Red dead-nettle

Red dead-nettle is an annual weed, either spring or winter type, belonging to the dicotyledon class, which germinates in autumn or spring.

A typical feature of this species is the reddish coloration on the stems (hence the name) and its characteristic strong odor.

It has a forked, taproot system. Stem height is 10 to 30 cm, quadrangular, branched at the base, hairy or bare on the edges. Cotyledons are oval, 4 to 8 mm long, bare, green, shallowly notched at the tip. True leaves are broadly ovate, opposite, short-hairy, dark green, and heavily serrated. It does not produce an inflorescence. Flowers are gathered in pseudo-whorls, purple, positioned at the top of the plant. The fruit is a light gray schizocarp, inversely ovate, blunt at the tip, with small white tubercles, and seeds are oval, smooth, and gray.

Occurrence and harmfulness

Red dead-nettle occurs in root crops, cereals, stubble fields, gardens, and ruderal sites. It grows on various soils but prefers humus-rich, nutrient-rich soils, more frequently found in manure-fertilized areas. Seeds are dispersed by ants.

Control

Mechanical method

The long period from planting to potato emergence is a critical time (ideal conditions for weed development), which can be used for mechanical treatments. These are carried out using available farm machinery, e.g., ridgers, light harrows, hoes, etc. Their main task is to create the most favorable conditions for rapid weed germination and then destroy them with subsequent cultivation operations. The highest effectiveness (even over 90%) can be achieved when weeds are in the seedling stage, but effectiveness drops to around 40% as weeds develop. This weed control system largely depends on weather and soil conditions. In very wet springs, the mechanical method is difficult to apply. On heavily infested fields, it can have the opposite effect as damaged couch grass runners are stimulated to even stronger growth and branching. Additionally, the mechanical system is the most costly and labor-intensive due to multiple passes. This method is most often used in small farms growing potatoes for self-sufficiency and in organic systems. 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 in their execution exposes the plantation to damage and increases the risk of pathogen transmission with sap and may excessively dry the soil (especially light soils).

Mechanical-chemical method

The most favorable and widespread weed control system. It allows saving 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 a herbicide treatment, after which no further cultivation is carried out. At this stage of vegetation, identification of existing weed species may be difficult; therefore, a treatment log, maintained by every professional user of plant protection products, can serve as the basis for herbicide application. It helps determine which weed species may pose a threat on a given field.

Chemical method

The most effective method of control is the use of herbicides, which achieve the highest effectiveness for this species when the weed is in the seedling stage; the more vigorous the growth, the lower the effectiveness. Herbicides in potatoes are applied in two main periods: before emergence and after emergence. Before using a herbicide, the label instructions must be read, which contain information on application timing and dosages. Some active substances in herbicides have medium effectiveness (average sensitivity of the weed species), and the control effect may be unsatisfactory, especially when the species occurs in high density or advanced growth stage.

On heavily infested fields, a non-selective herbicide containing glyphosate can be applied, called a simplified system. Its success depends on the presence of weeds, as glyphosate is absorbed only by leaves and cannot be applied to emerging potatoes (they would be destroyed!). Even potato sprouts just below the soil surface can be damaged (photo A, B). It is very important to adjust the timing of the treatment to weather conditions, mainly temperature and rainfall, which is often a decisive factor for high effectiveness. Pre-emergence herbicides applied too late, when potato sprouts are just under the soil surface, can damage the tubers after heavy rain. Some pre-emergence herbicides are registered for early growth stages when potato plants are about 5 cm high, e.g., prosulfocarb + metribuzin.

Active substances controlling red dead-nettle for pre-emergence application: chlormazon, fluorochloridone, linuron + chlormazon, metobromuron, metribuzin, metribuzin + chlormazon, pendimethalin, pendimethalin + chlormazon, prosulfocarb, prosulfocarb + metribuzin.

Post-emergence herbicides should be applied no later than the row closure stage to ensure coverage of weeds and prevent liquid from accumulating on potato plants. Due to the long pre-harvest interval, post-emergence herbicides cannot be used for the earliest harvest.

Active substances controlling red dead-nettle for post-emergence application: bentazon, metribuzin, and rimsulfuron. Metribuzin applied post-emergence may cause phytotoxic symptoms on some potato varieties.

Compiled by: Dr. Janusz Urbanowicz