Simple math: Zyra's Plants & Skill Points
Let's take a look at Zyra's 'W,' Rampant Growth which places and upgrades plants:
Skill points applied add a percentage health to the plants: RampantGrowthSkill = ( 10%, 20%, 30%, 40%, 50%)
Zyra's plants have a starting health of 5 hit points. So multiplying them together: StartingHeath x RampantGrowthSkill = ( 5.5 hitpoints, 6 hitpoints, 6.5 hitpoints, 7 hitpoints, 7.5 hitpoints )
What is the minimum damage output of a melee minion? 12 hitpoints.
It seems that Zyra's plants are one-hit kills no matter how many points are put into the skill, which begs the question why does the health skill progression exist?
https://s6.postimg.org/bhkbcmwcx/Heimerdinger_3.jpg Zyra's plants: http://gameinfo.na.leagueoflegends.com/en/game-info/champions/zyra/ Minion Damage: http://forums.na.leagueoflegends.com/board/showthread.php?t=3212572