Autodamage to the uneducated, that applies to poison ivy, poison oak, and regular nettles. If you don't know what it is, its going to get you because it doesn't look threatening or dangerous.

I am curious what changes or additions you would have to make to this plant to use it in your game (aside from adding dice values and such)

Well, I haven't really settled on any for the time being, but here's a few ideas.
1. Have the dew nettles produce a paralytic poison instead of stinging. Have a carnivorous amphibian, who is immune to the poison and likes to come out in the rain, prey upon the paralyzed victims. The amphibian would normally be not be much of a threat unless it's victim is paralyzed. Then have the amphibian's dung, victim's blood, or whatever be the perfect fertilizer for the dew nettles. Kind of a mutualistic relationship. The nettles would be valuable to healers as kind of a local anesthetic for patients.
2. The dew nettles are actually carnivorous plants and are trying to kill animals that come close. The preferred prey of the dew nettle is rodents. But if a person or something bigger were to die/fall unconscious on top of the nettles. They might chow down on whatever. The berries would be a natural rat poison.
3. Assuming that the dew nettles are annuals, they have a certain maturity point that is reached every year before they die. At that maturity point, the poison is eliminated and the berries are delicious and nutritious to eat (or contain an interesting magical effect or somehow are useful).

I will be honest, it seems like you are trying to make something that is quite simple artificially complicated.

Not every plant is going to be part of an elaborate scheme to harm PCs, nor is every plant existant for the benefit of PCs.

Your ideas aren't bad, this is just a stinging nettle that only stings when it is wet. Simple.

First, if I didn't make this clear earlier, the versions I proposed of your plants are exactly what you wrote. If I added or changed something, I wrote it in my ideas, otherwise they are exactly the same.

Second, every plant has a value... no matter how small and marginal. If the PCs want to harvest a field of hay with their time, I'm ok with giving them a few coppers or whatever from a local farmer (provided it takes less than one minute real time). If a lion were hiding in the hay, did I put the hay there just to get the PCs? The hay was just there, whether for the benefit or harm of the PCs depends on the situation. All three of my ideas could have nothing to do with the PCs. Again it really depends on what the plant is doing in your adventure. Are you trying to teach the PCs that they need a guide? Are you trying to strip a few health (hp) off of your PCs to keep their cockiness down? Are you just wanting a simple complication in the middle of a more complex crisis? Whatever the purpose for including the plant would be accomplished with whichever version of the plant we have discussed.

Finally, if you want to make poison oak that only works when wet, cool. Good for you, whatever makes you happy. Simple ideas are not always bad, but they're usually not terrific either. I like my props a little more interesting and dynamic. That's me. If it bugs you, I won't apologize. I want to be honest with you too. When you make something good. I will tell you about it. I love some of your previous ideas and have given you the votes to prove it. If you make a plain Jane idea... expect plain marks from me. If anything, I'm hoping to help you make better submissions, because I know you can.