Annual weeds

While there are at least 50 successful annual weed species in horticulture, this book can cover only a few examples that illustrate the main points of life cycles and control. Two species, annual meadow grass and speedwell, are described.

Annual meadow grass (Poa annua). Plant family - Poaceae (Graminae)

Damage. This species is a quite small annual (or short-term perennial) found on a range of ornamental and sports grass surfaces, on paths and in vegetable plots (see Figure 13.8). It is able to establish quickly on bare ground. It does not thrive on acid soils or those low in phosphates. Despite its relatively small size, it often emerges in sufficient quantities to smother crop seedlings. Its seed may be present as an impurity in commercial grass seed. Special selections of this species are used in seed mixtures for lawns.

Life cycle. Flowers can occur at any time of year and are usually self-pollinated. About 2000 seeds per plant are produced from April to September. Plants will flower and seed even when mown regularly. Seeds germinate from February to November with the main peaks in early spring and autumn. Some seed will germinate soon after their release; others can remain viable in soil for at least 4 years. This weed species can be the host of a number of nematode species that also attack important crops.

Spread. There is no obvious dispersal mechanism. Most seeds fall around the parent plant and become incorporated into the soil. Seeds may be carried around on boots and wheels of machinery. Worms may bring seeds to the soil surface in worm casts.

Weeds That Look Like Grass
Figure 13.8 Annual meadow grass plant

Control is achieved by a variety of methods. The physical action of hoeing normally controls the weed especially when it is in the young stage. Deep digging-in of seedlings and young plants is also usually effective. Mulching is effective against germinating weeds in flower beds and fruit areas. The amateur gardener can use the non-selective, non-residual herbicide, glufosi-nate-ammonium plus fatty acids, for control in situations such as ornamental beds containing woody perennials and in cane fruit. The professional horticulturalist may use paraquat or glyphosate for total chemical control, but these two chemicals should never be sprayed in the vicinity of growing crops. Care should be taken not to walk on grass after application of these chemicals. Chlorpropham may be used as a soil applied chemical on crops such as currants, onions and chrysanthemum.

Lawn Weeds
Figure 13.9 (a) Field speedwell seedling (b) Field speedwell plants

Speedwells (Veronicapersica and V. filiformis). Plant family - Scrophulariaceae

Damage. The first species, the large field speedwell (V persica) is an important weed in vegetable production, crowding out young crop plants and reducing growth of more mature stages. The second species, the slender or round-leaved speedwell (V filiformis), once considered a desirable rock garden plant introduction from Turkey, has become a serious turf problem.

Life cycle. The seedling cotyledons are spade shaped, while the true leaves are opposite, notched and hairy (see Figure 13.9) in both species. The adult plants have erect, hairy stems and rather similar broad-toothed leaves. V. persica produces up to 300 bright blue flowers, 1 cm wide, per plant. The flowers are self-fertile and occur throughout the year, but mainly between February and November. The adult plant produces an average of 2000 light brown boat-shaped seeds 2 mm across. The seeds of this species germinate below soil level all year round, but most commonly from March to May (see Figure 13.5), the winter period being necessary to break dormancy. Seeds may remain viable for more than 2 years. V. filiformis produces self-sterile purplish-blue flowers between March and May, and spreads by means of prostrate stems which root at their nodes to invade fine and coarse turf, especially in damp areas. Segments of this weed cut by lawnmowers easily root and further increase the species. Seeds are not important in its spread.

  1. Seeds of V. persica falling to the ground may be dispersed by ants. Seed of this species can be spread as contaminants of crop seed. V. filiformis does not produce seed. Its slow spread is mainly by means of grass-cutting machinery.
  2. Field speedwell (V. persica) is controlled by a combination of methods. The physical action of hoeing or mechanical cultivation, particularly in spring, prevents developing seedlings from growing to mature plants and producing their many seeds.

