For this build, ability wise:

Q: Activates your plants or deals a bit of damage on impacts towards champs/ minions/jg.
W: Active: able to grow 2 plants. Passive grow plants around you every so often.
E: Root champ/ minion/ jg/ etc.
R: Make enemy airborne + make plants in area enrage.

Make Q first for the max harassments you can deal with the plants around you + with the plants you get with W. You then want to max E to be able to root people more often.

Some combinations you can practice are:

E(root enemy)+..Q+W(Quickly).
E(root enemy)+..Q+W(Quickly) -> R

Pros:

Insane AP damage
Lots OF CC
Provides great pressure in team fights

Cons:

Squishy
Flash/ Zhonya reliant-- Zyra doesn't have any escape abilities so enemy can easily target you.

VS Comp 1: even matchup
Take.. STANDARD rune set.

VS Comp 2: easy lane / comp (soraka/ yummi)
Take.. VS SQUISHY LANE rune set.

VS Comp 3: hard lane (naut / leona)
Take.. VS ALL IN LANE rune set.

Boots

Sorcerer's Shoes-Provides good amt of magic penetration
Mercury's Treads-Good VS AP heavy comps
Plated Steelcaps-Good VS AD heavy comps


Items

Void Staff- Provides great AP + great amt of magic penetration. Especially early game.
Horizon Focus- Great item for Zyra as you can either damage or immobilize an enemy for the item to activate the passive. It's a much more cheaper alternative to rabadon.
Demonic Embrace- Great defensive item overall for what it provides in that cost. It has anti heal capabilities as well as defensive passives to give you armor and mr! Also there is an increase to the defensive passives for each champion you affect which zyra has a high chance of affecting almost all champs in team fights!


Items (Situational)

Banshee's Veil- Great MR item that grants you a spell shield for bursty comps.
Zhonya's Hourglass- Must have item for comps that contain assassins such as zed.
Morellonomicon- Good pick up for any healing comps. (Yummi/ Mundo/ Seraphine/ etc).
Rabadon's Deathcap- You can get this if you are ahead, or if you already have a kill lead yourself.. otherwise I would rather get Horizon Focus as it already provides a lot of AP. Though, it's much cheaper.

Flash+ Ignite pretty much take in almost every game as your root combos + Ignite can easily secure kills.

Warding is one of the most important aspects of the game! Since Zyra is pretty squishy, you do not want to wander around river alone. Though, you may if you have enough information from the map! Always keep an eye on the map and try to locate where the enemy Jungler is. If he's ganking top while the enemy mid is well.. is at mid while your lane is pushed. Go ahead and roam and try to get DEEP wards down! Below are some general hotspots to have wards to get early information on foot traffic.

One tip I always make a habit is to make sure to get 2X control wards each back no matter what!

Now to warding:

Early red side

Early blue side
(you have an option to put pink in pixel ward or in front of drag.. though they have a higher risk of being easily taken down

Drag red side

Drag blue side

Rift/ Baron red side

Rift/ Baron blue side

Thanks for taking a look <3