Garden weeds - Principles of Horticulture - page 212

way, flowering and releasing seed is prevented. It
should also be remembered that pieces of the stem
are able to re-establish in suitably moist conditions. In
container plants, weed seedlings should be removed
before seed production and dispersal begin. Covering
border soil or bare soils with a 15 cm deep mulch of
bark or compost will be effective in suppressing the
emergence of weed. There are several chemical
control options for this weed. For bare soil and the
side of paths, a choice of two contact chemicals,
fatty acids and pelargonic acid , are available. Where
weed infestations are high, the translocated chemical
glyphosate may be used. It is advised that the weed
is controlled by this chemical at the flowering stage in
order to have maximum effect. Extreme care should
be taken not to allow spray to reach garden annuals,
or any leaf growth on perennials as this chemical is
not selective and is capable of killing all plant species.
Glyphosate may be sprayed around the base of woody
plants so long as there are no emerging suckers
visible. Glyphosate is almost immediately neutralized
when in contact with soils, and therefore normally
presents no risk to garden species being planted into
soil that have recently been treated by this chemical.

Figure 17.7 Hairy bitter cress can quickly invade bare
ground and greenhouses

of container grown plants in garden centres may also
be covered by this species (and this is one means
by which the weed may be introduced into gardens)
(Figure 17.7). It is recorded up to an altitude of 1,500
m. It is a common weed of gardens, greenhouses,
paths, railways and waste ground.
Life cycle . Hairy bitter cress grows as an annual (or
biennial) plant. It flowers throughout the year but
peaks from March to August. It is self-pollinated.
Seed is most commonly released in May and June
(less often in September-October). A large plant
may release several thousand seeds. Weeding may
encourage this dispersal. High temperatures dry the
dispersed seed and induce the ripening process that
allows germination. Very few unripe seeds germinate
after dispersal. The peak time of germination is
between July and August and between November
and December. The seedlings, being frost-tolerant,
survive all but the severest of winters. The species is
able to complete its life cycle in as little as five weeks.
In fertile soils, the life cycle may take longer. The
dormant seeds of this species in the soil can lead to
a relatively persistent 'seed-bank' that emerges over
several years.
Spread . This is by means of the 'explosive' discharge
mechanism which can result in the seeds travelling
up to a metre away from the mother plant. Also, the
seeds become sticky when wet and can be spread on
tools and clothing.
Control . As with all annual weeds, it is important
to destroy the plant before seed can be produced.
Seedlings should be removed by cultural methods
such as cultivations and hoeing, among ornamentals
or vegetables, preferably when soils are dry. In this

Chickweed ( Stellaria media) and
mouse-eared chickweed ( Cerastium
fontanum )

These belong to the plant family Caryophyllaceae.
Damage and location . The first species is found in
many horticultural situations as a weed of bare soil,
among herbaceous and woody perennials, and in
vegetables, soft fruit and greenhouse plantings. It
has a wide distribution throughout Britain and Ireland,
growing on land up to altitudes of 700 m, and is most
important on nutrient-rich, heavy soils.
Life cycle . The seedling cotyledons are pointed
with a light-coloured tip, while the true leaves
have hairy petioles (Figure 17.8). 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 m 2 , 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.
An average of 2,500 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 six 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/