Damage

Problems caused by weeds may be categorized into seven main areas:

Competition 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.

Drainage (see Chapter 19) 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.

Machinery such as mowing machines and harvesting equipment may be fouled by weeds, such as knotgrass, that have stringy stems

Poisonous plants. 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 Figure 13.2 Ragwort, a poisonous weed to children and also contaminate mechanically harvested crops such as blackcurrants and peas for freezing.

Seed quality is lowered by the presence of weed seeds. For example fat hen can contaminate batches of carrot seed.

Tidiness 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.

Weed identification

As with any problem in horticulture, recognition and identification are essential before any reliable control measures can be attempted. The weed seedling causes little damage to a crop, but will quickly grow to be the damaging adult plant bearing seeds that will spread. The

seedling stage is relatively easy to control, whether by physical or by chemical methods. Identification of this stage is therefore important and with a little practice the gardener or grower may learn to recognize the important weeds using such features as cotyledon and leaf shape, colour and hairiness of the cotyledons and first true leaves (see Figure 13.3).

Chickweed (X1.5)

Bright green. Cotyledons have a light-coloured tip and a prominent mid-vein. True leaves have long hairs on their petioles

Groundsel (X1.5)

Cotyledons are narrow and purple underneath. True leaves have step-like teeth

Large field speedwell (X1.5) Cotyledons like the 'spade' on playing cards. True leaves hairy, notched and opposite

Chickweed (X1.5)

Bright green. Cotyledons have a light-coloured tip and a prominent mid-vein. True leaves have long hairs on their petioles

Creeping thistle (X 1.5)

Cotyledons large and fleshy. True leaves have prickly margins

Creeping thistle (X 1.5)

Cotyledons large and fleshy. True leaves have prickly margins

Groundsel (X1.5)

Cotyledons are narrow and purple underneath. True leaves have step-like teeth

Broad Leaved Weeds Vein

Small broad cotyledons. True leaves hairy and with pointed lateral lobes

Small broad cotyledons. True leaves hairy and with pointed lateral lobes

Broad Leaved Dock Cotyledon

Broad-leaved dock (X1.5)

Cotyledons narrow. First leaves often crimson, rounded with small lobes at the bottom

Figure 13.3 Seedlings of common weeds. Notice the difference between cotyledons and true leaves. (Reproduced by permission of Blackwell Scientific Publications)

Within any crop or bedding display, a range of different weed species will be observed. Changes in the weed flora may occur because of environmental factors such as reduced pH, because of new crops that may encourage different weeds to develop, or because repeated use of one herbicide selectively encourages certain weeds, e.g. groundsel in lettuce crops or annual meadow grass in turf. Horticulturists must watch carefully for these changes so that their chemical control may be adjusted. The mature weeds may be identified using an illustrated flora book, which shows details of leaf and flower characters.

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