Elm and maple are both considered high perishable. They rot, and quickly. Charing the exterior few mm is unlikely to do much either way. In your region, I assume cedar means eastern white cedar, which is a species of Thuja. It is moderately rot resistant, yes. Most of the people here on this forum talking about cedar are probably talking about eastern red cedar, which is a kind of juniper. It is considerably more rot resistant than Thuja.
That product is marked-up ferrous sulfate with a small quantity of tannins added. It is not a wood preservative chemical, it is a dye/colorant. Yes, technically, it does slightly slow down the rate of decay initially, but only under low moisture conditions. But that’s true of just plain tannins, of plain sulfur, of borax, or table salt, shoot, even of just plain jane lime or whitewash. When exposed to moisture, and especially when in contact with the ground, it will both get washed out and will chemically degrade and breakdown. The main commercial use of ferrous sulfate with respect to wood is as a colorant for artificial aging and weathering. It’s actually more commonly used as a lawn fertilizer because it’s a cheap form of iron.
By the way, it’s cheap stuff. You’re paying way too much for it. Here’s a 50 lb bag. https://www.tractorsupply.com/tsc/product/shur-crop-31-iron-sulfate. You can mix that with a little tannic acid powder, which home brewing places sell, usually for about 10 bucks a pound.
But, again, it’s not a true wood preservative. You might as well soak your logs in salt water or borax–it’ll be about as effective.
Store bought pressure-treated lumber no longer contains heavy metals, arsenic, or similarly dangerous materials, and hasn’t for decades. Most pressure treated lumber is treated with copper salts. Yeah, copper salts, pretty much the exact same stuff that organic gardeners spray directly onto their fruits and vegetables to keep fungal rots at bay. If you’ve sprayed your plants with Organic copper fungicides, you’ve released way more of those chemicals into your soil than a few posts of treated lumber ever will.
I’ll reiterate. CCA-treated wood, which is toxic, is not legally* available for purchase in retail big box stores, and hasn’t been in over twenty years. Modern pressure treated wood is treated with copper salts and is perfectly safe for ground contact in food-producing areas.
If you don’t mind dropping a good bit more money, you can use naturally rot-resistant wood. Being in Ontario, the decay rate is probably a lot lower than in other, warmer regions, and if you have very good drainage, that’ll also slow down the rate of decay.
*Correction: this is not actually true. While you can’t generally buy CCA lumber anymore, it’s not the result of a legal change. Rather, big lumber producers got together and decided to restrict their own product, hence why it is now rarely available to the public. Credit to @Audi_o_phile for the correction.
It’s actually grown for timber pretty extensive, but only in Europe. One-in-three trees in Hungry is American black locust. A lot of it gets used for flooring over there.
It’s not a timber tree here in its native US because of the locust borer. And because we have a surplus of excellent hardwood species and stocks. The American timber market is one of the best in the world, so it’s hard for locust to compete. But the borer is the main reason.
