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.
(Cirsium arvense). Plant family – Asteraceae
|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 13.11 Creeping thistle plant showing
(a) lateral roots, (b) seedling
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.
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.
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.
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
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.
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).
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.
). Plant family – Poaceae (Graminae)
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.
|Figure 13.12 Couch grass plant showing rhizomes
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.
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.
|Figure 13.13 Yarrow plant showing
(a) stolons, (b) seedling
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.
gardener can use the herbicide, glufosinateammonium
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
sprayed onto couch in fallow soils during active
weed vegetative growth, kill most of the underground rhizomes. In
established fruit, glufosinate-ammonium
Yarrow (Achillea millifolium
). Plant family – Asteraceae (Compositae)
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
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.
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.
Control of this weed may prove difficult. Routine scarification
of turf does not easily remove the roots. For the amateur
products containing 2,4-D
are used against
). Plant family – Asteraceae
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.
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.
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.
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
products containing the two translocated ingredients, 2,4-D
dicamba , are able to kill the stout penetrating root of the dandelion.
(Rumex obtusifolius). Plant family – Polygonaceae
This is a common perennial weed of arable land, grassland
and fallow soil.
|Figure 13.15 Broad-leaved dock showing
(a) swollen taproot, (b) seedling
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.
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.
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
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,
is effective against young
For the professional
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,
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.
Mixed weed populations
In the field a wide variety of both annual and perennial weeds may
occur together. The horticulturalist must recognize the most important
weeds in their holding or garden, so that a decision on the precise use
of chemical control with the correct herbicide is achieved. Particular
care is required to match the concentration of the herbicide to the weed
species present. Also, the grower must be aware that continued use of
one chemical may induce a change in weed species, some of which may
be tolerant to that chemical.