Field penny-cress

Field penny-cress is an annual weed that can occur as either a spring or winter type, belonging to the dicot class, with seedlings appearing in spring, autumn, and early summer.

A characteristic feature of this species is the smell released when crushing the leaves or stems, which strongly resembles garlic. This plant is poisonous!

It has a spindle-shaped root system with a whitish color. The stem reaches a height of 10–50 cm, is glabrous, slightly angled, and branched at the top. Cotyledons are oval-elliptical with a rounded tip. True lower leaves are inversely ovate, bluish-green underneath, and upper leaves are oblong, light green, notched or toothed, with a bluntly pointed tip, without hairs. The inflorescence is a raceme made up of small, white flowers with yellow stamens. The fruit is a winged silicle, round and flat, with a narrow, deep notch. Seeds are ovate, flattened, concentrically ribbed, and dark brown in color.

Occurrence and harmfulness

Field penny-cress is most commonly found in root crop (beet and potato) fields, rapeseed, cereals, gardens, and ruderal areas.

It prefers fertile, loamy, humus-rich soils, abundant in nutrients, mainly calcium carbonate, and is less common on lighter soils. Its harmfulness lies in its high seed production, as one plant can produce up to 1,000 seeds, which can remain viable in the soil for 10–20 years. Penny-cress has a wide germination temperature range from 2 to 30°C (it can germinate at 1°C). Additionally, this species heavily infests crops and can produce several generations per year.

Control

Mechanical method

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

Mechanical-chemical method

The most favorable and widely used weed control system. It saves on mechanical cultivations, which are used much less than in the purely mechanical system. After planting, 2–3 ridging operations are usually carried out, followed shortly before emergence by a herbicide treatment, after which no further cultivation is performed. At this stage of vegetation, identifying the weed species can be difficult, so a record of treatments kept by each professional user of plant protection products serves as the basis for herbicide application, allowing determination of which weed species may pose a threat on a given field.

Chemical method

The most effective control method is the use of herbicides, which are most effective on this species when the weed is at the seedling stage; the more developed the growth, the lower the effectiveness. Herbicides in potato crops are applied in two main periods: before emergence and after emergence. Before applying a herbicide, one should consult its label-instructions, which provide information about application timings and doses. Some active substances in herbicides have medium effectiveness (average sensitivity of the weed species); after application, control may be unsatisfactory, especially if the weed occurs in high density or is in an advanced growth stage.

On heavily infested fields, a non-selective herbicide containing 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 must not contact potato sprouts (which would be destroyed). Even potato sprouts just under the soil surface may be damaged (photographs A, B). It is very important to adjust the application time to weather conditions, mainly temperature and rainfall, which often determines the high effectiveness. Pre-emergence herbicides applied too late, when potato sprouts are just under the soil surface after heavy rainfall, may damage the crop. Some pre-emergence herbicides can still be applied in early developmental stages when potato plants are around 5 cm tall, e.g., prosulfocarb + metribuzin.

Active substances controlling field penny-cress for pre-emergence application: chlormazon, flufenacet + metribuzin, fluorochloridon, linuron, linuron + chlormazon, metobromuron, metribuzin, metribuzin + chlormazon, pendimethalin, pendimethalin + chlormazon, prosulfocarb + metribuzin.

Post-emergence herbicides in potatoes should be applied no later than the row closure stage to ensure weed coverage and proper spray deposition on potato plants. Due to the length of the pre-harvest interval, post-emergence herbicides cannot be used in very early harvest crops.

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