Weeds Of Cultivated Ground
Transcontinental and Intercontinental Migrations of Amaranthus
(Sauer 1967a )
Amaranthus is a cosmopolitan genus of annual herbs that includes about 50 species native to stream banks and seacoasts of many temperate and tropical regions. Some of the New World species became especially successful and widespread as agricultural weeds; the migrations of three of these will be sketched here. These three species are native to regions where maize was cultivated before Columbus arrived in America. In traditional American Indian agriculture, maize was interplanted with other crops in milpas, which were treated more like gardens than like Old World small grain fields. No weed seeds rode along on maize ears selected individually for seed grain, which was hand planted in hills that were hoed and hand weeded. Nevertheless, amaranths thrived and spread as milpa weeds. Farmers tolerated them as favorite sources of potherbs and edible seeds and eventually domesticated some amaranths as ancient grain crops, a story that is not relevant here. A healthy amaranth plant produces thousands of seeds, which in the weed
species, germinate gradually over decades whenever the soil is disturbed.
Amaranthus hybridus s.l. (including A. quitensis ) grows wild on riverbanks from eastern North America through the highlands of tropical America to temperate South America. It very likely became a milpa weed in Guatemala or Mexico when maize was first domesticated. It did not accompany maize to Europe in 1493, but finally arrived by means unknown in the early eighteenth century. During the nineteenth century, it became an abundant weed in southern Europe and northern Africa mainly in agricultural land, including maize fields; occasionally it escaped to river banks. North of the Alps, it has appeared repeatedly as an ephemeral ruderal but does not reproduce there. About 1900, A. hybridus appeared in California, South Africa, the Far East, and Australia and has continued to spread as an agricultural weed. In New South Wales, it has become much more abundant since 1960.
Amaranthus retroflexus grows wild on streambanks in eastern North America, as noted in Chapter 1. It is also a common weed in floodplain maize fields. In the mid-eighteenth century, Peter Kalm sent seeds from Pennsylvania to Linnaeus; a specimen grown at Uppsala, the type for the binomial, is still preserved in the Linnaean Herbarium in London. Linnaeus may have been responsible for inoculating Europe with the species by sending seeds to various botanical gardens. Lamarck noted it had escaped at Paris by 1783. By 1800, it was a common agricultural weed throughout much of the continent. Collections in major European herbaria show that during the nineteenth century, A. retroflexus spread as far as Norway, Portugal, the Azores, Algeria, Greece, Turkey, Syria, Astrakhan, and Afghanistan. Recently, Conard and Radosevich (1979) noted that maize yields in Italy and Austria were being seriously reduced by infestations of A. retroflexus that had developed atrazine resistance due to overuse of the herbicide. About 1900, A. retroflexus appeared in California and since then has arrived in China, Japan, and Australia. In New South Wales, it has become widespread since 1950.
Amaranthus powellii (including A. bouchonii ) grows wild in arroyos and desert washes in the Cordilleran system from western North America to the Andes. It was also a prehistoric milpa weed. In California and other far western states, as agriculture expanded in the nineteenth century, A. powellii became the most common weed amaranth in the region. It began to spread east of the Rockies about 1900 and has become a troublesome weed of maize and soybeans in the U.S. corn belt since 1940. Overseas, it showed up sporadically from the 1890s through the 1920s in Europe at scattered places from Scotland to Spain and from Sweden to Austria. For a long time, it remained a ruderal around docks, wool waste dumps, tanneries, grain elevators, and railroad yards. Since 1930, it has become a common weed in maize, potato, and other crops in the British Isles and on the continent; it reached Turkey after 1960. Meanwhile, A. powellii appeared in Australia, New Zealand,
and southern Africa in the late nineteenth century; only recently has it become abundant there. In East Africa, it first appeared in 1949 as a maize weed in Kenya. It invaded southern India after 1960.
