Weed Control With Round Up Weed Killer.

Round up weed killer contains a chemical called Glyphosate which is a systemic weed killer that enters the plant through the green areas i.e leaves and stems and travels within the plant. Glyphosate works by removing the chemical growth control clock within the plant so effectively the plant or weed outgrows itself. This is why often weeds or plants treated with Glyphosate and some time look healthier for the first few weeks after treatment be before collapsing and dieing.
Because the Glyphosate enters the plant mainly through the leaves its best applied on a dry day when the plant or weed is actively growing. So the plant or weed takes up as much of the active chemical Glyphosate as possible.
Glyphosate also has the advantage of not being to residual in the soil being broken down by soil bacteria over a number of days and so it is possible to grow plants in a treated area within a week or so.