Assuming it isn't engineered or laminated lumber of some sort, it should be perfectly safe.
Thanks for taking the time to answer my question. Most box springs that are available for upcycling are old. I am leaning towards going ahead and using it to make a raised bed for non edible plants. Thanks, again.
You can always lacquer the wood if you're concerned, but there's probably nothing to worry about.
. It may contain some flame-retardant(s) (eg, TBBA), but not enough to do much harm.