Chemical Spraying (Pest/Weed)

IMO it doesn't hurt to notify all residents prior to the application of fertilizers and other chemicals. People have allergies, sensitivities, and personal preferences when it comes to stuff like this. Notifying residents would probably more of a courtesy than a requirement per code or law.

I have kids and pets, so I like knowing when fertilizer will be applied to the common areas in my HOA. That way, I avoid these areas for a few days after the application.

I don't know how frequent you do sprayings for weeds and pests, but you should know when they will be done and would be able to provide advanced notice.

Signs, posters, and/or flyers (depends on what works for your HOA) could be posted 2-3 days in advance and left up 2-3 days after notifying residents of the sprayings. Residents can decide for themselves what they need to do.

It could even be written into contracts to make the contractor responsible for posting signage. That's what we do with our landscape contract. And notifying all residents would eliminate the need for you to personally call a handful of people.

We post notices around the property to notify residents about weed/lawn treatments.
Kids, animals and in this case someone with chemical sensitivities should be offered the consideration of notification.

Provide them with a schedule and a rain date and that should ha due it.

We had a few people who did not want their lawns sprayed. We only did the front yards. We just made arrangements for those yards to not be sprayed. The stuff is only bad for the first hour until it dries. That is when someone can track it in with their shoes. So we would try to announce and warn people before they got it on their carpet. We only sprayed a few times a year though.

The problem is the weather does not alway cooperate with a fertilizer application so a prepared schedule means little. Tough to notify someone even under the best of all conditions.

In our HOA, we had a similar issue. Our landscaping company does a spring broadleaf application (kills dandelions and a few other weeds). She demanded that they didn't go near her portion of the property. Well, 2 years ago, the contractors did and she claims they sprayed near her property and the chemicals landed on some of her plant materials. She asked/received $75 for plant replacement and soil remediation from the landscaping company. The next year, the same homeowner asked the common area immediately adjacent to the homeowner not be sprayed. The board said the common area is for everyone not just her family. Plus the weeds are so out of control (the homeowner thinks that the dandelions are "pretty"). Since then, we have insisted our landscaping company post notices 48 hours prior and to closely watch any applications. So far, they have done that.