Hey all, this is my first guide on Mobafire and I hope you guys can provide some feedback.
I've played a fair amount of games with Zyra and I feel like I have a fairly good handle on her. She suits my desired play style so I'm here to let you know of how I play her.

Pros

* great range
* very strong burst AND sustained damage
* great ultimate that will help greatly in a team fight
* has a great initiate

Cons

* need to land the skill shot skill to be effective
* ult needs to be timed well or the enemies will just walk out of it

Well my thought behind maxing her Rampant Growth last is that, if you pull off the skills, you won't really need the cool down reduction, and that's all you really need the leveling of the skill for.

My thought behind this word is that Zyra can make her opponents pretty useless thanks to her skill set. It's worked out quite well for me playing this way, so I hope it does for you as well.
So, my plan when playing Zyra is to attack my enemy from a far without the opponent being able to get near me.
For example start off with Grasping Roots then place a Rampant Growth near the target (*careful not to place it right beneath the target as this may trigger that champ to 'step on it' and burst it, not sure on this though) and then deadly bloom on that champion and Rampant Growth.
this combo works very well, you'll just need to anticipate the movement of the opponent well.
Be especially wary when playing against a highly mobile champion

Keep in mind the different types of plants Zyra can summon
the plant summoned when you use the Grasping Roots on Rampant Growth has a lower range but it slows the target hit.
and the plant grown by deadly bloom will have a larger range.

her passive is also a great skill, similar to Kog'Maw she will turn into a giant plant. In this form you will be given one shot you can fire in a straight line that deals true damage.
So it isn't the worst of ideas for you to be involved in the thick of the battle (while using your cc of course) since this way your shot has a better chance to penetrate more champions!