Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 49

the time of germination may occur between species characteristic of early
versus late spring (e.g., Figure 2.2). For some species, early and late spring are
apparently very different seasons. Third, although most species have times of
the year in which germination is most probable,at least some seeds germinate
over a wide range of other seasons. This occurs because the dormancy mecha-
nisms are less than perfect in matching germination to one particular season.
However, that variability provides adaptation to environmental unpredict-
ability. Finally, the species that have a very broad season of germination tend
to be small-statured, rapidly maturing plants, with broad-amplitude temper-
ature tolerance. Such species are well adapted to gardens and mixed high-
intensity vegetable farms where the cropping conditions are unpredictable,
and therefore, early catastrophic mortality is frequent.
A large comparative study by Forcella et al . (1997) illustrates the impor-
tance of understanding how dormancy affects the seasonality of seedling
emergence. By examining percentage emergence of several weed species in
relation to environmental conditions at 22 site and year combinations, they
identified soil temperature and moisture thresholds that induced secondary
dormancy in Setaria faberi , S. viridis , Polygonum convolvulus , P. pennsylvanicum ,
and Amaranthus spp. (mostly A. retroflexus ). Once the threshold for one of these
species has been passed during a growing season, the seeds are induced into
secondary dormancy. Few individuals are likely to emerge after that date, and
a grower can modify management plans accordingly.
Probably the principal utility of understanding periodicity of weed seed
germination is that it allows disruption of weed life cycles. Two approaches
are mentioned here and discussed in depth in Chapters 4,6,and 7.First,if the
time of germination is known for the dominant weeds in the seed bank, crop
planting dates can be adjusted so that either (i) the crop emerges before the
weeds and thereby obtains a competitive advantage, or (ii) weeds are allowed
to germinate and are then destroyed during seedbed preparation. Second, by
rotating between crops with radically different planting dates, a grower can
block the establishment and reproduction of particular groups of weeds in
any given year. Thus, for example, in a fall-sown grain crop, spring germinat-
ing weeds will either remain dormant or, if they do germinate, suffer heavy
competition from the already well-established crop. Those that do not germi-
nate are subjected to another year of mortality risk as seeds, reducing weed
pressure on later crops. Alternation of early spring grains or vegetables with
late spring- or summer-planted soybean or vegetable crops may be equally
effective in disrupting weed life cycles.