Agriculture Reference
In-Depth Information
fertilized turf. Nettles and chickweed prefer highly
fertile soils.
X The growth habit of a weed may infl uence its
success. Chickweed, creeping buttercup, slender
speedwell and creeping cinquefoil produce
horizontal (prostrate) stems bearing numerous
leaves that prevent light reaching emergent crop
seedlings or turf. Groundsel and fat hen have an
upright habit that competes less for light in the
early period of weed growth. Perennial weeds
such as bindweed, cleavers and nightshades are
able to grow alongside and climb up woody plants,
such as cane fruit and border shrubs, making
control diffi cult.
X Seed production may be high in certain species.
A scentless mayweed plant (perennial) may
produce 300,000 seeds, fat hen (annual) 70,000
and groundsel (annual) 1,000 seeds in a growing
season.
X Dormancy (see also p. 68) is seen in many weed
species. In this way, weed seed germination
commonly continues over a period of four or fi ve
years after seed dispersal, presenting the grower
with a continual problem. Groundsel is something
of an exception, since many of its seeds germinate
in the fi rst year.
X Perennial weeds with swollen underground
organs provide the greatest problems to the
horticulturist. This is especially so in long-term
crops such as soft fruit and turf because foliage-
acting and residual herbicides may have little effect
on these underground organs.
X Fragmentation of above-ground parts may be
important. A lawnmower used on turf which
has slender speedwell weed growing in it cuts
and spreads the delicate stems that establish
(like cuttings) in other parts of the lawn in damp
conditions.
X Greenhouse production generally suffers less
from weed problems because composts and
border soils are regularly sterilized, but weeds
such as sow thistle, chickweed (see Figure 17.8),
groundsel and hairy bitter cress may become
established.
Figure 17.6 Cleavers plant that produces 'sticky-bud'
fruit
X Seeds of chickweed and dandelion may be spread
by the moving water in ditches.
X The fruit of cleavers weed (Figure 17.6) stick
to clothes and hair of humans and animals in a
manner similar to 'velcro'. Chickweed seed is held
in a similar way. Groundsel and annual meadow
grass seeds become sticky in damp conditions and
are able to stick to boots and machinery wheels.
X Some of the seeds of groundsel, annual meadow
grass, yarrow and dock survive digestion in the
stomachs of birds. Chickweed and annual meadow
grass seed are also able to survive rabbit digestive
systems.
X Cut stems of slender speedwell are moved by
grass mowers.
X Ants carry around the seeds of speedwell.
X The underground horizontal roots, stolons and
rhizomes of perennial weeds such as thistle,
yarrow and couch, respectively, slowly spread the
weed from its point of origin.
X Ploughs and rotavators move around cut
underground fragments of weeds such as thistles,
yarrow, dandelion and couch.
X Commercial seed stocks can be contaminated with
seeds of weeds such as speedwells and couch.
17
Ephemeral weeds
Hairy bitter cress ( Cardamine hirsuta )
Other aspects of weed biology
X Soil conditions may favour certain weeds.
Sheep's sorrel ( Rumex acetosella ) prefers acid
conditions. Mosses are found in badly drained
soils. Knapweed ( Centaurea scabiosa ) competes
well in dry soils. Common sorrel ( Rumex acetosa )
survives well on phosphate-dei cient land.
Yorkshire fog grass ( Holcus lanatus ) invades poorly
This is an annual species of the Brassicaceae family.
Damage and location . It is common throughout
Britain and Ireland. It is often seen in gardens
and is particularly common on bare ground, in
greenhouses and at the side of paths. The compost
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