As BESW said, don't expect the game system to model reality.

That said, if you need to figure out stats for potato tea...

• It's an ingested poison. As the SRD says,

  Ingested poisons are virtually impossible to utilize in a combat situation. A poisoner could administer a potion to an unconscious creature or attempt to dupe someone into drinking or eating something poisoned. Assassins and other characters tend to use ingested poisons outside of combat.

  So, this really isn't likely to be a major combat advantage. In my view, a cheap and accessible plot-device poison isn't really a big deal. Also, how much exactly do you have to trick the target into drinking? From the sound of it, potato tea isn't nearly as potently concentrated as the expensive fantasy poisons in the game book.

• What's the save DC? Something that calls for a DC 12 Fortitude save, for example, is still going to mess up a lot of low-level characters, but most powerful people will be pretty much immune to it.

• What's the actual effect in game-mechanical terms? In the real world, it's pretty lethal, but lethal in the real world doesn't necessarily translate to "save or die." A poison that does 2d12 damage could kill a low-level commoner. Whereas a powerful character is just going to vomit it up but survive pretty much unscathed.

I'm not saying you should block the players by going out of your way to make it weak and useless. But don't make a game action super-powerful just because it could kill someone in reality. People were killed with regular ol' run-of-the-mill arming swords all the time. Yet they still do only 1d8 damage in D&D. Players characters and antagonists in D&D very quickly become more like action-movie heroes (or wuxia/super heroes, even) than real-world people as they attain levels or Hit Dice.

Scores a hit

RAW

Attack Roll

An attack roll represents your attempt to strike your opponent on your turn in a round. When you make an attack roll, you roll a d20 and add your attack bonus. (Other modifiers may also apply to this roll.) If your result equals or beats the target's Armor Class, you hit and deal damage.

It doesn't matter if your damage surpass DR or THP. Hit from your weapon take place if your attack roll greater or equal than target's AC.

Temporary Hit Points

RAW

Temporary Hit Points

Certain effects give a character temporary hit points. These hit points are in addition to the character's current hit point total and any damage taken by the character is subtracted from these hit points first. Any damage in excess of a character's temporary hit points is applied to his current hit points as normal. If the effect that grants the temporary hit points ends or is dispelled, any remaining temporary hit points go away. The damage they sustained is not transferred to the character's current hit points.

When temporary hit points are lost, they cannot be restored as real hit points can be, even by magic.

Even if you didn't surpass THP amount it's still counts as a solid hit. Supernatural ability or magic, that grants character/monster THP, just pull all damage from regular HP to THP. Any poison effects take place as intended, since there was damage to target. Because of THP target became tougher, not immune to certain amount of damage.

EDIT: Clarifying about THP and immunity

Q: But they are in fact immune to a certain amount of damage. If I gain 10 THP then take 5 damage, then have THP expire, I'm still at full health. Did I suddenly go from "injured" to "healed" by virtue of losing HP? Does the PC go from looking "beat up" to looking 100%?

A: @Gates VP - to clarify I need to quote this thing:

Hit points mean two things in the game world: the ability to take physical punishment and keep going, and the ability to turn a serious blow into a less serious one.

Immunity means there was no damage at all. In case of DR, if damage is too small to surpass certain amount - it's ignored. De-facto there was no damage at all. In case of THP you've recieved this damage. It was not ignored, it damaged you a little. But because of magic, or abilities it was not enough to hurt you seriously. To make an example:

False Life

You harness the power of unlife to grant yourself a limited ability to avoid death

You recieve damage, but because of magic you can live a bit longer, survive a bit longer.

In case of feats

Drunken Brawler (Combat)

When you drink a tankard of ale or strong alcohol, you take a –2 penalty on Reflex saving throws, but gain a number of temporary hit points equal to your level...

Have you ever heard expression that drunken people are tougher(not talking about stronger - it's a nonsense)? But in case of toughness they feel pain a bit less than sober. That helps them to endure some blows better.

Damage Reduction

RAW

Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury poison, a monk's stunning, and injury-based disease. Damage Reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact.

Attacks that deal no damage because of the target's damage reduction do not disrupt spells.

In that case you still score a hit, but because of some supernatural ability(or magic) your hit didn't do any damage to target. At all. Since there was no damage - there can't be any poison effects.

Why did I mention 'scores a hit' rules at the beginning

In both cases you should remember that poison works on 1 hit only. Even if you didn't surpass target DR or THP it doesn't really matter.

RAW

A poisoned weapon or object retains its poison until the weapon scores a hit or the object is touched (unless the poison is wiped off before a target comes in contact with it).