Dormancy and duration of viability

Although the seed production figures of an individual plant are impressive (Table 1.9), the total seed population in a given area is of greater significance. The soil seed reservoir reflects both past and present seed production, in addition to those imported from elsewhere, and is reduced by germination, senescence and the activity of herbivores (Figure 1.3). Estimates of up to 100,000 viable seeds per square metre of arable soil represent a massive competition potential to both existing and succeeding crops, especially since the seed rate for spring barley, for instance, is only approximately 400 m-2! Under long term grassland, weed seed numbers in soil are in the region of 15,000-20,000m-2, so conversion of arable land to long-term grassland offers growers a means of reducing soil weed-seed burden.

The length of time that seeds of individual species of weed remain viable in soil varies considerably. The nature of the research involved in collecting such data means that few

Table 1.9 Seed production by a number of common arable weeds and wheat (adapted from Radosevich and Holt, 1984 containing information from Hanf, 1983).

Weed

Common name

Seed production per plant

Veronica persica

Common field speedwell

50-100

Avena fatua

Wild oat

100-450

Galium aparine

Cleavers

300-400

Senecio vulgaris

Groundsel

1,100-1,200

Capsella bursa-pastoris

Shepherd's purse

3,500-4,000

Cirsium arvense

Creeping thistle

4,000-5,000

Taraxacum officinale

Dandelion

5,000 (200 per head)

Portulaca oleracea

Purslane

10,000

Stellaria media

Chickweed

15,000

Papaver rhoeas

Poppy

14,000-19,500

Tripleurospermum maritimum spp. inodorum

Scentless mayweed

15,000-19,000

Echinochloa crus-galli

Barnyard grass

2,000-40,000

Chamaenerion angustifolium

Rosebay willowherb

80,000

Eleusine indica

Goose grass

50,000-135,000

Digitaria sanguinalis

Large crabgrass

2,000-150,000

Chenopodium album

Fat hen

13,000-500,000

Triticum aestivum

Wheat

90-100

Within fields Between fields Between regions

Within fields Between fields Between regions

Wool trade -

Livestock (transported) -

Contaminated seed -

Irrigation water -

Manure

Combines -

Livestock (walking)

Soil wash -

Birds -

Tillage implements -

Wind

Explosive dehiscence -

Rain splash -

10-2 10-1 10° 101 102 103 104 105 106 107 Dispersal distance (m)

Figure 1.2 Some methods of weed seed dispersal with their estimated range in metres. Reproduced from Liebman M., Mohler C.L. and Staver C.P. (2001) Ecological Management of Agricultural Weeds. Cambridge University Press.

Human transport

Natural processes

Figure 1.3 Factors affecting the soil seed population (from Grundy and Jones, 2002).

comprehensive studies have been carried out, but those that have (see Toole and Brown, 1946, for a 39 year study!) show that although seeds of many species are viable for less than a decade, some species can survive for in excess of 80 years (examples include poppy and fat hen). Evidence from soils collected during archaeological excavations reveals seeds of certain species germinating after burial for 100 to 600 (and maybe even up to 1700!) years (0dum, 1965).

Dormancy in weed seeds allows for germination to be delayed until conditions are favourable. This dormancy may be innate and it is this that contributes to the periodicity of germination, as illustrated in Figure 1.1, In addition, dormancy may be induced or enforced in non-dormant seeds if environmental conditions are unfavourable. This ensures that the weed seed germinates when conditions are most conducive to seedling survival.

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