If you've ever played Heimerdinger mid, Zyra plays extremely similarly. However, Zyra's low cooldowns, low mana costs, and high damage make Zyra a (situationally) better and more fun pick.

Your main goal is to shove the enemy laner into tower. Her plants, combined with AoE Q & E abilities give you better wave clear than most Mid Laners. Zyra has surprising damage when you build her full AP.

Zyra's strongest suit is her survivability. Similar to Heimer, she can easily survive ganks, and many times even get one or two kills if the enemy jungler ganks. Her zoning control and long range plant auto attacks make it extremely difficult to engage on her, and getting close within Zyra's plant garden is always a bad idea. Her huge ultimate allows her to create a large damage field, and all plants within her ult are doubled in power, further allowing her to get a kill or two while dealing with ganks.

Zyra plays similarly to Heimerdinger but is not necessarily a substitute. She offers much more utility than Heimerdinger, as she's most commonly a support. She has long range, optimal zoning control, and great crowd control. Building Hextech GLP and Rhylai's Crystal Scepter allow her to kite, engage, or disengage with ease.

Zyra is best suited for a team that lacks Magic Damage and/or utility. Zyra mid is a great option if your support is someone like Soraka or Sona, or even Blitzcrank. Zyra is also a great choice if you have a hyper ADC such as Caitlyn/Jinx, an AD toplaner like Fiora/Darius, and an AD jungler like Jarvan/Lee Sin. She offers a huge amount of magic damage and a great amount of consistent zoning and crowd control.

Zyra mid, at least as I would describe her, is a fun alternative to champs like Ziggs and Heimer, that excel in pushing and poke. She's great into squishier mages but can struggle into an assassin.

Blind picking Zyra can be scary in the case that the enemy picks an assassin like Fizz or Zed, however most people would assume that a blind pick Zyra is going support. So, they may just pick a squishy mage that Zyra has no problem with.

Zyra's best strength is her ability to survive, or even triumph enemy ganks. If your getting ganked, simply place all your plants close to you, throw out your E near the enemy jungler, and if you must, use your ultimate. Even if Zyra is extremely vulnerable, it is NEVER a good idea to enter her garden of plants, and using her ultimate atop her plants will double the strength of them. You can usually count on 1. getting a kill, 2. getting 2 kills, or 3. surviving when an enemy jungler ganks you.

Zyra isn't a good roamer. It is always a great idea to roam on a champion that hard-pushes. So, if your lane is stagnant, the waves are fully pushed in, or your laner is roaming, it's a good idea to roam as well. She doesn't have very high mobility so a roam from Zyra can seem sort of slow or end up not being the most useful, but being present in a ranked game is the best thing to do. And if, say the enemy bot lane has no vision, a Zyra roam from behind (tri-bush or river bush if the enemy is fully pushed) can be deadly.