YANBU, if a lemon tree is not something you have said you wanted or could be assumed to be a perfect gift to match your personality or interests. A random lemon tree could be a bit of a shock when historically you have been given and thrilled with, say, chocolate.
I personally would love DH to think of giving me a lemon tree, but that is just me. We live in a good climate for it, and in his Texan way of saying "I was raised by tree-hugging, tofu-eating, VW Bus-driving, Californian Deadhead hippies". And quite proud of it, BTW. I had a very, very interesting childhood 
It is Mothers's day here as well in the US. But this year we are quite poor, and DH's health hasn't been well (he has a degenerative condition) so I lowered the proverbial bar and told them I dont want presents, please. Make me a nice card, pack a lunch and a bottle of wine and let's spend the day at the pool. It was 96F here yesterday and the pool is my best friend!