I think there is some bacteria that will help get rid of the chemical they will absorb it and others will eat them. The ameoba and parameciams and protozoa that we learned about in 9th grade. Eventually the microbes will ameliorate. I like that word ameliorate the soil. It's your garden. Put what you want there. Maybe if you take some good soil from a place that you know grows good plants and sprinkle it on top it will rain down the good microbes. Maybe some good Organic fertilizers at the plant nursery will be good to sprinkle and make the soil good again. Don't they have something called starter fertilizers? To get the good things going again? I think rain eventually takes it away and it dissapates in the air and water and water vapor and the microbes.

January 2023. The Niwaki cloud pruning continues after I skipped last year somehow. What a difference six years of growth makes. A cloud pruning master I am not. I'm in no hurry to reach a final form and probably not brutal enough. I decided I wanted fluffier clouds. It is time to push the Fothergilla away from the Niwaki. They are getting crowded. That can wait until spring. I can dig a bunch of Fothergilla starts to plant elsewhere. I'm much better at herding cats.

Let's not get too nit-picky!! We are not talking about top pruning.....we are talking about the removal of a couple of extremely tall but almost leafless stems. The impact of their removal will have a negligible effect on the overall vitality and vigor of the plant. And it is also well established that pruning actually stimulates plant growth if done correctly and at the appropriate time, NOT resulting in "less of it than there would have been without the pruning."!! It's the OP's choice - leave or remove. But in all my years as a consulting horticulturist, I have never had the removal of strong, almost epicormic vertical growth be a detriment to overall plant growth and vigor.

" The old stems from the previous year aren't wood " Sorry but that is just wrong!! They most certainly are woody and stating otherwise is a reflection of a major lack of botanical understanding. Hydrangeas are caning shrubs, constantly generating new stems from the root crown. If you do not remove the dead or nonviable ones, you will wind up with a big congested mess! Routine garden maintenance is rarely considered as slight as a mole hill but it is far less than a mountain!!