Herbalism

Rarity:

You should limit how often the PCs can search for a particular herb. I would suggest once per hour in a forest region, once per day in any other region, or only once per square mile. If a player knows any hints as to an herb's location, grant them advantage on their Survival check.If a plant is usually found near a waterfall, or in shady areas, or near certain animals, for instance.

Common: Found with a DC 13 Survival check. Identified with a DC 10 Nature check.
Uncommon: Found with a DC 17 Survival check. Identified with a DC 15 Nature check.
Rare: Found with a DC 22 Survival check. Identified with a DC 20 Nature check.
Unique: A Rare herb found in a unique biome (as seen in Part 1)

Using Herbs

Using an herb consumes it.

Magical Uses

Using an herb for its magic does not require that you have access to spells, but it does require you to have proficiency in Nature.

Herbs can be used magically in several different ways, most of which you might expect from your mad aunt with the pointed hat who lives in a cottage in the woods. Such ideas include (but are not limited to):

Putting herbs under a pillow while you sleep
Making emblems, talismans, and fetishes from bundles of herbs to anoint the target of a spell
Satchels filled with prepared herbs to hide under a floorboard
Throwing powdered herbs at a target or sprinkling it into a circle
Burning or boiling herbs for people to inhale the steam or smoke
Making pigment for a dye to paint the target's skin

Now for some actual mechanics. The Nature DC is what is required to properly use the herb. Not rolling high enough usually doesn't backfire; it just has no effect.

Warding

Nature DC 16, 2 Uncommon Herbs

When using the herbs as a spell component in a Magic Circle spell, it can also impose disadvantage against creatures of an alignment of your choice (good, evil, chaotic, or lawful) attacking those within the circle. Such creatures may still enter the circle, but must make a CHA saving throw to do so. You can also ward a 5-foot square using these herbs to prevent ethereal creatures from entering that square unless they succeed at a DC 11 CHA saving throw.

Detection

Nature DC 14, 1 Common Herb, 1 Uncommon Herb

Using the herbs on a creature or area can detect the presence of good, evil, or magic (your choice).

Dreamcatching

Nature DC 10, 2 Common Herbs

The affected creature is warded from nightmares, fearful visions, the Nightmare spell, and in general has a peaceful night's sleep even with much on their mind.

Causing Nightmares

Nature DC 14, 1 Rare Herb, 1 Uncommon Herb

An affected creature has an unrestful slumber and awakens with one level of exhaustion. This cannot cause a creature to die from exhaustion.

Divination

Nature DC 12, 2 Uncommon Herbs

You gain a vague vision of the past, present, or future, usually in the form of a metaphor. Leave this to the discretion of the DM.

Boosting Abilities

Nature DC 18, 2 Rare Herbs

When you cast Enhance Ability using these herbs as a spell component, it increases the chosen Ability Score by 2 in addition to its other effects. Otherwise, these herbs can cast a normal iteration of Enhance Ability.

Embalming

Nature DC 16, 1 Rare Herb, 2 Uncommon Herbs

A creature embalmed in these herbs is prevented from being raised as an undead. Alternatively, slightly different herbs can prepare a corpse for being raised as a Mummy. When casting a Resurrection spell on an embalmed corpse, this reduces the value of the required diamond by 200 gold pieces.

Rare Spell Components

1 Unique Herb

Boost Spell: Nature DC 16. Increase the spell's effective slot level by 1.
Strengthen Spell: Nature DC 18. Increase the spell's DC by 1.
Empower Spell: Nature DC 15. Increase the spell's damage by one die or 1d6, whichever is larger.
Lengthen Spell: Nature DC 13. Double the spell's duration.
Extend Spell: Nature DC 12. Double the spell's range (melee spell attacks gain Reach).
Twin Spell: Nature DC 18. Double the spell's area or target one additional target.
Inevitable Spell: Nature DC 16. The spell cannot be countered.

Medicinal Uses

You must have proficiency in both Nature and Medicine to use herbs for medicinal purposes.

There are a few ways to prepare an herb for use in medicine:

Having the patient drink a prepared tincture, decoction, or infusion of the herb
Soaking the patient in an herbal bath
Having the patient inhale vapors from a burning or boiling herb
Applying a poultice, paste, or balm to the patient.
Having the patient eat the herbs raw
Or else maybe one of the magical uses above for a more spiritual kind of healing.

The Nature DC is what is required to properly use the herb. You may substitute a Medicine check here instead if you find that your players don't use that skill enough. Not rolling high enough usually won't injure the patient further, but incorrectly identifying an herb might make a wound worse or even poison them.

Healing Herbs

Weak Healing Herbs: Nature DC 10, 1 Common Herb. Heals 1d4 over a short restModerateHealing Herbs: Nature DC 15, 1 Uncommon Herb. Heals 2d4 over a short rest.
Strong Healing Herbs: NatureDC 19, 1 Rare Herb.Heals 4d4 over a short rest.

Medicinal Herbs

Weak Medicine: Nature DC 14, 1 Uncommon Herb and one Common Herb. Gives +1 on a creature's next saving throw against a disease.
Strong Medicine: Nature DC 19, 1 Rare Herb. Gives +4 on a creature's next saving throw against a disease.

