Both Spectrum Weed and Grass Killer and Roundup are nonselective
herbicides, meaning they kill most or all plants, including grass.
They should be used before a lawn is planted or to kill weeds in
spots or in specific areas such as beside sidewalks. Young weeds
are easier to kill with postemergent herbicides. They do not work
well if weeds are stressed by drought or have begun to produce
seeds.
Spectracide Weed and Grass Killer destroys plants on contact. It
kills only that part of the weed with which it has direct contact,
and it does not kill underground rhizomes or tubers. If weeds grow
back from rhizomes or tubers, several applications may be needed to
kill weeds. Roundup is a systemic killer; it moves inside the plant
with water and nutrients. It is useful for larger weeds, but it may
take longer to kill weeds.
Diquat dibromide, commonly called diquat, is the active
ingredient in Spectracide Grass and Weed Killer. Spectracide is
sprayed on plant leaves or an entire plant; it kills them by
causing them to dry out quickly. Diquat kills only that part of the
plant that it physically touches, and it does not leave trace
amounts in the soil, water or on plants. Diquat is widely used to
control the flowering of sugar cane, to kill seed crops and potato
vines and to control aquatic weeds
Glyphosate, the active ingredient in Roundup, mimics and blocks
part of a molecule called phosphoenolpyruvate, commonly called PEP,
that is central to the metabolism of both plants and animals.
Glyphosate moves within a plant, killing all of it, and it is
useful for killing older weeds. It kills all annual and perennial
plants including broad-leaved weeds such as dandelions, weedy
grasses, sedges and woody plants. Roundup is used among a variety
of crops and on non-cropland.
