I like most of this guide, but I choose a slightly different build.

I choose movement and mana regen over the extra AP boost. Zyra is a utility champ with vision and low cooldown soft aoe cc. I prefer to be all over the map with a seemingly endless supply of mana able to run to and defend turrets swiftly and smoothly. Zyra's damage comes from dealing multiple shots with her plants and e->q combos, but usually the extra AP won't make much of a difference like it would with burstier champs. I prefer to play her as a peeler and let my teammates finish the opponents off.

Runes: Mixture of flat and scaling Mana Regen runes, seals, and marks. Movement Quints
Masteries: 9 in offense, 21 in utility, including the movement speed boost at the end

Skills: e, w, q, e, w, r, (Max E for snare length, or W for CDR and more seeds, depending on the situation)

Items:
Buy wards whenever you have money, buying at least 1 at a time, but primarily on build until after rylai's

Trinket: Warding Trinket - vs annoying champs that hide in the brush - Thresh, Blitz, Nidalee, Nami
Sweeping Lens - vs teemo or if you want to sweep out wards (not as useful as oracle's was)
Money Items - I rarely waste time on these. I

(Start) (21 points in utility, including extra gold at start):
1 Faerie Charm
3 red potions
2 blue potions
2 stealth wards


1. Tear of the Goddess
2. Boots of Mobility * - I find mobility extremely useful on Zyra.
3. Fiendish Codex
4. Rylai's Sceptor * - Core

Finish with the rest in any order:
- Rabadon's Deathcap - Core
- Morellonomicon vs Lifesteal champs or Healing champs
- Liandry's vs Health building champs
- Banner of Command - 15% more damage from plants if you aren't up against lifesteal champs
- Void Staff - vs Magic Resist
- Ruby Sightstone - core for support champs
- Upgraded Trinket - (I choose the vision ward trinket vs the regular ward trinket)
- Boot upgrade - Captain

Zyra can melt squishy champs like ADCs and most APC's. She can snare and destroy a yi jungle who tries to run up on her. She can use her plants to detonate Teemo's plants and then kill him. She can peel tanks and snare any stunned opponent to ensure kills. In fact there are very few champs she can't "deal" with. Even at a nice full build, the following battles will be tough if not impossible:
Nasus
LeBlanc
Fizz
Jax

She can't do anything to Taric or Alistar in support lane

<3 Luv your Mid lane build, but I still have something to ask though, is it really good to max W first than E in Zyra Support? O.o

I love the mix of items, runes, masteries...a very deadly combo...although very risky because she's a glass cannon...very damaging, but very easy to break.