Why is it that the summer months we spray herbicides, and they don’t work for some weed species. 

For example, during the hot time of the day and when desert weeds are stressed for moisture even the highest rate of Glyphosate sometimes is not effective.
Some desert weeds develop physical characteristics and growth habits that equip them to survive hot and dry conditions. We have seen that some species have different response to herbicides in cool areas than our low desert. 

Weeds in the desert have a thicker skin to preserve water. The stomates, which are the pores found in the epidermis of leaves, stems, and other organs, controlling the rate of gas exchange between the internal air spaces of the leaf and the atmosphere are closed¹. Plant transpiration is reduced, and this interferes with the translocation of herbicides in the weeds.

Take as an example Purslane, which has very succulent leaves and stems and can survive incredibly high hot and dry conditions. Then when weather is more favorable it comes out of survival mode to start growth again at the presence of moisture.

Other perennial species have underground structures that help them survive such as Nutsedge and bermudagrass have tubers, and rhizomes.
Lundkvist² stated:” The conclusion is that environmental conditions that promote growth rate of the weed stand will also increase the herbicide effect. The results were based on data from two field experimental series performed at six sites in southern Sweden during 1991–1994”.



 References:
1. https://simple.wikipedia.org/wiki/Stomata
2. A. Lundkvist,
Predicting optimal application time for herbicides from estimated growth rate of weeds,
Agricultural Systems,
Volume 54, Issue 2, 1997,