Copper sulphate is water soluble, and at the concentrations needed for root pruning, if absorbed by the plant will kill it.
Hmm, I presumed that copper hydroxide was also soluble (and only kept from dissolving by the latex paint). Apparently it's insoluble. Okay, well there goes that idea Unless I can dig up some sodium hydroxide somewhere to convert it...
Oh hey wait, I have some cement... calcium hydroxide should precipitate out zinc hydroxide....
Would not be easier for you to get some Copper oxyclorure? It's the most common copper fungicide here in Europe.
Not in Iceland it wouldn't
Seriously, just as an example: do you know the grand total number of pesticides I've found in all the garden centres here? Guess.
The answer: two. I'm not joking. Both pyrethroids. You can't even buy diatomaceous earth here.
I could buy things online, but shipping and customs costs would be quite high, and customs may give me trouble for shipping in chemicals. I could buy them in the US when on vacation next, but I imagine that the TSA might read me the riot act for bringing in chemicals. Shipping takes weeks regardless, and who knows when my next US vacation will be. But I have copper sulphate and cement today
The reaction seems to have gone pretty well (it was quite noticeably reacting). One difficulty (vs. using sodium hydroxide, which I don't have onhand) is that calcium sulphate (plaster of paris / gypsum) can be - depending on the form - either well, or only poorly, soluble in water. So I don't know how much I washed away during the filtration; I suspect some of it (possibly even most of it) is left behind. Not like calcium sulphate should be a problem in the latex paint, it just means I should use more of the powder. The key part is that I did 3-4 filtration/rinse cycles, so that should have gotten the vast majority of any water-soluble copper compounds out.
I'm drying it now.
One nice advantage: the copper sulphate was large coarse crystals that I would have had to try to crush fine with a mortar and pestle. The insoluble precipitate here is an extremely fine bluish "clay".
Trivia: one neat thing you can do with copper hydroxide is to dissolve it in ammonia; this makes Schweizer's reagent, which can dissolve cellulose
You can then precipitate the cellulose out as a fine powder by adding acid.