I am curious as to how far herbalism kits can be pushed.
In the PHB (p. 153), it says about the herbalism kit:
This kit contains a variety of instruments such as clippers, mortar
and pestle, and pouches and vials used by herbalists to create
remedies and potions. Proficiency with this kit lets you add your
proficiency bonus to any ability checks you make to identify or apply
herbs. Also, proficiency with this kit is required to create antitoxin
and any potion of healing.
In XGE (p. 82), it says about the herbalism kit:
Arcana. Your knowledge of the nature and herbs can add insight on your magical studies that deal with plants and your attempt to
identify potions.Investigation. When you investigate an area overgrown with plants, your proficiency can help you pick out details others might miss.
Nature and Survival. When you travel in the wild, your skills make it easier to identify plants […]
Identify Plants. You can identify most plants with a quick inspection of their appearance and smell.
This is great when you're looking to make antitoxins and potions of healing/penicilin and all the good stuff, but I am curious to know about the other side of herbology; such as identify poisonous berries, mushrooms, herbs and all the other remedies that herbalists need to know about to avoid making a poison by mistake…. wouldn't proficiency in herbology not help in that way too?
More specifically, given that nature itself is a two-sided coin, with medicinal plants offering hallucinogens, paralysis, poison, inflammatories, etc, wouldn't a PC who is proficient with herbalism kits also be able to create plant/nature-based poisons and potions of special effects much like one proficient with poisoner's kits?
As a player, this can give some interesting options, such as mixing potions of healing with potions of sleep applied to a large gaping wound to help prevent the wound from reopening right away or using potions based on mushrooms to coat your weapon/ammunition and cause the enemy to trip out and cast/swing wildly, or even numb their bodies so they can no longer fight and/or talk…
Though I agree that these would have to be plant-based, and thus most likely would not consider wasp farming or toad slime, I'm wondering why the PHB and XGE only focus on the positive benefits of herbology in this matter…
Best Answer
It would not be wise
The Poisoner's Kit is designed to do exactly what you want to do and fairly safely. The Assassin rogue's 3rd-level Bonus Proficiencies feature (PHB, p. 97) grants them proficiency with the Disguise Kit and Poisoner's Kit. So by making the Herbalism Kit more powerful, you are weakening the Poisoner's Kit and the Assassin subclass.
An example of what could happen:
Let’s look at a level 3 Scout rogue with an Herbalism Kit vs. a level 3 Assassin rogue with a Poisoner's Kit.
At Level 3, a Scout rogue gets the Survivalist feature (XGtE, p. 47):
By being proficient in the Herbalism Kit, you know all beneficial and harmful plants and you have expertise in finding them. The Kit also has leather gloves for harvesting the poison; leather gloves basically make you immune to poison (sarcasm). And because healing with plants is just the other side of the coin to hurting with plants, you are able to make poison. In fact, you are proficient in making healing potions and likely plant-based sleeping aids - nice, you just created a synthetic Drow Poison without ever having to have heard of Drow Poison or the Drow. You could even make a super deadly poison with no cure! ...Wait, why is the room spinning and so cold?
At level 3, an Assassin rogue gets the Bonus Proficiencies feature, which grants them proficiency with the Poisoner's Kit. Per XGtE p. 83, the Poisoner's Kit allows you to apply poisons and create them from various materials. You can identify poisonous plants and animals. And it lets you handle poisons: "Your proficiency allows you to handle poison without risk of exposing yourself to its effects."
In the above example the Herbalism Scout outclassed the Poisoner Assassin - but without a proficiency in a Poisoner's Kit, you will likely be poisoned during the harvesting, creating, and applying of the poison.