Weeds that can spread vegetatively when controlled mechanically

Uncontrolled weeds may reduce crop yield by as much as 80%.

Introduction

Weeds that are difficult to control frequently do not depend on above-ground seeds alone to spread, but can also re-establish itself by means of rhizomes, tubers and subterraneous flowers situated on burrowing runners that produce seeds under the ground.

The following weed species have one or more of these characteristics:

Johnson grass (Sorghum halepense (L.) Pers.)

Johnson grass (Figure 1a and 1b) is a perennial grass that can reach a height of 2.5 m. The species has branched rhizomes from which erect culms grow (Figure 1a). If cut off mechanically, pieces of the rhizomes can form new roots (Figure 1b) which eventually leads to the formation of a new plant. The culms are glabrous, with nodes that are sparsely hairy. The leaf-sheaths are striate and glabrous. The glabrous leaf-blades can be 3 cm wide and reach a length of 60 cm. The inflorescence is a large open panicle (Figure 1b) that can reach a length of 40 cm.

Couch Paspalum (Paspalum distichum L.)

Couch Paspalum has a creeping habitus (Figure 2a) and spreads by means of rhizomes and many-noded stolons (Figure 2b). Mechanical cultivation cuts the rhizomes in pieces, all of which can form new roots and eventually new plants. The multi-noded, flattened culms grow erect and can reach a height of 25 cm. The culms, leaf-sheaths and leaf-blades are glabrous. The inflorescence consists of two racemes (Figure 2b), which often become deflect with maturity.

Common couch (Cynodon dactylon (L.) Pers.)

Common couch has a creeping habitus (Figure 3a) and spreads by means of rhizomes and many-noded stolons. If cut off mechanically, pieces of the rhizomes can form new roots (Figure 3b), which eventually leads to the formation of a new plant. The culms grow erect and can reach a height of 40 cm. The culms, leaf-sheaths and leaf-blades are glabrous. Sometimes the upper leaf surface is hairy. The inflorescence consists of three to seven spikes arranged sub-digitately (Figure 3b).

Mat sandbur (Cenchrus incertus Curtis)

Mat sandbur is a semi-prostrate to erect grass (Figure 4a) that can reach a height of up to 90 cm. The culms, leaf-sheaths and leaf-blades are glabrous. If cut off mechanically, pieces of the culms can form new roots (Figure 4b), which eventually leads to the formation of a new plant. Few hairs are present where the leaf-sheath and leaf-blade meet. The leaf-blades can be 1.5 cm wide and reach a length of 25 cm. The inflorescence is a false spike of up to 10 cm long, and consists of a number of densely packed sessile burs (Figure 4b).

Field bindweed (Convolvulus arvensis L.)

Field bindweed (Figure 5a and 5b) is a prostrate or climbing weed with stems that have minute hairs, are striate and can reach a length of 3 m. The leaves can be either glabrous or sparsely covered with minute hairs on both sides. The flowers are axillary on long penduncles. If cut off mechanically, pieces of the stems can form new roots, which eventually leads to the formation of a new plant.

Benghal wandering jew (Commelina benghalensis L.)

This is a multi-branched soft herb (Figure 6a), that can reach a height of 40 cm. The burrowing runners produce subterraneous flowers and seeds (Figure 6b). Mechanical cultivation severs the runners from the mother plant and drags them along the work row, thus spreading the seeds. The leaves are situated alternately on the stems, which can be trailing or erect. The stems are covered with fine hairs. Along the margin of the leaf-sheath either red or colourless hairs occur. The above-ground flowers have inky blue petals and are situated in a pocket shaped, folded, nearly sessile spathes.

Yellow nutsedge (Cyperus esculentis L.)

Yellow nutsedge (Figure 7a and 7b) is herbaceous and normally reaches a height of 30 cm. The species has scaly rhizomes that grow downwards and produce small tubers at the apices (Figure 7b). Mechanical cultivation severs the tubers from the mother plant and drags them along the work row, thus spreading them. The stem is erect. Both the stem and leaves are smooth and glabrous. The leaves are shiny and have prominent central grooves (Figure 7b). The inflorescence is a compound umbel (Figure 7b).

All these species can cause major problems and are very expensive to control in vineyards farmed organically or bio-dynamically (use of herbicides not allowed). These species compete with grapevines, especially young or newly planted vines, for water and nutrients.

FIGURE 1. (a) The habitus and (b) leaves, rhizome with new root and inflorescence of Johnson grass (Sorghum halepense (L.) Pers.).

FIGURE 2. (a) The habitus and (b) rhizome with roots and inflorescence of couch Paspalum (Paspalum distichum L.).

FIGURE 3. (a) The habitus and (b) the leaves, rhizome with new root and inflorescence of common couch (Cynodon dactylon (L.) Pers.).

FIGURE 4. (a) The habitus and (b) leaves, base of the culms with roots and inflorescence of mat sandbur (Cenchrus incertus Curtis).

FIGURE 5. (a) The habitus, flowers and (b) the leaf-shape of field bindweed (Convolvulus arvensis L.).

FIGURE 6. (a) The habitus and (b) runners with subterraneous flowers, the leaves and inflorescence of Benghal wandering jew (Commelina benghalensis L.).

FIGURE 7. (a) The habitus and (b) leaves with prominent central grooves, rhizome with small tuber and inflorescence of yellow nutsedge (Cyperus esculentis L.).

Summary

Johnson grass (Sorghum halepense (L.) Pers.), couch Paspalum (Paspalum distichum L.), common couch (Cynodon dactylon (L.) Pers), and mat sandbur (Cenchrus incertus Curtis) are grasses that are frequently found in the vineyards of South Africa and can spread vegetatively when controlled mechanically. Other species that have the same ability are field bindweed (Convolvulus arvensis L.), Benghal wandering jew (Commelina benghalensis L.) and yellow nutsedge (Cyperus esculentis L.). All these species can cause major problems and are very expensive to control in vineyards farmed organically or bio-dynamically (use of herbicides not allowed). These species compete with grapevines, especially young or newly planted vines, for water and nutrients.

Acknowledgements

The Agricultural Research Council for funding, as well as Soil and Water Science at ARC Infruitec-Nietvoorbij for infrastructure and technical support.

– For more information, contact Johan Fourie at [email protected].