It's not good for trees to have nails knocked into them - the circulatory system of trees is just underneath the bark, so its impossible to avoid penetrating that. The odd nail shouldn't kill the tree, but for preference, bands which fit tight round the tree are better as supports, you often see them where bird boxes have been erected as a means of attaching them to trees, though the bands should be loosened off a bit each year or two. Difficult if you want to hang pots though.

Instead of small pots, why not use hanging baskets, just throw a length of thin rope or strong twine over a branch,
tie the two ends together, and just hang the basket/basket,s on them, doesn,t damage the tree, and you don,t need to adjust it.
Personally I,m not a fan of hanging baskets, or pots of anything , hanging off anything, I think they,re more trouble than they,re worth, but that,s just my opinion,
everyone to their own. Derek.

I personally agree with Derek and would not hang anything off a tree, but then I like my garden to look reasonably natural and trailing geraniums in a tree would look weird in our garden. Please do not put nails into the tree as Bamboo has explained the circulatory system of the tree is just below the bark.

And if you really must, don't use copper ones.

I do not see why anyone needs to put nails into a tree Steragram!

Perhaps some sort of climber would give you the effect that you're after?