Common hemp-nettle

Common hemp-nettle is an annual spring weed from the dicotyledonous class, germinating in spring and early summer.

A characteristic feature of this species is the strong, rough hairiness of the plant, from which its name derives. It has a well-developed taproot system. The stem reaches a height of 10 to 100 cm, is thickened at the nodes, strongly branched and hairy. Cotyledons are round to oval, 5 to 8 mm long, hairless, green, rounded at the tip and slightly notched. True leaves are ovate-lanceolate, hairy, green, crenate or serrated and pointed at the tip. It does not form a distinct inflorescence. Flowers are arranged in false whorls, purple, pink or cream in colour. The fruit is a round-ovate, slightly flattened, dark grey schizocarp with dark grey pigmentation, and the seeds are oval.

Occurrence and harmfulness

Common hemp-nettle occurs in root crops, cereal crops, gardens and ruderal habitats. It grows on loamy and sandy soils, humus-rich, with good water relations. It does not tolerate dry soils or those poor in nitrogen.

Control

Mechanical method

The long period from planting to potato emergence is a critical time (ideal conditions for weed development) that can be used for mechanical control. Treatments are carried out using machinery available on the farm, e.g. ridgers, light harrows, weeders, etc. Their main task is to create the most favourable 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 at the seedling stage; however, as weeds develop, effectiveness drops to about 40%. This weed elimination system is highly dependent on weather and soil conditions. In years with very wet springs, the mechanical method is difficult to apply. On heavily infested plantations it may have the opposite effect, as damaged couch grass rhizomes are stimulated to even stronger growth and tillering. In addition, the mechanical system is the most costly and labour-intensive due to the need for multiple passes. Most often, this method is used on small farms producing potatoes for self-supply 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 treatments should not exceed 2–3, and any delay increases the risk of crop damage, pathogen transfer with plant sap, and excessive soil drying (especially on light soils).

Mechanical and chemical method

The most favourable and widely used weed control system. It allows savings on mechanical cultivation, which is used much less frequently than in the purely mechanical system. After planting, two or three ridging operations are usually carried out, and shortly before emergence a herbicide treatment is applied, after which no further cultivation is performed. At this stage of vegetation, identifying weed species may be difficult; therefore, the basis for herbicide application may be a treatment log, which should be kept and stored by every professional plant protection product user. Based on it, potential weed threats in a given field can be determined.

Chemical method

The most effective control method is the use of herbicides, which achieve the highest effectiveness for this species when the weed is at the seedling stage; the more vigorous the growth, the lower the effectiveness. In potatoes, herbicides are applied at two main timings: before emergence and after emergence. After deciding to apply a herbicide, the label-instructions should be consulted for information on application timing and rates. Some active substances contained in herbicides show medium effectiveness (medium sensitivity of a given weed species), and after their application the control effect may be unsatisfactory, especially when the weed occurs at high density or is in an advanced growth stage.

On heavily weed-infested plantations, a non-selective herbicide containing the active substance glyphosate can be applied, the so-called simplified system. The basis for its success is the presence of weeds on the plantation, as glyphosate is absorbed exclusively through leaves, and potato emergence must not have occurred (they would be destroyed!). Even potato sprouts located just below the soil surface may be damaged (photo A, B). It is very important to adjust the timing of the treatment to weather conditions, mainly temperature and rainfall, which often determines high effectiveness. Pre-emergence herbicides applied too late, when sprouting potatoes are just below the soil surface, may damage them after heavy rainfall. Among pre-emergence herbicides, there are also registered products that can still be applied in early growth stages when potato plants are about 5 cm tall, e.g. prosulfocarb + metribuzin.

Active substances controlling common hemp-nettle for pre-emergence application: clomazone, flufenacet + metribuzin, linuron + clomazone, metobromuron, metribuzin + clomazone.

There are no active substances intended for the control of common hemp-nettle after potato emergence!