With total respect to all of the previous responses (which included good
suggestions), I offer the following.

Your neighbor may not even have been aware of what they were doing (though
the word "Chem" in their name ought to be telling). Do talk to your
neighbor about the issue.

Additionally . . .

If you are inside city limits, take your concerns to City Council. Have
your comments prepared in writing to give them a written copy (and to
ensure you say what you intend to say). There is often a time limit of 3
minutes for each citizen, so clarify prior to attending.

If not inside the city limits, take it to the County Commission, same way.

In most communities in our electronic age, all meetings are taped and
shown on the local city/county televisions channels. You are most
definitely not the only one in your community concerned about this. The
more people that speak up about it, the more chance of something being
done.

I see your issues as including:
- Damage to your previously chemical-free garden. If you wanted chemicals,
you'd buy at the grocery store. The chemicals drifting into your garden
are now a permanent part of your soil and the food grown in it; they can
say they dissipate all they want to . . . they go somewhere! (I say that
as a downwinder from Hanford's 1950s "harmless" radioactive releases!)
- There is potential damage to anyone with breathing problems or immune
system problems with the spray drifting into your yard or onto public
property (sidewalks/streets).
- There is potential damage to small children playing in your yard,
especially at the time of spraying. Their systems are not as able to deal
with this type of thing as an adult . . . don't let anyone ever tell you
that "there is no harm" because no one knows!

Approach it all in a very logical and professional (unemotional) manner
with facts, not opinions. The time for the opinion is at the end when you
summarize but have it all supported with previous facts, which are
primarily your property did receive a chemical which you did not authorize
and you have damage as a result, as well as concerns.

I can assure you if this were my yard, my 3-year-old and 5-year-old
granddaughters would very much figure into how I handled this. I would
include a letter to the neighbor voicing my complaint and concerns, as
well as one to the City Manager, the Mayor, and each member of City
Council. The offending company would also get a letter as would the
County Health Department. In other words, no possible responder would be
forgotten, and this would get as much attention as I could have it get.
No one has the moral right to spray chemicals onto your property, either
accidently or purposefully. All chemicals intended for spraying include
cautions about drifting/air movement direction, so the applicators cannot
claim ignorance. The only sure way to ensure against drifting is to not
spray the stuff.

Nearly 20 years ago, I hired someone to mow the lawn at the office. A few
days later, I noticed the grass dying around the planter under my sign as
well as next to the sidewalk. I confronted him about this. He had put a
weed-killer (casaron) on my lawn with neither my permission nor my
knowledge. Needless to say, he did no more work for me. The worst part
of it was that he really didn't feel he did anything inappropriate,
pouring poison onto my property. In my simple mind, mowing and spreading
a toxic agent is not the same thing.

Good luck and my sympathies to your plants.

Glenna