Antitoxins

Weak Antitoxin: Nature DC 12, 2 Common Herbs. Cures a creature affected by a poison with a saving throw DC of 13 or less.
Strong Antitoxin: Nature DC 17, 1 Rare Herb. Cures a creature affected by a poison with a saving throw DC of 18 or less.
Perfect Antitoxin: Nature DC 20, 1 Unique Herb. Cures a creature affected by any poison.

Stop Bleeding

Nature DC 10, 1 Common Herb

An herb that when applied to a bleeding wound will quickly seal the wound (1 action) and prevent it from bleeding further. Prevents any damage from bleeding and will heal 1d4 HP for a creature below half its maximum hit points.

Treat Burns

Nature DC 12, 1 Uncommon Herb

Heals a creature for 1d8 HP over a minute but only if they were dealt fire damage since their last rest.

Treat Frostbite

Nature DC 12, 1 Uncommon Herb

Heals a creature for 1d8 HP over a minute but only if they were dealt cold damage since their last rest.

Awakening

Nature DC 15, 1 Rare Herb

Returns an unconscious creature to consciousness in one round with a quick sniff of this herb. If the creature had 0 HP, it gains 1 HP.

Regeneration

Nature DC 20, 1 Unique Herbs, 1 Rare Herb

These herbs will, through some medical miracle, help reattach or regrow lost limbs over the course of one month. The herbs must be reapplied each week, and each use consumes one batch of herbs, so be sure to stock up.

Restore Vitality

Nature DC 14, 2 Uncommon Herbs

These herbs will restore a creature's ability score to their full value in one minute. [Most ability drain in D&D 5e lasts until the victim finishes a rest. If you want such herbs to be more vital, you can have such drains last permanently and have herbs heal them over a rest.] Different herbs are required for different abilities.

Cure Conditions

Nature DC 17, 1 Rare Herb, 1 Common Herb

These herbs can help treat simple conditions like those that a Lesser Restoration spell could restore, other than the poisoned condition (that's what antitoxins are for). The condition is removed if the herbs are applied over a long rest.

Alchemical Uses

Using an herb to brew any type of potion requires proficiency in Alchemist's Tools.

Most alchemical uses of herbs involve either powders, potions, pastes, or oils. Messing up an alchemical item has the same sorts of risks as a potion backfiring.

Burning Balm

Nature DC 13, 2 Common Herbs

When applied to the skin, this substance causes a creature to take 1d4 radiant damage each round they are exposed to direct sunlight. In addition, they become vulnerable to all other forms of radiant damage. This lasts for 4 hours.

Poisons

Weak Poison: Nature DC 16, 2 Common Herbs. 2d8 poison damage. DC 11 CON save halves.
Moderate Poison: Nature DC 18, 1 Rare Herb. 4d8 poison damage. DC 16 CON save halves.
Strong Poison: Nature DC 20, 1 Unique Herb. 8d8 poison damage. DC 20 CON save halves.

Stonebiter Acid

Nature DC 15, 1 Rare Herb, 1 Uncommon Herb

This acid is harmless on organic matter, but burns away stone and metal. A small vial will melt through one square foot of stone or 6 square inches of metal. It deals 4d6 damage when thrown at a construct creature made of either stone or metal.

Tanglefoot Bag

Nature DC 13, 2 Common Herbs

Toss this satchel of herbal goop at an enemy up to 20 ft. away to cover them in sticky ooze, restraining them until they free themselves with a DC 12 STR check.

Everburn

Nature DC 10, 1 Common Herb

Toss these herbs in a campfire and it will burn for 24 hours without needing to put more fuel in the fire.

Sleep Powder

Nature DC 15, 2 Common Herbs

A sprinkle of the herb can potentially knock out a creature for 2d4 minutes unless they succeed at a DC 12 CON saving throw. The creature can make a new saving throw if attacked or if someone tries to awaken them. Additionally, the powder can be snuck into a creature's food or drink for a more potent effect, increasing the save to 14 and lasting 2d4 hours.

Drugs

Once a creature becomes addicted to a drug they are compelled to take the drug again as whenever possible, gaining one level of exhaustion each day that they do not take the drug. Drugs can have various effects like inducing hallucinations, relaxing the body, or focusing the mind but usually involve varying degrees of euphoric high. You can come up with those effects on your own, as they will likely not have too much mechanical impact on your game. I would suggest having the high last for 10x1d8 minutes for any given drug, and have it increase CHA by 1 and decrease DEX and WIS by 1 each for the duration. You could have more powerful drugs compound these effects or increase a different ability (other than DEX or WIS). Look in the 3e Book of Vile Darkness for other ideas on drug use in D&D.

Weak Drug: Nature DC 14, 1 Uncommon Herb. Addiction CON DC 6.
Moderate Drug: Nature DC 16, 1 Rare Herb. Addiction CON DC 10.
Strong Drug: Nature DC 18, 1 Unique Herb. Addiction CON DC 15.

Truth Serum

Nature DC 15, 1 Common Herb, 1 Uncommon Herb

A creature that drink this tincture is compelled to give truthful responses to questions asked of it. It can resist this urge with a DC 12 WIS saving throw. Only one saving throw can be made per question.