Whether you’ve been camping, hiking, or simply driving among forested roads, you might have noticed purple paint on trees. A thick swatch might be seen painted along the bark, but why?

A purple-painted tree is a sign warning people against trespassing on the property. Yes, it’s legitimate.

Now, you might be wondering why you can’t just put up a sign. You can. When it comes to longevity, signs can be blown off trees (or fence posts which are also often painted purple), be bleached by the sun over time, and undergo wear and tear that could make it less effective. A giant streak of purple? That’s memorable.

As for why purple? First, it’s a striking and bold color, but second, it’s not common outdoors like greens and yellows. If you see purple along a fence or tree line, you’re likely to notice it.

The paint color as a property line marker was first recognized in Arkansas in 1989, but since then, it has spread to over a dozen states including Texas, Georgia, and Illinois. just to name a few. Yes, in states where purple paint is recognized as a property marker, it is considered trespassing if you go beyond it.

The next time you’re out for a hike or driving down a country road, be on the lookout for purple fences or trees. You now know exactly why they’re there.