Agriculture Reference
In-Depth Information
integrated management of weed populations, especially if crop rotation pro-
vides some years in which the crop is removed early in the maturation period
of the weed.
The capacity of wandering perennial species to produce vegetative propa-
gules is also large. For example, single tubers of Cyperus esculentus planted in
California and Zimbabwe grew into clones that in one year produced 6900
and 17700 tubers, respectively (Tumbleson & Kommedahl, 1961; Lapham,
1985). Unlike seed production, which is necessarily preceded by a period of
vegetative growth, vegetative reproduction in wandering perennials often
begins early in life.Production of new tubers in Cyperus esculentus may begin as
early as 3 weeks after tubers sprout (Bell et al ., 1962). Adventitious buds form
on the roots of Euphorbia esula and Cirsium arvense within 1-2 and 6-8 weeks of
seedling emergence, respectively (Selleck, 1958; Bakker, 1960). Consequently,
the number of potential individuals produced is roughly proportional to the
size of the plant, and tends to increase exponentially when interference is
absent (Lapham,1985).As a result,vigorous competition from a crop is impor-
tant for reducing vegetative reproduction of wandering perennials
(HÃ¥kansson, 1968; HÃ¥kansson & Wallgren, 1972). Strategies for mechanical
control of wandering perennials are discussed extensively in Chapter 4.
Dispersal of seeds and ramets
The spatial scale of dispersal events
Various seed dispersal mechanisms work on greatly differing spatial
scales (Figure 2.7). Dispersal of seeds by rain splash or explosive dehiscence of
the fruit is generally not effective for more than a few meters, unless assisted
by wind.At the other extreme, contaminated crop seed and other agricultural
commodities have regularly transported weeds many hundreds of kilometers
and have been major sources of intercontinental weed introductions.
Three categories of scale can be usefully distinguished for purposes of
managing the spread of weeds: dispersal within fields, between fields, and
between regions. Some natural dispersal processes operate primarily at the
within-field scale (Figure 2.7).For a weed to spread long distances,other agen-
cies must come into play. Wind, birds, mammals, and water may transport
seeds between fields. Natural processes, however, rarely transport species
between regions or continents, which is a large part of why regional floras
were highly distinct prior to human commerce.
If a newly arrived weed persists within a field, its spread is likely because
both natural processes and machinery will tend to disperse the species out
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