Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 53

that were prompted to germinate in response to seedbed preparation without
cueing germination of many additional seeds. In contrast, a deep cultivation
tends to bring up seeds that are then prompted to germinate by disturbance-
related cues. Dynamics of the seed bank in response to tillage is discussed
further in Chapter 4.
The germination response of weeds to soil disturbance can also be used to
induce inappropriate germination. For example, species with broad seasonal-
ity of germination can be stimulated to establish at times that are unsuitable
for survival to reproduction, thereby depleting the seed bank. A more
common application is to use shallow cultivations with intervening rests
before planting to flush out and kill many of the weeds that would otherwise
establish with the crop. Use of this “false seedbed” method is analyzed in
Chapter 4.
Not all species of weeds are sensitive to germination cues associated with
soil disturbance. Most of these are relatively large seeded species (Table 2.4,
below) that presumably have sufficient resources in the seedling stage to
establish in the face of some competition from established vegetation. Many
have hard, impermeable seed coats that prevent water uptake and germina-
tion, or other dormancy mechanisms that prevent germination until the seed
coat is physically altered (Table 2.4).In the field,temperature extremes or des-
iccation typically break physical dormancy (Baskin & Baskin, 1998 a ,pp.
114-20). Response to these factors spreads germination over several years
and, to some extent, also cues germination to appropriate times of the year.
Some large-seeded weeds also have innate physiological dormancy mecha-
nisms (Wareing & Foda, 1957). Thus, large-seeded weeds have mechanisms
that match germination to appropriate environmental conditions, but only a
few (e.g., Solanum viarum - Akanda, Mullahey & Shilling, 1996) sense the
removal of competitors through a strong response to light, alternating tem-
perature, or nitrate.
Survival of weed seeds in the soil
The seeds of most annual and stationary perennial weeds persist in
the seed bank for at least a few years, and many remain viable for decades if
conditions are favorable. Excavations from dated archaeological strata indi-
cate that some agricultural weed species, including Chenopodium album ,
Stellaria media , and Lamium purpureum , probably remain viable for several
hundred years (Odum, 1965), although movement of younger seeds into the
strata by soil animals cannot be excluded with certainty. However, the cool,
moist, dark, undisturbed environments of such sites are highly favorable to