|Figure 13.1 Chickweed
and sowthistle crowding
Problems caused by weeds may be categorized into seven main areas:
between the weed and the plant for water, nutrients and light
may prove favourable to the weed if it is able to establish itself quickly.
A large cleaver plant (Galium aparine),
for example, may compete for a
square metre of soil. The cultivated plants are therefore deprived of their
major requirement and poor growth results. The extent of this competition
is largely unpredict able, varying with climatic factors such as temperature
and rainfall, soil factors such as soil type, and cultural factors such as
cultivation method, plant spacing and quality of weed control in previous
seasons. Large numbers of weed seeds may be introduced into a plot in
poor quality composts or farmyard manure. The uncontrolled proliferation
of weeds will inevitably produce serious plant losses.
|Figure 13.2 Ragwort, a
(see Soil water
) depends on a free flow of water along
ditches. Dense growth of weeds such as chickweed may seriously reduce
this flow and increase waterlogging of horticultural land.
such as mowing machines and harvesting equipment may be
fouled by weeds, such as knotgrass, that have stringy stems
Ragwort (see Figure 13.2), sorrel and buttercups are
eaten by herbivorous animals when more desirable food is scarce. Also,
poisonous fruits of plants such as black nightshade may be attractive
to children and also contaminate mechanically harvested crops such as
blackcurrants and peas for freezing.
is lowered by the presence of weed seeds. For example fat
hen can contaminate batches of carrot seed.
is important for a well-maintained garden. The amenity
horticulturist may consider that any plant spoiling the appearance of
plants in pots, borders, paths or lawns should be removed, even though
the garden plants themselves are not affected.
Alternate hosts of pests and diseases.
Pests and diseases are commonly
harboured on weeds. Chickweed supports whitefly, red spider mite
and cucumber mosaic virus in greenhouses. Sowthistles are commonly
attacked by chrysanthemum leaf miner. Groundsel is everywhere
infected by a rust which attacks cinerarias (see Figure15.9). Charlock
may support levels of club root, a serious disease of brassica crops. Fat
hen and docks allow early infestations of black bean aphid to build-up.
Speedwells may be infested with stem and bulb nematodes.