Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 144

further subdivided according to the scheme of Håkansson (1982). The effect
of tillage on a wandering perennial depends on whether the implement
reaches the storage roots or rhizomes, or merely severs the vertical branches.
Similarly, the presence or absence of a taproot, bulb, or similar storage organ
influences the response of nonwandering species to tillage. Annuals with a
phenology that is out of phase with the tillage operation behave as perennials.
For example,an autumn-germinating annual presents well-established,over-
wintering plants to a spring tillage operation and thus responds as would a
perennial.
Perennials like Asclepias syriaca and Convolvulus arvensis in which a large pro-
portion of the perennating roots or rhizomes lie below the plow layer are sus-
ceptible to tillage primarily through exhaustion of reserves. Because the
rhizome lies deep in the soil, a long, usually large-diameter vertical shoot is
required to get the growing point to the soil surface. Consequently, replace-
ment of shoots that are removed by tillage requires a substantial investment
of energy. However, because the dormant buds on the deep rhizomes are
immune to all except extraordinarily deep tillage, substantial control of a
population by tillage alone may require fallowing the field in order to perform
several operations (see below).
Although weeds with shallow perennating roots and rhizomes can be
uprooted and chopped by tillage implements, this often has only a short-term
benefit since rhizome fragments develop new shoots and new roots grow to
supply exposed pieces of shoot. Moreover, separation of buds onto a multi-
tude of small root or rhizome fragments can actually increase the productivity
of some weeds by releasing buds from apical dominance (Håkansson, 1968 b ;
Håkansson & Wallgren, 1972 a , 1976; Bourdôt, Field & White, 1982). In addi-
tion, if the implement drags fragments around the field, tillage may effec-
tively disperse the weed, as well as propagate it. Nevertheless, as detailed
below, shallowly wandering perennials are susceptible to a variety of mechan-
ical measures.
Some perennials spread by rhizomes but perennate by means of a bulb or
tuber. These species may pose particular problems, because the tubers tend to
survive tillage intact and the substantial food storage allows emergence after
deep burial. Cyperus rotundus , which has been labeled the world's worst weed
species (Holm et al ., 1977), is of this type. However, this and similar species
may be attacked by tillage that destroys the shoot shortly after emergence, or
that exposes the tubers to desiccation (see subsequent sections).
The effect of tillage on nonwandering perennial weeds depends on
whether the species has a taproot or equivalent storage organ. Uprooting
species with a taproot is relatively ineffective (Table 4.2) unless the root can be