Ecology and management of an exotic weed Phalaris minor - Invasive Plants: Ecological and Agricultural Aspects - page 161

To identify P. minor as an invasive weed, it should escape its boundaries and
must reproduce at a considerable distance from the parent plant. During our
field visits, P. minor was mainly restricted to cultivated fields and rarely
observed in abandoned fields or on roadsides (Fig. 1b). However, we are not
sure whether the weed has escaped its boundaries; perhaps growers have
dropped seeds of P. minor on the ground while transporting the weed after
manual removal from the field (Fig. 1c). Therefore, the question of identifying
P . minor as an invasive weed still needs further investigations.
What makes a favorable habitat for P. minor ?
The resource fluctuation hypothesis explains habitat susceptibility to invasion
[15, 16]. An increase in unused resources makes a community susceptible to
invasion [16]. An increase in resources occurs when resource use declines or
when there is an increase in resource supply [16, 17]. Disturbance, such as
tillage or herbicides, releases resources and therefore is a key determinant in
invasion [18-21].
P. minor thrives under moist, nutrient-rich conditions [22]. For example, the
dry weight of P. minor increased by 42% when nitrogen was increased from
30 to 120 kg ha -1 while wheat biomass increased by only 9% [23]. Cultivated
fields therefore have unused resources that P. minor uses to establish and
thrive. Disturbance such as tillage further aids in its establishment; for exam-
ple, plowing brings seeds of P. minor up to the soil surface.
Mustard ( Brassica campestris, B. napus or B. juncea ) and wheat are the pre-
ferred winter crops in northern India. The reduced success of P. minor in mus-
tard fields compared to wheat is likely due to mustard's vigorous growth and
lower requirement for irrigation compared to either P. minor or wheat.
Farmers in the wheat-rice belt prefer to cultivate dwarf wheat varieties due
to their high potential yield. P. minor , however, grows taller than dwarf wheat
(Fig. 1d) and thus is highly competitive for light. Paul and Gill [24] showed
that height, dry matter, yield and number of nodes on primary shoot of P.
minor are considerably reduced when grown with tall wheat varieties com-
pared to semi-dwarf and dwarf varieties. These authors suggested that semi-
dwarf wheat varieties should be cultivated, because they combine weed smoth-
ering potential and reasonable grain yield.
In summary, the habitat created by wheat cultivation is more suitable to P.
minor establishment and survival than the habitat in mustard crops. Resources,
particularly nutrients and light, are important factors for P. minor establish-
ment and growth . Allelopathy has been suggested as a mechanism for its inter-
ference to weed species [25-27], but we do not have any empirical evidence
to support the hypothesis that mustard could interfere with P. minor growth
and establishment through release of allelochemicals. This aspect needs fur-
ther experimentation.