Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 216

Flooding fields when crops are not present
Flooding can severely reduce the germination, growth, and survival
of weed species unadapted to anaerobic conditions and can be used in certain
agroecosystems to suppress perennial weeds before or after periods of crop
production. McWhorter (1972) reported that flooding soil for 14 days when
water temperature was

C prevented establishment of the perennial
grass Sorghum halapense from rhizomes. Because flooding is practical on thou-
sands of hectares of level land in the lower Mississippi River valley that are
infested with S. halapense , McWhorter (1972) suggested that excellent control
might be obtained by flooding for two weeks during summer months, when
water temperatures are high; substantial control might also be obtained by
flooding for four weeks in March or April, which would permit crop produc-
tion during the summer of the same year.
The combination of flooding and subsequent ice-encasement, which can
kill plants because of the accumulation of CO 2 and other toxic metabolic prod-
ucts,may also be used to manage certain weed species.Ransom & Oelke (1983)
reported that fall flooding and subsequent freezing killed all corms of Alisma
triviale , an important broadleaf perennial weed infesting cultivated fields of
wild rice ( Zizania palustris ) in northern Minnesota. Flooding alone failed to
suppress corm viability of A. triviale; exposure to freezing temperatures and
ice-encasement were also required.

20 °

Water management in rice production systems
Water management is a key factor influencing the density, productiv-
ity, and species composition of weeds infesting rice, the staple food for about
one-half of the world's population. Rice can be grown under a wide range of
environmental conditions (Purseglove, 1985, pp. 161-99), but most rice is
grown with flood irrigation for at least a portion of the crop cycle.Because rice
is tolerant of flooding but many weed species are not, differential responses
between rice and associated weeds to moisture conditions comprise an impor-
tant component of weed management for the crop. Planting may consist of
broadcasting or drilling seeds into dry soil followed by irrigation (dry-
seeding); broadcasting seeds into flooded fields (water-seeding); or transplant-
ing young seedlings into wet or flooded soil. In addition to raising soil levees
or bunds around rice fields to retain water, water retention may be improved
by sealing wet soil (puddling) with a harrow or animal treading before plant-
ing the crop.
The potential weed flora of rice fields includes terrestrial, semiaquatic, and
aquatic species of monocots and dicots. The actual abundance of different