Agriculture Reference
In-Depth Information
step-like teeth (Figure 17.12). The adult plant has an
upright habit and produces as many as 25 yellow,
small-petalled flower heads. Flowering occurs in all
seasons of the year. Each flower produces about
45 column-shaped seeds, 2 mm in length, densely
packed in the fruit head. As can be seen in Figure
17.11, the seeds may germinate at any time of the
year, with early May and September as peak periods.
Since there may be more than three generations of
groundsel per year (the autumn plants surviving the
winter) and each generation may give rise to 1,000
seeds, it is clear why groundsel is one of the most
successful colonizers of cultivated ground. Its role as
a symptomless carrier of the wilt fungus
Verticillium
increases its importance in certain crops such as
tomatoes and hops.
Spread
. The seeds bear a mass of fine hairs. These
hairs, in dry weather, can parachute seeds along on
air currents for many metres. In wet weather the
seeds become sticky and may be carried on the feet
of animals, including humans. The seeds survive
digestion by birds and thus can be transported in this
way.
Control
. A combination of control methods may be
necessary for successful control.
Cultural
control
is by hoeing or cultivation, particularly in spring
and autumn to prevent developing seedlings from
flowering. Care should be taken not to allow uprooted
flowering groundsel plants to release viable seed.
The gardener can use the herbicide
glyphosate
for
control in uncultivated areas, and in such situations
as ornamental beds containing woody perennials, and
in cane fruit. Great care is needed to avoid spraying
foliage of garden plants.
Figure 17.13
Annual meadow grass adult plant
it often emerges in sufficient quantities to smother
crop seedlings. Its seed may be present as an impurity
in grass seed bought at the garden centre. (Special
selections of seed containing this species may
sometimes be deliberately used by seed companies
as a component of turf seed mixtures.)
Life cycle
. Flowers can occur at any time of year and
are usually self-pollinated. About 2,000 seeds per plant
are produced from April to September. Plants will
flower and seed even when mown regularly, because
the weed is able to flower at a plant height of less than
1 cm. Seeds germinate from February to November,
with the main peaks in early spring and in autumn.
Some seed will germinate soon after their release;
others can remain viable in soil for at least four years.
This weed species can be the alternate host of a
number of nematode species such as root knot
nematode (
Meloidogyne minor
), which is increasingly
Annual weeds
There are at least 50
successful
annual weed species
found in gardens. This topic can cover only a few
examples that illustrate the main points of life cycle,
spread and control. Two species, annual meadow
grass and speedwell, are described below.
Annual meadow grass (
Poa annua
)
This belongs to the plant family Poaceae (Graminae).
Damage and location
. This species is a fairly small
annual (or short-term perennial) found in herbaceous
and woody perennial borders, on sports grass
surfaces, on paths and in vegetable plots (Figure
17.13). It is quite often seen on golf greens where its
flowering heads and its yellowish green foliage make
it conspicuous. It does not thrive on acid soils or those
low in phosphates. Despite its relatively small size,
