How does damage versus Zyra's Plants work?

Zyra's plant summons all have 6 hitpoints, yet they often survive multiple hits from champions.

I assume this means they are similar to Wards (3 hp, 1 hp per attack), but the plants seem able to take more than 1 hp of damage at a time, so it's not an identical mechanic.

What determines how much damage Zyra's plants take?

Solution 1:

All plants can killed by 3 AAs from Champs or one single target spell and one AA.

Minion Damage is treaten diffrently, but from a siege minion one shot equals one bar. Turrets instantly kill the plant (one shot).

EDIT:

• Single-Target spells do 4 damage each (2 bars).

• AOE abilites deal 1.5 damage each. (per tick if applicable, like Morgana's tormented soil)

• DoTs deal .5 damage each tick.

• smaller mobs (including neutrals) do 1 damage per auto attack (1/2 a bar) .

• bigger creeps (big wraith, big golem etc) and champions do 2 damage (1 bar) per auto attack.

• Towers kill the plants instantly (6 damage).

EDIT2:

From Reddit: Zyra's Plants Take Damage in Extremely Buggy Ways

Solution 2:

In addition to Jacimovski's excellent answer, Morgana's pool takes off half an hp "bar" per tick from the plant, so it kills it in 6 ticks. This seems really relevant given Zyra's role as a mage (ie mid).

Then again I've found it best to just ignore the plant they cast near me and drop my pool on the minions, then last hit the plant for a free 5g.