Comigration of Cereal Crops and Weeds to California
(Barrett 1983; Robbins 1940)
Old World small grain agriculture, since its Neolithic origins, has depended on mass harvesting and sowing of seed from a mixed population of more or less domesticated cereals and associated weeds. Spanish wheat fields contain such mixtures today. During the colonial period, Spanish weeds inevitably went along with wheat to the New World. Eventually, perhaps via both Mexico and Chile, Spanish wheat and weeds even reached California. The eighteenth-century adobe bricks of California missions contain seeds of black mustard, Brassica nigra , and other European weeds that probably were accidentally carried in the seed wheat. Curiously, the red poppy, Papaver rhoeas , one of the most common Spanish grainfield weeds, has not naturalized in California wheat fields, although the species is grown as an ornamental.
Rice cultivation began in California with seed introduced from Japan in 1912–1915. Various Japanese rice weeds still grow around the Sacramento Valley experiment station where rice was first planted. Some of these weeds have not spread from there. Two species of barnyard grass, Echinochloa oryzoides and E. phyllopogon , which evidently came with the imported rice seed, have spread as major weeds throughout California rice fields. A close relative, E. crus-galli , also native to Asia, arrived in California earlier; it has been a common weed there since the mid-nineteenth century in various crops. Echinochloa oryzoides and E. phyllopogon are almost entirely confined to flooded rice fields. They are believed to have evolved as weed mimics of rice in ancient paddies. Hand weeding caused any genetic changes that made the weed harder to eliminate from the crop to be favored by unintentional selection.
Both species now have seedlings that look almost identical to rice seedlings; both flower and mature their grain synchronously with rice. The seeds are not as heavy as rice but are two to three times as heavy as ordinary E. crus-galli seed. They resemble rice closely enough to pass inspection in certified seed. Both species produce seed with weak dormancy that germinates fully along with planted rice, unlike E. crus-galli . Also like rice and unlike E. crus-galli , the weed mimics are adapted to conduct O2 from the air down to flooded root systems. In retrospect, immigration of these weeds could have been barred by meticulous seed sorting, but they passed routine screening.
Sequential Migration of Glycine and Setaria from China
to North America
(Fairbrothers 1959; Pohl 1951)
A species of bristlegrass, Setaria faberi , noted above as having arrived in Finland with soybeans imported from China, arrived in North America long after the crop. Soybeans were introduced repeatedly from 1765 to 1900, but did not become an important crop until World War I when a phenomenal expansion began that culminated with their regular rotation with maize throughout the U.S. corn belt.
The first record of Setaria faberi in North America was on Long Island, New York, in 1925. During the next 20 years, it spread east of the Appalachians along railroads and roads and in scattered urban sites from New England to the Carolinas. For a long time, it was not recognized as distinct from a related weed, Setaria viridis , which had arrived long before.
Setaria faberi ceased to be a minor ruderal and became a major agricultural weed when it finally reached the corn belt. It was first recorded in Illinois in 1941; by 1950 it was exceedingly abundant across Illinois and Indiana, frequently blanketing soybean fields so densely that the crop could hardly be seen. It was first recognized in Iowa in 1949, and by 1950 it was in 36 Iowa counties and had spread to neighboring states. It grows as an annual in both phases of the standard crop rotation, but is more of a problem in soybeans than in the taller maize.
Comment
Human dispersal of weed seeds may be irrelevant in some situations, as where riparian species are preadapted to colonize cultivated ground within their natural seed shadows. Human dispersal was obviously crucial in cases of weed migration overseas, as in the case of Setaria faberi , and weeds have commonly gone abroad as stowaways in shipments of grain or cotton. Another pattern, not discussed above, is escape of a deliberately introduced crop to become a noxious weed. For example, Tientsin jute or velvetleaf, Abutilon theophrastii , was a fiber crop, domesticated in Asia, which was introduced to the British colonies in North America. Before American cultivation was abandoned in the late nineteenth century, Abutilon had been widely planted by corn belt farmers. It survives there as a major weed of both maize and soybeans, resistant to herbicides and causing losses of hundreds of millions of dollars annually (Spencer 1984). Similarly Johnson grass, a variety of Sorghum halepense , has been widely planted in North America as a fodder crop and has escaped to become a troublesome weed in other crops.
In Central America in 1960, a Mennonite colony struggling to establish itself in Belize planted Johnson grass for the dairy cattle, setting in train a permanent weed problem. The seeds are spread by birds and cattle and invade new land as fast as the forest is cleared (Sawatzky 1971).