Question
My sister has river rock surrounding her pool and back deck. Just spent three days pulling weeds. Do you know the best way to get apply a weed killer so it penetrates the soil through the rocks? The soil is hard and dry. Should I water first, then spray the weed killer then water again?

Answer
Hi Karen, Glyphosate is a non-selective herbicide you can spray over the rock and kill anything there.  If does nothing to the soil, so weeds can come back by seed or by spreading into the area.  You could add a pre-emerge like Dimension to stop seeds for 90 days or so, but you may still have grass spreading into the area.  Spraying once every couple of weeks on anything green should keep the area clear.
Or, you can spray Pramitol over the area and sterilize the ground for a year.  Nothing will grow there.  Grass may creep out on the rocks, but it will not root in.  Pramitol would be my first choice. You'll have to use enough water to get the product to the soil surface, but use caution not to flood the area that may carry it into desirable plants.  I use it in my rock walkways that are bordered with zoysia with great success.
Jim