Mary Ann,
That info on temperature is from the radio program I mentioned. The subject comes up regularly in the spring when they are talking about getting rid of perennial weeds. I never plan on using it during the summer, just spring and fall. We had so much rain this spring and then it warmed up quickly so didn't have many days when could spray.
Found this at
http://www.ehow.com/about_5415...
Cool weather and frost also reduce the ability of Roundup to kill weeds. Likewise, the effectiveness of Roundup declines at temperatures above 85 degrees Fahrenheit.
The best temperature to apply Roundup is between 60 degrees and 85 degrees.
and this at
http://donelsongateway.org/bro...
Certain herbicides have a tendency to volatize (change from liquid to gas) when
air temperatures reach 85 degrees and higher. The commonly used broadleaf weed
herbicide called 2,4-D is one of these. Applied as a liquid spray, any of the
liquid that is not absorbed into the target plant has the potential to turn into
a gas during periods of hot weather. Also, any liquid that contacts warm, bare
soil can volatize. During hot days this process can occur within minutes and
continue for several hours or even days. Dicamba, another commonly used
broadleaf herbicide does not easily volatize, but may remain in the soil for a
period of several weeks. Dicamba does not bind to soil particles, but is highly
soluble in water, and because of this factor, may enter into exposed surface
roots of trees. The herbicide Roundup (glyphosate), which can kill both
broadleaf and grassy weeds does not easily volatize, but does bind to soil
particles. Because Roundup binds to soil particles, it is rapidly broken down by
microbes in the soil and rendered harmless. However, caution should be used when
applying Roundup around trees to control weeds as it can be absorbed into trees
through wounds or openings in roots and bark if sprayed onto these areas of the
plant.
So what does herbicide injury look like?. You might see very small
adventitious leaves appearing on the branches of plants. The leaves will appear distorted and leaf veins and leaves will not fill out and grow normally. The plant may be alive, but leaf development will greatly impacted. Plants will eventually grow out of this condition and develop normally, but this may take some time.
I saw this on some myrtle about 12 feet from where I did a lot of spraying of quack (Johnson?) grass with Roundup on a 86-90 degree day. So I learned my lesson. It did grow out. I wouldn't care if it killed the myrtle, but just beyond was an iris bed.
