GARDENING; 10 Stubborn Plants That Bedevil Americans
By JOAN LEE FAUST
Published: August 4, 1991
EVERYONE who grows plants confronts weeds at one time or another. Some weeds are pretty to look at and some are just plain nuisances. Usually when weeds appear in someone's garden, the plants are frowned upon and the garden is relegated to an unkempt category. Besides, weeds compete with desirable plants for soil moisture and nutrients.
Other gardeners are tolerant of weeds, however, and consider them nothing more than plants out of place. After all, they admit, for every weed or unwanted plant there is someone in the world who likes it and covets it.
Not many serious gardeners view plants this way. In fact, a recent publication of the United States Agricultural Research Service contains an article called "10 Weeds We Could Live Without." The research service says about 1 of every 10 species of plants is considered to be a weed. Scientists believe that there are 30,000 weed species, with about 1,800 of them causing serious economic losses in crop production.
About 70 percent of the worst weeds in the world are said to grow in the United States. And American farmers spend an estimated $2.1 billion annually for herbicides, plus another $938 million in application costs. The expenditure for these herbicides accounts for more than 65 percent of all pesticide sales in this country.
The newer approach taken by the researchers is to use post-emergent herbicides, those that act on the plants themselves rather than the pre-emergents that are put on the ground to kill the seed. The findings are that the treatment of the weeds above ground is a boon for the environment.
Post-emergent herbicides are believed to be deactivated in the soil, broken down by microbes without the residue effect caused by other chemicals. This research is also leading to answers of why the weeds are killed and opening new discoveries in particular plant pathogens.
What are those 10 weeds that Americans could live without? Although all of them are plagues that farmers battle, many of them are weeds quite familiar to the home gardener. And one curious thing about them is that eight are plants that have been introduced from other lands, while only two are native to the United States.
The two native species that have grown to become weeds are pigweed, an amaranthus that is known to the Great Plains especially but is a familiar weed in almost any garden. If left to grow without any interference, it can become five feet tall.
The other native plant that has become a weed in this country is the common ragweed, which is found mostly in the Eastern and North Central states. It can reach great heights, too, if unwatched, and its pollen causes hay fever for about 35 million people in the nation.
The other eight weeds, introduced to this country, have become naturalized pest plants and serious troublemakers for many who raise plants for profit, nurserymen as well as farmers. Probably one of the worst is that pretty little, but aggressive, plant, annual morning glory. Any of these Ipomoea species are aggressive in fields, even though many admire their lovely flowers. It is one of the toughest plants to get rid of, because it has a strong clinging habit.
Its cousin, field bindweed, is also on the weed hit list and has the distinction of being one of the worst weeds in the world. Field bindweed can be found in 47 of the 48 contiguous states (all but Florida). One of the researchers described this weed as an "out of control morning glory." Its roots go down about six feet.
Canada thistle is another nuisance, and a tough one, because it has a deep root system. It is also perennial, as is the dandelion, and tops broken off only leave the root to remain and grow for another season. The common cocklebur is in the same nuisance category, a bushy plant with prickly seeds.
Lamb's-quarters, which was introduced from Europe, is a thin-leaved annual weed highly susceptible to drought. If it has enough moisture, however, it will thrive aggressively and become a nuisance.
Although many landscapers may not recognize it as a weed, nutsedge is becoming an increasing problem. It is a tough-leaved perennial grass with unattractive blades. Although it is rampant in the southern portion of the country, it does make its appearance in the colder states and may be due to invasive spreading by poor quality seed mixes. It needs to be routed out of lawns as soon as it appears, lest it take hold by its aggressive creeping tendrils.
Although the last two weeds are not seen much in the Eastern states, they are problems elsewhere. Velvetleaf, an abutilon that was introduced from India, is so named because it has velvet hairs on its many branched stems. This weed is a pest in soybean and corn fields. Wild oats, another aggressive field weed, causes problems for wheat farmers in the Northern Plains states.