I am a gardener first and a smith second. I'm pondering making some plant markers, which would essentially be a stake with a flattened upper half, which could be engraved with "Tomato", "Kale", or even simple pictures of the produce in question.

I'm wondering how long forged iron, coated in a beeswax/BLO/turpentine mixture, will remain reasonably rust-free. I don't need it pristine, I just need it to not completely rust over so that the engraving stays visible. 

I think I'd want to renew the coating every year after harvest.  I'd expect a BLO covering to make it through a growing season.

GNM

Is there any need to strip before re-coating, or do you just make sure the piece is thoroughly clean and dry?

There have been conversations here concerning heavy metals being present in boiled Linseed oil.  I was at first skeptical, so I googled it.  There is quite a good possibility that any BLO that you wish to use or already have will have those present as a drying agent.  I bought a can myself, but I did not find a listing on this particular can of what all the constituents of the product were.  I wouldn't use it for anything in my garden.  A good, weather resistant paint is what I would use.

Thank you! I missed that discussion. I will look into paints. 

You are quite welcome.

What about powder coating? 

Linseed oil, also known as flaxseed oil or flax oil (in its edible form), is a colourless to yellowish oil obtained from the dried, ripened seeds of the flax plant (Linum usitatissimum). The oil is obtained by pressing, sometimes followed by solvent extraction.  Having a high content of di- and tri-unsaturated esters, linseed oil is susceptible to polymerization reactions upon exposure to oxygen in air. This polymerization, which is called "drying", results in the rigidification of the material.  (Wikipedia)

Powder coating is a great idea until it gets a ding or scratch.  Then the barrier is compromised and you have rust and the powder coating starts flaking off.  The only way to repair it is to remove it (usually sand blasting) and apply another protective barrier.  Paint when dinged or scratched and you just clean the rust and recoat.

Plant labels for the garden are easy.  Tongue depressors and a sharpie marker will last for a season.  Heavy wire (coat hangers) and aluminum roof slashing cut into 1 inch or 1-1/4 strips, with a hole drilled into one end and hung on the loop in the wire will last several seasons.  Aluminum is soft so you can use the pointed end of a nail to mark the name on the tag.  When was the last time you replaced the flashing on your roof.  The flashing moving when the wind blows acts as a scarecrow for many birds and aminals.

I'd powder coat or use a 2 part epoxy.

Frosty The Lucky.

I don't garden but I have some shepherds crooks with lanterns hanging on them in the Celtic garden that I coated with BLO years ago and they are still rust free.

I can't control the wind, all I can do is adjust my sails. ~ Semper Paratus

If you use BLO I'd just clean of the loose dirt and recoat.  I doubt that any nasty additives would be present in large enough quatities to be any sort of health concern.

GNM

I have made years ago something for my wife. I painted it with cheap metal paint.

Still looks good 

My grandfather used to be a concrete guy and he used rebar to hold stuff up in the garden. Unpainted or coated or anything.

They last very long, they are rusty, but they last very long