The amateur gardener can use the herbicide, glufosinate-ammonium plus fatty acids for control in situations such as ornamental beds containing woody perennials and in cane fruit.

For the professional grower, total contact herbicides, such as paraquat, may be sprayed to control the weed on paths or in fallow soils. Soil-acting chemicals such as chlorpropham on crops such as lettuce, onions and chrysanthemums kill off the germinating weed seedling. A contact, foliar chemical such as clopyralid, may be sprayed on to brassicas, young onions, and strawberries to control emerging seedlings.

The slender speedwell (V.filiformis) represents a different problem for control. Physical controls such as regular close mowing and spiking of turf removes the high humidity necessary for this weed's establishment and development.

The amateur gardener has difficulty controlling this weed with the turf herbicides available. The amateur or professional grower can control the weed in a turf seedbed using a total, contact chemical such as glufosinate-ammonium plus fatty acids, a few weeks before sowing the turf seed. This 'stale seed bed' method leaves the turf to establish relatively undisturbed by weeds. Organic growers may remove stale seedbed weeds by hoeing. Professional growers can use a selective contact chemical such as chlorthal dimethyl which is effective against this weed in established grass.

Perennial weeds

Five species (creeping thistle; couch; yarrow; dandelion and broad-leaved dock) are described below to demonstrate the different features of their biology (particularly the perennating organs) that make them successful weeds. The flowering period of these weeds is mainly between June and October (see Figure 13.10), but the main problem for gardeners and growers is the plant's ability to survive and reproduce vegetatively.

Creeping thistle (Cirsium arvense). Plant family - Asteraceae (Compositae)

Creeping thistle

Couch grass

Yarrow

Broad-leaved dock

Creeping thistle

Couch grass

Yarrow

Broad-leaved dock

■■ ■ ■

F M A SPRING

M J J SUMMER

A S 0 AUTUMN

N D J WINTER

Figure 13.10 Perennial weeds: periods of flowering. Most flowers and seeds are produced between June and October. Annual weeds commonly flower throughout the year. However the slender speedwell flowers only between March and May

Figure 13.10 Perennial weeds: periods of flowering. Most flowers and seeds are produced between June and October. Annual weeds commonly flower throughout the year. However the slender speedwell flowers only between March and May

Damage. This species is a common weed in grass and perennial crops, e.g. apples, where it forms dense clumps of foliage, often several metres across.

Life cycle. The seedling cotyledons are broad and smooth, the true leaves are spiky (see Figure 13.11). The mature plant is readily recognizable by its dark green spiny foliage growing up to 1 m in height. It is found in all areas, even at altitudes of 750 m, and on saline soil. The species is dioecious, the male plant producing spherical and the female slightly elongated purple flower heads from July to September. Only when both sexes of the plant are within about 100 m of each other does fertilization occur in sufficient quantities to produce large numbers of brown, shiny fruit, 4 mm long. The seeds may germinate beneath the soil surface in the same year as their production, or in the following spring, particularly when soil temperatures reach 20°C. The resulting seedlings develop into a plant with a taproot which commonly reaches 3 m down into the soil.

  1. Seeds are wind-borne using a parachute of long hairs. The mature plant produces lateral roots which grow out horizontally about 0.3 m below the soil surface and may spread the plant as much as 6 m in one season. Along their length adventitious buds are produced that, each spring, grow up as stems. Under permanent grassland, the roots may remain dormant for many years. Soil disturbance, such as ploughing, breaks up the roots and may result in a worse thistle problem.
  2. The seedling stage of this weed is not normally targeted by the gardener/grower. The main control strategy is primarily against the perennial root system. For both amateur and professional horticulturalist, cutting down plants at the flower bud stage when sugars are being transferred from the roots upwards is a physical control measure that partly achieves this objective.

The amateur gardener can use another physical control, removing roots by deep digging. The amateur gardener is also able to use herbicide products which contain a mixture of dicamba, MCPA and mecoprop-P (all of these are translocated down to the roots).

