A broad range of weed-killing herbicide products are sold under the brand name Roundup. The primary active chemical ingredient that kills weeds is glyphosate. Some formulations contain other chemicals, such as pre-emergent herbicides, to further control weeds in a landscape. Apply Roundup herbicides when weed foliage is dry to the touch, and preferably when rain is not expected within four hours.
Roundup is sold or mixed as a water-based solution. The concentration of glyphosate in the solution affects efficacy of the herbicide. Do not spray weeds with Roundup when the foliage is already wet or overly damp with rainwater or dew. The water on the plant foliage buffers the tissues and dilutes the chemical before it can be fully absorbed into the plant. Roundup acts by being absorbed into the leaves and then transported in the vascular system to kill stems and roots.
When to Apply
Spray Roundup onto weeds on windless days when the temperature is above 15.6 degrees Celsius. Do not schedule spraying if rain is expected. High ambient humidity can slow the drying of the Roundup solution on the plants, and any mist or rain can wash off the chemicals, thereby diminishing the herbicide's effect. Don't spray if the foliage is still wet from rain or irrigation. You can spray several hours before a rain as long as you are confident the Roundup solution -- which is applied heavily to douse and wet the entire weed -- will air-dry before rains arrive.
Rainy Weather Considerations
When multiple thunderstorms or repeated light showers are expected over the course of a day or week, it will be best to wait to spray weeds with Roundup. Traces of chemicals that remain wet will be washed away or diluted on weeds when mists or downpours occur. Postpone the herbicide treatment to a dry day. The worst scenario is applying the herbicide, and having it wash off. Then you must purchase more Roundup and spend more time spraying.
Carefully read the labels on Roundup products. Look for specific information about timing of application during rainy weather. Some products, such as Roundup Weed & Grass Killer Super Concentrate, are fast-drying and reputed to be dry to the touch and fully absorbed in 30 minutes. Other Roundup formulas may not have special additives or concentrations that ensure fast, complete absorption of chemicals into the weeds before or after rains.
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