amaranth leaves are high in iron, with some varieties containing between 111 and 284 milligrams per 100 grams. The leaves also provide substantial levels of calcium, magnesium, and zinc. The grain itself is rich in manganese, providing over 100% of the recommended daily intake in a single cooked cup.
Controlling Amaranth as an Agricultural Pest
The aggressive nature of weedy amaranth species (pigweeds) makes them a severe problem for commercial farming. These weeds exhibit rapid growth rates; Palmer amaranth can grow up to three inches per day under optimal conditions. This fast growth allows the weed to quickly outcompete and shade slower-growing crops, significantly reducing yields.
A single mature amaranth plant can produce hundreds of thousands to over a million seeds, contributing to a massive and persistent seed bank in the soil. Furthermore, many species have developed resistance to common herbicides, including glyphosate, which complicates management efforts. Integrated management strategies are necessary, combining chemical applications with non-chemical methods like deep tillage to bury seeds, the use of competitive cover crops, and manual removal of plants that escape other treatments.