When it comes to PvP you have to know your champion, lane, and purpose. Many have failed to recognize most of these things. They go in expecting their ganking and aggressiveness to win matches. With Zyra there is no other way to play her than carefully. She is so dangerous that you should be dead scared of her. Funny ting is most people either underestimate her or lack the knowledge to play her. I have found that when I go into a match and run mid lane. That my team often thinks they have to help me push. It is likely that the jungler will show up to try and eliminate my competition. Which more often then not just scares them away. I have never gotten a kill early game because the jungler tried to help. So with that being said here is how you play Zyra on mid lane effectively.
1: Know your build, she is all about ability power. Don’t forget wards in the river, better to know before it is too late to act.
2: Your plants are the most dangerous thing in your arsenal, make sure they get to lvl 5 fast.
3: When going for a champion kill, make sure you are lvl 6 and that they can’t get by the turret before the plants get a chance to do the damage. With your ultimate available make sure you set your ult before the plants and then drop all the plants you can. Then use your vines to trap the champion. Giving your pets that extra few seconds means lights out for the enemy. This tactic is hard to beat unless you have flash then they might escape. However in most cases if you plan ahead. You can eliminate flash by using the trap and plant tactic. Many will panic and flash away wasting their flash early and giving you the advantage.
4: Remember that turrets will aim for the plant immediately and kill it so keep your plants out of turret range. However you can use the plant for a final shot beyond a turret which is effective at killing the running champion.