Gold 4 support Zyra main here. http://www.lolking.net/summoner/na/21897854#profile
Oh Damn. All this time and I finally get to talk about my plant bitch fetishes.
Zyra is a mage. Plain and simple. She has incredibly low move speed AND health pool (like bottom 3 in the game I think.) But the bitch does them deeps.
Runes wise I don't try and cover her weaknesses, instead I amp her strengths. Hybrid pen reds/armor yellow/ap blues and Quints. 9/0/21 is my usual mastery set up. I always run exhaust on my supports. Just my own habit. Personal preference. Start E-->W-->Q before maxing E first then Q and finally W, R when able.
Her power spikes are at level 2, 3, 5, 6 she has great pressure before then gaining her power spikes almost strictly on items. Those being sorcerer shoes/haunting guise then later once I've finished liandrys and finally at rylais. Yes, those are my support items.
When in lane getting lvl 2 is important. Her incredible squishyness and lack of escape means that you need to play aggressive other wise you're just going to wilt (ha!) under harass if you don't gain the upper hand early. Remember, you automatically gain 1 seed when you level W the first time. Landing a level 2 E/W can win lanes instantly, truly.
Being aggressive means you need to ward heavily because of likely ganks and the before mentioned lack of escape. If you don't see the help coming, it's either death or flash usually. By being pushed on her (do to stray spell dmg and uncontrollable plant attacks on minions will often cause this) you usually won't be warding lane brush, so trinket wards can focus in tri and river brush.
Thankfully plants are amazing, seeds too. Seeds have a further cast range then wards do, give vision briefly upon being cast in brush before staying put for 30 sec and revealing anyone who walks over them. These are great for when you have gaps in your trinket cool down and are still pushed for, particularly, tri brush or other areas.
Never KEEP 2 seeds on you for long. Sometimes you won't have an angle to throw an E or land a Q and your seeds will build up. If this is happening be sure to drop extra seeds in various brush and get that next seed CD going.
When fighting tank supports in lane your mission is to chip them out so bad that they can't afford to dive off a hook or something. Force their pots early and assert yourself.
When at level 3 and beyond. If you have 2 seeds available, work on getting both types out. Hit the snare with seed 1 before following up with the Q and seed 2. The ranged dmg can get another 2-3 shots on a retreating target which really adds up early in games.
First 3 items will be sorcerer haunting guise, sightstone and sorcerer shoes depending on how you're doing. The more ahead you are, rush the guise. The extra hp on her should not be overlooked as well as the dmg is brings. Keep in my, if you are ahead (kills are frequent on her, it's ok to feed it) and choose to rush the spell pen portion of the build you MUST STILL BUY WARDS you are the support here. Vision is big and if you decide to snowball your lead aggressively then green wards should be in your inventory too.
Her plants proc liandrys, people aren't used to the extra dmg and no one respects the plants dmg. It's wonderful. The rylais helps with her inherent squishyness and allows her to actually survive initials bursts in fights and lets her ranged plants to slow as well, increasing not only her dmg from liandrys but her cc too.
Her ultimate rocks and for as much damage as I rave about dmg, her R can be used to reset fights and disengage as well as break people in half if you're ahead. Not to mention that it heals all plants back to full while giving them an att spd steroids. Yes please!
The big nerf to her was the W passive CD reduction going from 20% to 10% some time back which hurt quite a bit. Crucible is very good item on her to but I tend to get it later in games.
I'm rambling like a jackass at this point and am a bit all over the place. Please follow up if I Sucked at explaining or if you'd like any more specific thoughts.
Cheers Yoshi