I'm not 30 yet, but I can tell you that as Zyra support, I go for the coin
for the edge in mobility and I play agressive denial of gold for enemy carries. As soon as Zyra hits level 2, you have the ability to throw a turret down on the other side of the minions, forcing MANY champs to back off or trade poorly.
If you hit 6 before they do (because you've gotten the kill or assist), then it might be over for them because your ult just destroys teams that lack mobility. they're going to lose a turret.
On the other hand, if your team is behind, you have unbelievable poke range and harassment tools. Yesterday in Hexakill when I and a carry were dominating a lane against 4, we saw that the bottom lane was feeding hard. despite pushing to turret without losing any ground, it was now 2 to 18 (and that wasn't my worst game believe it or not). I did my best to deny shrines and tried to come in and help (but you can't do anything if your allies charge right past your plants and chase right into the turrets). I called out in /all for the enemy team to come and end it once it got to 5 and 80, and showed them just how obnoxious Zyra can be under turret support. by the end of the game, I, as a support character, had 6 kills, each of them solo 100%-0 against their hardest fed champs. Zyra harasses strong enough that even the most tanky champs question whether they can dive for it.