While there are at least 50 successful annual weed species in
horticulture, this book can cover only a few examples that illustrate the
main points of life cycles and control. Two species, annual meadow
grass and speedwell, are described.
Annual meadow grass
). Plant family – Poaceae (Graminae)
|Figure 13.8 Annual meadow grass plant
This species is a quite small annual (or short-term perennial)
found on a range of ornamental and sports grass surfaces, on paths and
in vegetable plots (see Figure 13.8). It is able to establish quickly on
bare ground. It does not thrive on acid soils or those low in phosphates.
Despite its relatively small size, it often emerges in sufficient quantities
to smother crop seedlings. Its seed may be present as an impurity in
commercial grass seed. Special selections of this species are used in
seed mixtures for lawns.
Flowers can occur at any time of year and are usually selfpollinated.
About 2000 seeds per plant are produced from April to
September. Plants will flower and seed even when mown regularly. Seeds
germinate from February to November with the main peaks in early spring
and autumn. Some seed will germinate soon after their release; others can
remain viable in soil for at least 4 years. This weed species can be the host
of a number of nematode species that also attack important crops.
There is no obvious dispersal mechanism. Most seeds fall
around the parent plant and become incorporated into the soil. Seeds
may be carried around on boots and wheels of machinery. Worms may
bring seeds to the soil surface in worm casts.
is achieved by a variety of methods. The
physical action of hoeing normally controls the
weed especially when it is in the young stage. Deep
digging-in of seedlings and young plants is also
usually effective. Mulching is effective against
germinating weeds in flower beds and fruit areas.
gardener can use the non-selective,
non-residual herbicide, glufosi-nate-ammonium
plus fatty acids,
for control in situations such as
ornamental beds containing woody perennials and in
cane fruit. The professional
horticulturalist may use paraquat
for total chemical control,
but these two chemicals should never be sprayed in the
vicinity of growing crops. Care should be taken not
to walk on grass after application of these chemicals. Chlorpropham
may be used as a soil applied chemical on crops such as currants, onions and chrysanthemum.
(Veronica persica and V. filiformis
family – Scrophulariaceae
|Figure 13.9 (a) Field speedwell seedling
(b) Field speedwell plants
The first species, the large field speedwell (V.
persica) is an important weed in vegetable production,
crowding out young crop plants and reducing growth
of more mature stages. The second species, the
slender or round-leaved speedwell (V. filiformis), once
considered a desirable rock garden plant introduction
from Turkey, has become a serious turf problem.
The seedling cotyledons are spade shaped,
while the true leaves are opposite, notched and hairy
(see Figure 13.9) in both species. The adult plants
have erect, hairy stems and rather similar broadtoothed
leaves. V. persica produces up to 300 bright
blue flowers, 1 cm wide, per plant. The flowers
are self-fertile and occur throughout the year, but
mainly between February and November. The adult
plant produces an average of 2000 light brown boatshaped
seeds 2 mm across. The seeds of this species
germinate below soil level all year round, but most
commonly from March to May (see Figure 13.5), the
winter period being necessary to break dormancy.
Seeds may remain viable for more than 2 years. V.
filiformis produces self-sterile purplish-blue flowers
between March and May, and spreads by means of
prostrate stems which root at their nodes to invade
fine and coarse turf, especially in damp areas.
Segments of this weed cut by lawnmowers easily
root and further increase the species. Seeds are not
important in its spread.
Seeds of V. persica falling to the ground
may be dispersed by ants. Seed of this species can be
spread as contaminants of crop seed. V. filiformis does
not produce seed. Its slow spread is mainly by means
of grass-cutting machinery.
Field speedwell (V. persica) is controlled
by a combination of methods. The physical action
of hoeing or mechanical cultivation, particularly in
spring, prevents developing seedlings from growing
to mature plants and producing their many seeds.
gardener can use the herbicide, glufosinate-ammonium plus fatty acids
in situations such as ornamental beds containing
woody perennials and in cane fruit.
For the professional
grower, total contact herbicides, such as paraquat,
may be sprayed to control the weed on paths or in fallow soils. Soilacting
chemicals such as chlorpropham
on crops such as lettuce, onions and chrysanthemums kill off the germinating weed seedling. A contact,
foliar chemical such as clopyralid,
may be sprayed on to brassicas,
young onions, and strawberries to control emerging seedlings.
The slender speedwell (V. filiformis
) represents a different problem for
control. Physical controls such as regular close mowing and spiking of
turf removes the high humidity necessary for this weed’s establishment
gardener has difficulty controlling this weed with the turf
herbicides available. The amateur
grower can control
the weed in a turf seedbed using a total, contact chemical such as glufosinate-ammonium plus fatty acids,
a few weeks before sowing
the turf seed. This ‘stale seed bed’
method leaves the turf to establish
relatively undisturbed by weeds. Organic growers may remove stale
seedbed weeds by hoeing. professional
growers can use a selective
contact chemical such as chlorthal dimethyl
which is effective against
this weed in established grass.