The professional grower can use the technique of deep ploughing to expose and dry off roots. Products containing the three active ingredients mentioned in the last paragraph are also available to the professional. Effectiveness of herbicide translocation to the roots is greatest when applied in autumn, at a time when plant sugars are similarly moving down the plant.

Couch grass (Agropyron repens). Plant family - Poaceae (Graminae)

Damage. This grass, sometimes called 'twitch', is a widely distributed and important weed found at altitudes up to 500 m. It is able to quite rapidly take over plots growing ornamentals, vegetables or fruit.

Lawn Weed Identification
Figure 13.11 Creeping thistle plant showing (a) lateral roots, (b) seedling
Lawn Weeds Looks Like Grass

Figure 13.12 Couch grass plant showing rhizomes

Life cycle. The dull-green plant is often confused, in the vegetative stage, with the creeping bent (Agrostis stolonifera). However, the small 'ears' (ligules) at the leaf base characterize couch. The plant may reach a metre in height and often grows in clumps. Flowering heads produced from May to October resemble perennial ryegrass, but, unlike ryegrass, the flat flower spikelets are positioned at right angles to the main stem in couch. Seeds (9 mm long) are produced only after cross-fertilization between different strains of the species, and the importance of the seed stage, therefore, varies from field to field. The seed may survive deep in the soil for up to 10 years.

Figure 13.12 Couch grass plant showing rhizomes

Annual Rye Grass Unmowed Height
Figure 13.13 Yarrow plant showing (a) stolons, (b) seedling

Spread. Couch seeds may be carried in grass seed batches over long distances. From May to October, stimulated by high light intensity, overwintered plants produce horizontal rhizomes (see Figure 13.12) just under the soil; these white rhizomes may spread 15 cm per year in heavy soils, 30 cm in sandy soils. They bear scale leaves on nodes that, under apical dominance, remain suppressed during the growing period. In the autumn, rhizomes attached to the mother plant often grow above ground to produce new plants that survive the winter. If the rhizome is cut by cultivations such as digging or ploughing, fragments containing a node and several centimetres of rhizome are able to grow into new plants. The rapid growth and extension of couch plants provides severe competition for light, water and nutrients in any infested crop.

Control is achieved by a combination of physical and chemical methods. In fallow soil, deep digging or ploughing (especially in heavy land) exposes the rhizomes to drying. Further control by rotavating the weed when it reaches the one or two leaf stage disturbs the plant at its weakest point, and repeated rotavating will eventually cut up couch rhizomes into such small fragments that nodes are unable to propagate.

The amateur gardener can use the herbicide, glufosinate-ammonium plus fatty acids for control in such situations as ornamental beds containing woody perennials and in cane fruit.

For the professional horticulturalist, a translocated herbicide such as glyphosate, sprayed onto couch in fallow soils during active weed vegetative growth, kill most of the underground rhizomes. In established fruit, glufosinate-ammonium is recommended.

Yarrow (Achillea millifolium). Plant family - Asteraceae (Compositae)

Damage. This strongly scented perennial, with its spreading flowering head (Figure 13.13), is a common hedgerow plant found on most soils at altitudes up to 1200 m. Its persistence, together with its resistance to herbicides and drought in grassland, makes it a serious turf weed.

Figure 13.14 Dandelion plant growing in turf

Life cycle. The seedling leaves are hairy and elongated, with sharp teeth (Figure 13.3). The mature plant has dissected pinnate leaves produced throughout the year on wiry, woolly stems, which commonly reach 45 cm in height, and which from May to September produce flat-topped white-to-pink flower heads. Each plant may produce 3000 small, flat seeds annually. The seeds germinate on arrival at the soil surface.

  1. Seeds are dispersed by birds. When not in flower, this species produces below-ground and above-ground stolons which can grow up to 20 cm long per year. In autumn, rooting from the nodes occurs.
  2. Control of this weed may prove difficult. Routine scarification of turf does not easily remove the roots. For the amateur and professional, products containing 2,4-D and mecoprop are used against yarrow.

