Summer annual weeds are plants that germinate mainly in spring or early summer and complete their entire life cycle within a single growing season. They grow, flower, and produce seeds before dying in autumn or with the first hard frosts. The next generation survives as seeds in the soil. Many species favour warm soils; their emergence depends on factors such as temperature, soil moisture, and light availability.
Typical examples include gallant soldier, common lambsquarters, black bindweed, and various annual grasses, such as barnyardgrass and foxtails. In agricultural and horticultural crops, they can compete with crop plants for water, nutrients, light, and space. Many species produce large numbers of seeds, some of which remain viable in the soil for years. Where control is necessary, it should ideally take place at an early growth stage and before seeds mature, to limit competition and prevent further accumulation of seeds in the soil seed bank.
Proso millet is an annual, warm-season species. In addition to being cultivated as a crop, it occurs as a volunteer plant, an escaped cultivated form, or a weed-adapted population. As a summer annual grass weed, proso millet germinates mainly in spring and summer when soil temperatures are sufficiently high. Seedlings emerge primarily from shallow soil layers. The species uses the C₄ photosynthetic pathway and can develop rapidly under warm conditions with high light availability. Compared with cultivated forms, weed-adapted populations may exhibit greater seed shattering and more pronounced seed dormancy. However, these traits vary considerably among populations. Proso millet occurs primarily in summer annual crops, such as maize, soybean, and other row crops. As a weed, it competes with crops for water, nutrients, and light. Potential yield losses depend particularly on weed density, the timing of weed emergence relative to the crop, and site conditions. Heavy infestations can also hinder harvesting and contaminate the harvested crop.
Growth habit: Summer annual grass with an erect to ascending growth habit; often branched at the base.
Plant height: Usually approximately 30–150 cm, depending on genotype, site conditions, and competition.
Root system: Fibrous root system with numerous adventitious roots arising from the stem; no stolons or rhizomes.
Culms: Rounded, jointed at the nodes, and usually hollow; hairiness varies.
Leaves:
Arranged alternately in two rows along the culm.
Leaf blades flat, linear to broadly lanceolate, with pointed tips.
Leaf blades and sheaths often distinctly hairy; the degree of hairiness varies.
Ligule: A short fringe of hairs.
Auricles: Absent.
Inflorescence: A branched panicle, usually approximately 10–30 cm long; ranging from loosely spreading to contracted, depending on the form, and sometimes drooping at maturity.
Spikelets:
Small, ovoid with pointed tips, and usually approximately 4–5 mm long.
Usually contain a lower sterile or male floret and an upper bisexual floret.
Lower glume distinctly shorter than the spikelet.
Fruit: A rounded to ovoid caryopsis, usually approximately 2–3 mm in size, enclosed by a hardened lemma and palea.
Grain colour: Whitish, yellowish, reddish, brown, or nearly black, depending on the form.
In weedy populations, plant height, branching, panicle shape, and grain colour can vary considerably. None of these traits alone reliably distinguishes weedy populations from cultivated forms.
Field mint (Mentha arvensis L.) is a perennial plant species belonging to the mint family (Lamiaceae). Vegetative reproduction through underground rhizomes is particularly important for its persistence and spread in agricultural fields. Tillage can fragment and disperse these rhizomes, creating new patches of infestation. Sexual reproduction through seeds is also possible. Field mint prefers moist to periodically wet, nutrient-rich soils. In arable fields, it frequently occurs in areas with poor drainage or soil compaction. Its presence may indicate elevated soil moisture, but does not in itself provide conclusive evidence of waterlogging or compaction. As a weed, it competes with crops for water, nutrients, and light. At high densities, it can impair crop development and complicate field management. Field mint forms part of the vegetation characteristic of its habitat and also has ecological value, as its flowers provide food for various insects.
Growth habit: A perennial herbaceous plant with erect to ascending, often branched shoots.
Height: Usually about 10–60 cm, depending on growing conditions.
Underground organs: Creeping rhizomes with buds that produce new shoots; roots may develop at the nodes.
Stem: Square in cross-section, usually hairy, and sometimes tinged reddish.
Leaf arrangement: Opposite and decussate, with successive pairs arranged at right angles.
Leaves: Short-stalked, ovate to elliptical, usually about 2–6 cm long. The margins are serrated, and the tips are usually pointed. The degree of hairiness varies.
Inflorescence: Flowers occur in separate false whorls (verticillasters) in the axils of the upper foliage leaves. The shoot tip usually bears leaves rather than a terminal inflorescence.
Flowers: Small, usually pale lilac to pink, occasionally whitish. The fused corolla is approximately 4–5 mm long and has four nearly equal lobes.
Calyx: Bell-shaped and hairy, with five short, triangular teeth.
Fruit: A schizocarp that can split into four small, single-seeded nutlets when mature.
Scent: When crushed, the leaves release a characteristic mint scent due to their essential oils.
The axillary flower whorls and leafy shoot tip are important identification features. Since mints frequently hybridize and are morphologically variable, individual characteristics are not always sufficient for reliable species identification.
Common groundsel (Senecio vulgaris) is an annual plant species belonging to the daisy family (Asteraceae). Originally native to Eurasia and North Africa, it is now established in many temperate regions worldwide. The species preferentially colonizes nutrient-rich, frequently disturbed, open habitats. It occurs particularly in vegetable and ornamental crops, plant nurseries, and arable fields. Under favourable temperature and moisture conditions, it can germinate almost year-round and produce several generations per year. Reproduction occurs through seeds. Self-pollination enables seed production even when pollinator availability is low. The seeds are dispersed primarily by wind. In crop stands, common groundsel competes for water, nutrients, and light. Its economic importance depends on its population density, the crop, and the timing of its emergence. The species contains pyrrolizidine alkaloids, which can cause toxic effects, particularly liver damage, following metabolic activation. Contamination of harvested products, such as herbs or teas, as well as animal feed, is therefore a concern.
Growth habit: Annual herbaceous plant with an upright, often branched habit.
Height: Usually 10–30 cm, reaching approximately 50 cm under favourable conditions.
Root system: Taproot with numerous fine lateral roots.
Stem: Often ribbed, green to reddish-tinged, and hairless to sparsely covered with cobweb-like hairs.
Leaves:
Alternately arranged.
Oblong, irregularly lobed to deeply pinnately divided, with toothed margins.
Lower leaves usually stalked; upper leaves sessile, with auriculate bases that partially clasp the stem.
Hairless to sparsely covered with cobweb-like hairs.
Inflorescence: Small, narrow, cylindrical flower heads arranged in loose, branched clusters.
Flowers:
Yellow, bisexual disc florets.
Ray florets are absent in the typical form; forms with short ray florets occur occasionally.
Involucral bracts: A single row of inner bracts of approximately equal length, accompanied by additional short outer bracts; dark to black tips are characteristic, particularly on the outer bracts.
Fruits: Small, elongated, ribbed, and shortly hairy dry, indehiscent fruits, botanically termed cypselae, often also referred to as achenes.
Pappus: A tuft of white hairs attached to the fruit that facilitates wind dispersal.
