"If" your plants/trees are in your backyard in such a way as to where they can't be seen, or identified, without physically going into your backyard, then there would/should be no way they would know what you have. Barring your giving them blanket permission, by agreeing to their HOA bylaws, to enter your property for the purposes of inspection, they would have no legitimate way to be there to see what you have. If they want to know they would have to ask your permission and failing to receive that I'm thinking they would have to try to obtain some type of "administrative search warrant." I would expect that if they chose to go that route they would have to go before a judge, or similar official, apply for it and see if they got it. None of that would be immediate and would give you time to take the "necessary" steps to mitigate the situation.
Long and short of it is "Don't take it lying down." Like some have said, HOAs "can" be a collection of bullies that when confronted, like a bully, will, at times, tuck their tails and slink away. A good offense can frequently be a great defense." Hit them where it hurts!!