Dandelion (Taraxacum officinale). Plant family - Asteraceae (Compositae)

Damage. This species is a perennial with a stout taproot. It is a weed in lawns (see Figure 13.14), orchards and on paths. Several similar species such as mouse-ear hawkweed (Hieracium pilosella) and smooth hawk's beard (Crepis capillaris) present problems similar to dandelion in turf.

Life cycle. Seedlings emerge mainly in March and April. Flowers are produced from May to October. An average of 6000 seeds is produced by each plant. Most seeds survive only one year in the soil, but a few may survive for five years. Mature plants can survive for 10 years.

  1. Seeds are wind dispersed by means of tiny 'parachutes' and may travel several hundred metres. They are also able to spread in the moving water found in ditches and by animals through their digestive systems. The plant may regenerate from roots, after being chopped up by spades or rotavators.
  2. Physical removal of the deep root by a sharp trowel is recommended, but this leaves bare gaps in turf for invasion by other weeds. For the amateur gardener and professional groundsman, products containing the two translocated ingredients, 2,4-D and dicamba, are able to kill the stout penetrating root of the dandelion.

Broad-leaved dock (Rumex obtusifolius). Plant family - Polygonaceae

Damage. This is a common perennial weed of arable land, grassland and fallow soil.

Life cycle. The seedling cotyledons are narrow (see Figure 13.3). Seedling true leaves are often crimson coloured. The mature plant is readily identified by its long (up to 25 cm) shiny green leaves (see Figure 13.15), known to many as an antidote to 'nettle rash'. The plant may grow 1 m tall, producing a conspicuous branched inflorescence of small green flowers from June to October. The seed represents an important stage in this perennial weed's life cycle, surviving many years in the soil, and most commonly germinating in spring. Like most Rumex spp, the seedling develops a stout, branched taproot (see Figure 13.15), which may penetrate the soil down to 1 m in the mature plant, but most commonly reaches 25 cm. Segments of the taproot, chopped by cultivation implements are capable of producing new plants.

  1. The numerous plate-like fruits (3 mm long) may fall to the ground or be dispersed by seed-eating birds such as finches. They are sometimes found in batches of seed stocks.
  2. High levels of seed production, a tough taproot and a resistance to most herbicides present a problem in the control of this weed. For the amateur gardener, in turf, physical removal of the deep root by a sharp trowel is recommended, but this leaves bare gaps in turf for invasion by other weeds. A product containing dicamba, MCPA, and mecoprop-P is effective against young plants.

For the professional horticulturalist, attempts in fallow soils to exhaust the root system by repeated ploughing and rotavating have proved useful. Young seedlings are easily controlled by translocated chemicals such as 2,4-D, but the mature plant is resistant to all but a few translocated chemicals, e.g. asulam, which may be used on grassland (not fine turf ), soft fruit, top fruit and amenity areas, during periods of active vegetative weed growth when the chemical is moved most rapidly towards the roots.

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Responses

  • couch grass
    There are several studies about couch grass, about their chemical composition and its health benefits. It’s been centuries since researchers have found uses of couch grass for folk medicine in the form of decoction and infusions as diuretic, tonic, sedative, analgesic, and as a coating agent in dermatitis. The use of this grass has been known worldwide.
    7 years ago
  • Tom
    How to kill dandelion weeds?
    7 years ago
  • Alyx Reid
    How to control weeds in flower beds?
    7 years ago
  • Luisa Esposito
    What is the size, quantity and viability of grass seeds in annual and perennial grass species?
    7 years ago
  • dominik
    Which rotovator will cut easily through dense weeds?
    7 years ago
  • gebre asmara
    How to control creeping field speedwell?
    7 years ago
  • ambrogio
    How to kill v. persica?
    2 years ago

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