(Stellaria media). Plant family – Caryophyllaceae
This species is found in many horticultural situations as a
weed of flowerbeds, vegetables, soft fruit and greenhouse plantings. It
has a wide distribution throughout Britain, grows on land up to altitudes
of 700 m, and is most important on rich, heavy soils.
The seedling cotyledons are pointed with a light-coloured tip
while its true leaves have hairy petioles (see Figure 13.5). The adult plant
has a characteristic lush appearance and grows in a prostrate manner over
the surface of the soil; in some cases it covers an area of 0.1 square metres,
its leafy stems crowding out young plants as it increases in size. Small
white, five-petalled flowers are produced throughout the year, the flowering
response being indifferent to day length. The flowers are self-fertile.
|Figure 13.5 (a) Chickweed seedling (b) C hickweed plant
An average of 2500 disc-like seeds (1 mm in diameter) may result from
the oblong fruit capsules produced by one plant. Since the first seed
may be dispersed within 6 weeks of the plant germinating and the plant
continues to produce seed for several months, it can be seen just how prolific the species is. The large numbers of seed (up to 14 million/ha)
are most commonly found in the top 7 cm of the soil where, under
conditions of light, fluctuating temperatures and nitrate ions, they may
overcome the dormancy mechanism and germinate to form the seedling.
Many seeds, however, survive up to the second, third and occasionally
fourth years. Figure 13.6 shows that germination can occur at any time
of the year, with April and September as peak periods. Chickweed is
an alternate host for many aphid transmitted viruses (e.g. cucumber
mosaic), and the stem and bulb nematode.
|Figure 13.6 Annual and perennial weeds : periods of seed germination.
Note that chickweed,
groundsel and field speedwell seeds germinate
Many other species are more
(Reproduced by permission of
Blackwell Scientific Publications)
The seeds are normally released as the fruit capsule opens
during dry weather; they survive digestion by animals and birds and may
thus be dispersed over large distances. Irrigation water may carry them
into channels and ditches.
This weed is controlled by a combination of methods. Physical
controls include partial sterilization of soil in greenhouses while hoeing
in the spring and autumn periods prevents the seedling from developing
and flowering. Mulching is effective against germinating weeds.
gardeners can use the non-selective, non-persistent herbicide, glufosinate-ammonium plus fatty acids
for control in such situations
as ornamental beds containing woody perennials, and in cane fruit.
horticulturalists use pre-emergent contact sprays such as paraquat
applied before a crop emerges. A soil-applied root-acting
herbicide such as propachlor
is used on a crop such as strawberries
before weeds germinate. A foliage-acting herbicide such as linuron
applied for chickweed control in potatoes.
). Plant family – Asteraceae (Compositae)
|Figure 13.7 (a) Groundsel seedling
This is a very common and important weed, particularly on
heavy soil. Its high level of seed production and the ability of its seed to
germinate soon after release lead to dense mats of the weed. It grows on
both rich and poor soils up to almost 600 m in altitude.
The seedling cotyledons are narrow, purple underneath,
and the first true leaves have step-like teeth (see Figure 13.7).
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. Produces about 45 column-shaped seeds,
2 mm in length densely packed in the fruit head. As can be seen
in Figure 13.5, 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
generation surviving the winter), and each generation may give
rise to a thousand 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, e.g. tomatoes and hops.
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.
A combination of control methods may be necessary for
successful control. Physical control is by hoeing or by mechanical
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.
gardener can use the herbicide, glufosinateammonium plus fatty acids,
for control in such situations as
ornamental beds containing woody perennials, and in cane fruit.
horticulturalist can use contact herbicides, such
to control the weed in all stages of growth on paths
or in fallow soil (but never on garden plants or turf). Soil-acting
chemicals such as propachlor
on brassicas kill off the germinating
seedling. An established groundsel population, especially in a
crop such as lettuce (a fellow member of the Asteraceae family)
requires careful choice of herbicide to avoid damage to the crop (see