I live in a suburban area, and my property borders a high school. The school has a large campus, with basketball courts, baseball fields, football fields, and parking areas. There isn’t a clear border between my land and the school. In fact, I realize that I’ve been planting bushes and flowers on land that is technically on the school’s property, and not on mine. I began planting there at least 25 years ago, and I started because the prior owner of my house also planted there – probably for at least a decade before that. I’ve heard that one can get legal title over neighboring land by using it for a long period of time. Can I “claim” that portion of the school’s property as my own?
In most states, planting trees and flowers can constitute what's known as adverse possession of property. As long as the plantings are open and obvious for anyone to see – particularly when it comes to the true owner of the property – the trespasser is giving notice to the world of a claim over that land.
This is particularly true if the trespasser is appearing there every single day, pruning leaves and watering plants, while the true owner is doing nothing to improve the land. One of the primary policy purposes behind adverse possession is to ensure that the individuals who are dutifully improving land are rewarded with legal title; owners who are ignoring their land and failing to evict trespassers are punished.
Here, it sounds like your conduct over the bordering patch of land meets all of the legal requirements for basic adverse possession – open, actual, hostile, adverse, and continuous for whatever time period is set by your state's statute (25 years is more than enough in most states).
However, your legal claim against the school will likely run into a major roadblock: government-controlled property is generally immune from adverse possession claims. This is generally true regardless of whether the property is federal, state, or local. Assuming the school is public, this could defeat your claim for title.
Moreover, a lawsuit would alert school officials to the situation, and they are not likely to look kindly upon it. You might be ejected from the land.
Your better move is most likely to avoid bringing a legal action, and simply continue using the land as your garden until someone objects. That way you get the benefit of the property without the expense and hassle of a fight over it. It seems pretty clear that school officials simply do not realize, or currently care, that this patch of land is part of their campus.