Common amaranth

Redroot pigweed is an annual weed from the class of dicotyledons, which germinates in late spring and early summer.

A typical feature of this species is the gray-green color of the upper leaf surface, while the edges and underside are reddish.

It has a spindle-shaped, thick, and strongly branched root with a slightly pink hue. The stem, 10 to 100 cm tall, is erect, single or branched, ranging from light green to reddish, with short, rough hairs. Cotyledons are linear, 7 to 12 mm long, hairless, green on top and reddish underneath, often with a blunt tip. True leaves are rhombic-ovate, gray-green, reddish on the edges, alternate, slightly pointed at the tip or blunt, with a serrated margin. The inflorescence is a dense, slightly prickly spike. The flowers are small, inconspicuous, located at the nodes at the end of the stem, greenish, with prickly perianth leaves. The fruit is a single-seeded, smooth capsule containing a lid. The seed is lens-shaped, round, brownish-black, smooth, and highly glossy.

Occurrence and Harmfulness

Redroot pigweed occurs in root crop fields, maize, gardens, and ruderal sites. It grows on various soils, mainly lighter, humus-rich, nitrogen-rich soils, and is a light-loving species. It is troublesome, grows very quickly, has developed forms resistant to some triazine herbicides, is drought-resistant (root system length up to 1.5 m), reduces yields, and complicates harvesting.

Control

Mechanical method

The long period from planting to potato emergence is critical (ideal conditions for weed development), which can be used for mechanical treatments. These are carried out using farm equipment, e.g., ridgers, light harrows, finger weeders, etc. Their main task is to create the most favorable conditions for rapid weed germination, followed by destruction through subsequent cultivation operations. The highest effectiveness (over 90%) can be achieved when weeds are in the seedling stage, but as they grow, effectiveness drops to around 40%. This weed control system is highly dependent on weather and soil conditions. In very wet springs, mechanical methods are difficult to apply. On heavily infested fields, it may have the opposite effect, as damaged couchgrass runners are stimulated to stronger growth and branching. Additionally, the mechanical system is the most labor- and cost-intensive due to multiple passes. This method is usually used on small farms growing potatoes for self-sufficiency or organically. Before potato emergence, 3 to 6 treatments should be carried out at 6–8 day intervals. After emergence, the number of cultivations should not exceed 2–3, and any delay may damage the crop and increase the risk of pathogen transfer via sap and may excessively dry the soil (especially light soils).

Mechanical-chemical method

The most effective and widespread weed control system. It saves on mechanical cultivations, which are used much less than in the purely mechanical system. After planting, double or triple ridging is usually performed, followed shortly before emergence by herbicide application, after which no further cultivation operations are carried out. At this vegetation stage, identifying weed species may be difficult, so the basis for herbicide application can be a treatment log, maintained and stored by every professional user of plant protection products. Based on this, the species of weeds that may pose a threat in a given field can be determined.

Chemical method

The most effective method is the use of herbicides, which achieve the highest efficacy when the weed is in the seedling stage; the more vigorous the growth, the lower the effectiveness. Herbicides in potatoes are applied at two basic timings: before emergence and after emergence. Before using a herbicide, its label-instruction must be consulted, which contains information on application timing and doses. Some active substances in herbicides have moderate efficacy (average sensitivity of the weed species), and the control effect may be unsatisfactory, especially when the weed occurs in high density or advanced growth stage.

On heavily infested fields, a non-selective herbicide containing the active substance glyphosate can be applied, known as the simplified system. Its success depends on the presence of weeds, as glyphosate is absorbed only through the leaves and cannot be applied to potato sprouts (they would be destroyed!). Even potato shoots just below the soil surface can be damaged (photographs A, B). Timing the treatment according to weather conditions, mainly temperature and rainfall, is crucial for high effectiveness. Pre-emergence herbicides applied too late, when potato sprouts are just below the soil surface, may damage them after heavy rain. Among pre-emergence herbicides, some products can still be used in early developmental stages when potato plants are about 5 cm tall, e.g., prosulfocarb + metribuzin.

Active substances controlling redroot pigweed for pre-emergence use: fluorochloridon, linuron, metobromuron, pendimethalin, pendimethalin + chlometuron, prosulfocarb + metribuzin.

Post-emergence herbicides in potatoes should be applied no later than the time of row closure, ensuring weed coverage and preventing pooling of spray solution on potato plants. Due to the long pre-harvest interval, post-emergence herbicides cannot be used in the earliest harvest crops.

Active substances controlling redroot pigweed for post-emergence use: metribuzin and rimsulfuron. Metribuzin applied post-emergence may cause phytotoxic reactions on some potato varieties.