Flash is a must for obvious reasons...
You can choose between Ignite, Teleport, and Exhaust
I like ignite for securing kills early game but you can also run teleport if you plan on farming and scaling up to late game. Exhaust is also a good option if you are fighting an assassin like Diana, Zed, Fizz, Talon, etc...

Zyra's first item can vary depending on the match up. If you are fighting an AD assassin, it is wise to buy Zhonya's Hourglass for that slight tankiness during laning phase. Overall, Morellonomicon is an excellent item for laning phase since it lets Zyra spam her abilities with very minimal mana issues. However, Zyra's three core items are Rylai's, Liandry's, and Void staff. These three items let her scale amazingly into late game, since her plants will be shredding all champions tanky and squishy. When looking at Zyra's abilities and AP ratios, you are probably wondering how can she possibly be any good late game with her ****ty AP scaling, but trust me, when you buy the three core items, you will see how big of a threat she is in team fights.

Many people like to max E first since it lets you root the enemy for a longer period of time but I think Q is the best option since it is easier to land, costs less mana, and has a lower cooldown. You can poke your enemy laner more efficiently when maxing Q first.

Deathfire Touch might do more damage than Thunderlord's Decree late game, but I like to take Thunderlord's decree because it offers nice burst damage especially since it is very easy to proc thanks to her plants.

I don't think I have to explain the quints and marks to you guys. But I prefer scaling armor and mr runes because they gives Zyra some tankiness, when coupled with Liandry's and Rylais. Especially since Zyra's mr does not scale at all. Also, despite having low mobility, Zyra can poke and farm safely from a distance pre-level 6 so her squishiness early game should not be an issue. And once you hit level 6, scaling armors and mr are already outscaling flaat armor and mr runes.

I don't think I have to explain the quints and marks to you guys. But I prefer scaling armors and mr because it gives Zyra some tankiness, when coupled with Liandry's and Rylais. Especially since Zyra's mr does not scale at all. Also, despite having low mobility, Zyra can poke and farm safely from a distance pre-level 6 so her squishiness early game should not be an issue. And once you hit level 6, scaling armors and mr are already outscaling flaat armor and mr runes