Honestly, I think she's a little OP.
Q: The AP ratio on the damage is fine, but drop the base armor lowered towards the end. Make it, like, 5/10/15/20/25 (+1 for every 75/100 APM) armor for 2/2.25/2.5/2.75/3 seconds. CD of 10. Otherwise, it's even better than Sona's W's buff. Or halve the amounts and durations and allow for MR as well. (Since she does Magic Damage)
W: The DOT is fine, but the period after leaving the brush is too long. Make it last only 2 seconds after leaving. (Even Cass's poison doesn't last that long.) The snare is also too long, .5/.75/1/1.25/1.5 is a good amount, considering most CC doesn't last longer than 3 seconds and especially not AoE CC. CD is fine, but the slow might be a little much. (Swain's AoE snare is 2 seconds, and he's an AP carry.)
E: Really OP. Drop the stacks to 2 per ally, and let her charge the casts instead of have a CD timer to make it more fun and promote not-dying, with a 1-2 second CD between stack casts. Like Teemo's shrooms. But on a 10-15 second timer. Stacks don't add up while dead and on revive have 2 stacks. The AD boost is a little high on the length of time, it should be decreased to something like 3-4 seconds. (Soraka's heal is 4 seconds for her armor.) And the cool down is too low. At max level it should be 10 seconds. (Since it heals for so much and can be stacked, this is making it better than Soraka's heal on CD.)
R: The percentage per AP is too high for the amount of AP. Make it 1% per 100 or 0.5% per 50. You can decrease the armor boost by 5 per herb and make it for MR as well, something I see her missing on this list and that her allies will need. The duration is also too long. It should be 4/4.5/5. If that. The mana cost could be higher, I'd say at 250 or 300 at all levels or 200/250/300. And the CD is too long, 150/125/100 would be good times.