Actually the shrub one was on Car Talk the other day (speaks the Car Talk veteran) so I didn't post, aren't I nice, since I had just heard the answer. I love these kinds of things. When I was in grad school in the early 90's we could still stump people with this old chestnut:

A man and his son are in a terrible car crash. The father is killed instantly and the son is rushed to the hospital for life-saving surgery. Upon seeing the patient, however, the doctor exclaimed: "I can't operate on this boy! He's my son!" - How can that be? (I told you it was old!)

I'll post something more difficult as soon as I remember one. Meanwhile, does anyone remember the How do the cannibals and the missionaries get across the river puzzler? I don't want to get it wrong... and it's a toughie! And I've more recently seen a slightly different version of it. Will ferret.