Pigweed (April 29, 2015)
Pigweeds are some of the most common summer annual broadleaf weeds in the low deserts. Although they are often lumped together, there are 4 different species of pigweed that are common here and more than 10 species that occur as weeds in California and Arizona. Their growth habits and response to herbicides are similar. It is easy to identify them by physical characteristics but one species of pigweed can hybridize with another and become less distinguishable.
Palmer Amaranth: (Amaranthus palmeri) is probably the most common pigweed species found in this region. It is very aggressive and fast growing and can become 6 feet tall or higher if uncontrolled. It has one thick stem and several lateral branches. The leaves are lance shaped, hairless and have distinctive white veins on the underside. It has flowering tassels that become stiff and spiny. This species has become resistant to Glyphosate in many parts of the county.
Redroot Pigweed: (Amaranthus retroflexus) is probably the second most common pigweed species. It is shorter and the seed heads are smaller, in clusters and have stiff spine-like scales. It has leaf hairs on the margins and the veins are often reddish. The lower stems are often reddish. This species will hybridize with Palmer Amaranth and become less distinguishable.
Tumble Pigweed: (Amaranthus albus) is very different from Palmers or Redroot. It grows lower to the ground and has many branches that turn upright. The leaves are much smaller and narrower. The numerous stems are light green rather than red. The seed heads are small, spiny and at the base of the leaves rather than in long terminal spikes. When mature, the branches are sticky, stiff bristles that break off at the ground and tumble with the wind.
Prostrate Pigweed : (Amaranthus blitoides) is very similar to Tumble Pigweed but the stems are more prostrate, grow close to the ground and form mats. The stems and leaves are smaller and reddis
This question comes up every year. The answer is that it is unlikely. Glyphosate binds strongly to soils, especially those that are fine textured. Glyphosate is a systemic herbicide and it must be absorbed into the plant to work. If it is bound to soil it is no longer active. This is still the case if treated soil is deposited onto the foliage. Dirty water that contains soil particles also reduces the activity of glyphosate. We have conducted trials in the greenhouse where we blew treated soil unto foliage and seen no symptoms. This is not the same for all herbicides. Oxyfluorfen (Goal, Galigan), for instance, does not adhere well to soil and it can “lift off” readily from the soil with water and injure crops it comes in contact with.
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