The approach suggested by many other answers here (insist on returning or paying for the trees, and internally notify your boss or HR) has a couple of possible downsides:
- Most importantly, the contractor may be upset about their gift being ungraciously rejected.
- Less importantly, you end up either out of pocket and/or without any trees. (This is not the primary issue here, but hey, it'd still be nice to keep the trees you wanted!)
If you think that this was truly meant to be a bribe, then those downsides are an acceptable price of doing the honourable thing. However, it seems like this was a financially trivial gift (a quick Google suggests we're talking about tens of dollars) from a party with whom you had an existing relationship. You are in the best position to judge, but it sounds like in their minds it may have been nothing more than an innocent show of appreciation for your previous work, with no more corrupt intent than saying "thank you" or providing snacks or beers when you visit their offices.
Even in that case, it's still worth covering yourself. But you can do so tactfully and tactically in a way that doesn't risk hurting your - or your company's - relationship with the contractor. Send an email along these lines, and CC your boss or HR or whoever you think it's appropriate to report gifts to in your company:
Dear Mr Contractor,
Thank you for the surprise gift of the three Bonsai trees! I never imagined when I mentioned wanting them previously that I would return home to find them in my garden - it was very thoughtful of you!
I've CCed Bob, since our policies require me to report gifts to my line manager.
All the best,
I would not voluntarily reject the gift or insist on paying for it without being asked to do so by a manager or by HR. That would simply cause embarrassment and administrative overhead for the contractors, which they may well resent - especially since the cost in labour for them of sorting out such a fuss will likely be greater than the total value of the gift!
- Makes clear - to all parties - that you didn't intentionally solicit the gift.
- Clearly notifies your company of the gift, so you can't be accused of hiding it later.
- Eliminates any potential for blackmailing you with your earlier email, by disclosing to your company that you mentioned wanting the trees.
- Openly commits your company to a friendly outlook on the gift, rather than a hostile one that treats it as a bribe, deterring any overzealous HR people from getting angry about it and burning bridges with the contractor.
- Clues in the contractor to the fact that there was some risk of this gift going badly wrong, and that sending unsolicited gifts to business associates can be a bad idea.
Of course, if you think the contractor is corrupt and was trying to bribe you, this approach may not sit right with your conscience; you may want to make a big deal out of it and try to get your company to cut ties with the contractor. But if you think this was an innocent gift and just want to mitigate the risk of being seen to have solicited bribes, a casual email like this seems like it achieves that with the least risk of fallout.