Agriculture Reference
In-Depth Information
several centimetres of rhizome are able to grow into
new plants. The rapid growth of couch plants creates
severe competition for light, water and nutrients in
any infested crop.
Control
. This is achieved by a combination of
cultural
and
chemical
methods. In bare 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 they are unable to propagate.
The gardener can use the herbicide
glyphosate
for
control in such situations as bare soil and ornamental
beds containing woody perennials, and in cane fruit,
but
not
in growing crops. Great care is needed to
avoid spraying foliage of garden plants.
Creeping thistle (
Cirsium arvense
)
This belongs to the plant family Asteraceae
(Compositae).
Damage and location
. 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 spiky (Figure 17.18). The
mature plant is readily recognizable by its dark green
spiny foliage growing up to a metre in height. It is
found in all areas, even at altitudes of 750 m, and
on saline soil. The species is dioecious (see p. 100),
the male plant producing spherical and the female
slightly elongated purple flower heads from July to
September. Only when the two sexes of plants 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
that commonly reaches 3 m down into the soil.
Spread
. 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, causing a large dense patch of thistles.
Along their length are produced adventitious buds
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.
Figure 17.17
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
base of each leaf are a distinguishing feature of couch.
The plant may reach a metre in height and often
grows in clumps. 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.
Spread
. Couch seeds may be carried in grass seed
batches over long distances. From May to October,
stimulated by the high light intensity, overwintered
plants produce horizontal rhizomes (Figure 17.17)
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 the underground nodes
that remain suppressed during the growing period
and do not produce new stems. But, in 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
