Legal Question in Criminal Law in Virginia

The legality of the entry onto your property depends to some extend on the particular circumstances (layout, size of yard, location and accessibility of entrances, etc....), but in general I would say no. If they did not see the plant until they had actually entered your property and began to look around then the search would be illegal in my opinion. If they were able to see it in plain view before they actually entered your property that would be another thing.

The circumstances here seem pre-textual to me. Even if we assumed, for the sake of argument, that the fact that some crimes were committed in a neighborhood therefore gives the police the right to go traipsing around everyone else's backyard and invading their privacy to determine if they were also a victim of crime, it doesn't make any sense that they would need to do that in this case. What exactly was the purpose of the "courtesy check" I wonder? To see if your air conditioning unit had been stolen and you just hadn't realized it yet? In 95 degree temps, that's not exactly the sort of thing that would go unnoticed. In other words, it sounds to me like they made it up.

Hopefully you did not admit to knowledge that the marijuana plant was growing. Since marijuana is a weed that grows naturally in this climate, it would be nearly impossible for them to convict you of anything without an admission (a single plant does not exactly = a grow operation). I would advice you not to speak or otherwise cooperate with them in any way. Don't answer their questions, don't consent to any more searches, don't say anything at all to them except "I do not want to talk to you without a lawyer."