Q.Palm Trees Dying
My gardener put Round Up on weeds around these Washingtonian palms, then we had torrential rains for a week. They are showing signs of yellowing leaves that collapse and the green leaves look dull. I think the rain washed the Round up down into the dirt surrounding the roots.
Is there anything I can do to save them?
The California Department of Pesticide Regulation has licensing and educational materials that are quite good on safe and effective use issues.
You may consider suggesting to your gardener that he get certified or licensed as a pesticide applicator if he is not already. Besides getting legal, he will learn to be responsible and professional, and learn about new generation alternatives to old school pesticide use. There is a tremendous amount of new research and development on organic program compatible pest and disease methods and materials.
Ask him to check out FinalSanO contact herbicide as an alternative to RoundUp. http://certisusa.com/pest_management_products/herbicide/final-san-o.htm
There is not too much you can do to counteract the effects of herbicide poisoning in palms like this. Flushing with copious water as a soil drench is a common recommendation, but I'm not sure that this is appropriate in your case. The rains and waterlogging of the normally dry soil, and reduction of aeration to the roots could be a contributing factor for the disorder. So I would hold off on watering any more at this time.
You are very observant to note the physical differences in leaf turgidity and discoloration; from the photos its hard to see a significant effect, but I know these palms and I kind of see what you mean.
Sinescense, wilting and yellowing of older palm fronds is of course normal and may be part of the picture of what is going on with the lower leaves. Nutritional deficiencies cause chlorosis (yellowing of foliage). But that wouldn't usually appear suddenly, it would take some time to develop.
It is unusual for a common RoundUp product with glyphosate as the active ingredient, to affect a plant through root uptake, even with heavy water like you had. I haven't seen it, but I suppose its possible. But I would think not likely unless the gardener used an extraordinary amount or high concentration of Roundup. If it is the result of a normal weed killer application rate, I would not expect it to kill palms of this size and maturity, and I would expect them to recover.
But Roundup has different formulations, including RoundUp QuickPro with glyphosate and diquat, and a new Round Up for Lawns with active ingredients that are broadleaf weed killers. If you know which product with what percent of active ingredient he used and at what concentration (how many ounces of the product per gallon of water) it would help for diagnosis.
You can consider contacting the Riverside County Agricultural Commissioner's office in Coachella Valley, and ask for the biologist overseeing pesticide use and enforcement in the county. They will investigate and advise on best course of action.
But I warn you that you could lose your gardener. If found in offense or misuse, at a minimum he could be fined and would not be happy with you for reporting him. At maximum he could be ordered not to use pesticides in his operation without a Qualified Applicator Certificate if he doesn't have one. Repeated violations can increase severity of penalties.