How It Works
When you spray Roundup on weeds, it settles on the leaves. Aided by surfactants in the chemical mixture, glyphosate permeates the leaves and enters the the plant tissue that transports food. It then migrates to areas of growth activity and combines with an enzyme essential to growth. As a result, the plant wilts and dies. Because it enters through the green, leafy parts of a plant, Roundup is only effective against plants that have broken the surface. It binds so tightly to soil that the soil essentially neutralizes it. Some Roundup products contain other herbicides to kill germinating seeds and roots; these products are clearly labeled.
What It Kills
No plant is safe from Roundup unless it has been genetically modified to resist glyphosate. The plants that are most vulnerable are those with large leaves, because the leaves provide more surface area where the herbicide can be absorbed. Woody plants with small, waxy leaves may be somewhat less vulnerable to Roundup products with weaker concentrations of glyphosate but will succumb to high-concentration mixtures. Roundup even kills freshly cut tree stumps. Plants can store glyphosate for months, so a bush in the path of a fall spraying program may exhibit signs of disease in the spring when its sap begins to run.
Handling Roundup Safely
To avoid spraying desirable plants, spray on a windless day and protect nearby plants with shields made from cardboard or plastic. Even a small amount of overspray can cause damage, and you can't predict when the damage will become apparent. You can neutralize any overspray that settles by dousing the affected plant with plenty of clean water.
Effects on Humans
The Environmental Protection Agency considers glyphosate to be mildly toxic to humans and animals. Research results published in the April 2013 edition of the journal "Entropy" state that glyphosate interferes with the same enzymes in human intestinal bacteria that it does in plants. The study proposes a connection between glyphosate and a number of ailments ranging from eczema and skin allergies to Parkinson's disease, liver disease and cancer.