Can I Use Roundup on Wet Weeds?
Although many weed killers contain glyphosate, Roundup is the original glyphosate-based weed killer. You administer it by spray, and when it settles on a plant's leaves, the plant draws the glyphosate into its system and eventually dies. Water on the leaves can potentially interfere with the ability of glyphosate to kill a weed in three ways. It can weaken the concentration of the herbicide, wash it off prematurely or neutralize it.
The Action of Glyphosate
When you spray Roundup on a weed and the spray settles on the leaves, glyphosate migrates through the leaves into the phloem, and from there it travels to nodes of new growth and interferes with a key growth enzyme. The weed must continually grow new branches and leaves to stay alive, and because glyphosate curtails this, the weed eventually wilts and dies. Glyphosate is a post-emergent weedkiller, which means it only works on weeds that have broken ground, and it is most effective when the weeds are growing quickly. It is also nonselective -- it kills desirable plants as well as weeds.
The Hard Water Effect
Most products contain glyphosate salts rather than free glyphosate molecules. This makes the glyphosate water-soluble and keeps it available to react with the growth enzymes in the plant. Salts used in most products are those formed with sodium, potassium and isopropylamine. Some of the chemicals in hard water, particularly calcium and magnesium, are more strongly positive than the ions in the product and attract the negative glyphosate molecules to form more stable compounds, thus reducing their effectiveness. Rain water and well water can both contain an abundance of these elements, and if either has already soaked the leaves, it can essentially neutralize whatever Roundup you spray.
Spraying in Wet Weather
Apart from how rainwater can neutralize glyphosate, a sufficient amount of water on the leaves will probably wash off the glyphosate before it has a chance to enter the plant. This is particularly likely if you can see water running off. Water can also dilute the concentration of glyphosate in the product to the point of ineffectuality. If runoff isn't a problem, you can counter both dilution and hard water by either spraying more of the product or by using a different product that has a higher concentration of glyphosate.
Death in the Morning Dew
One of the evils of glyphosate -- at least from the standpoint of weeds -- is that it is most effective on weeds that are happy and growing quickly. The phloem is active and respiration is complete, both of which allow the herbicide to work more effectively. Weeds tend to be happy in the morning, just before the dew on their leaves has evaporated under the morning sun, and this can be a good time to spray them. There is no conclusive evidence that dew either inhibits or enhances the effect of the glyphosate in Roundup.