I am sorry, but I am posting from my smurf account that does not have zyra, but I do play her a lot.
's burst is all in her timing. Mostly in the E+Ult combo rather than Q+seed combo.
The basics of
is the both the Q and E have a, what I like to call, Appearance Time - The time in which it takes for a spell to show up and do damage. Because the Seeds have no cast time and no Appearance Time (or a very short one) the Basic Combos are:
1. Q then IMMEDIATELY press W on THE EXACT SAME SPOT as the Q was. By the time it comes up, it will come up with a plant and also do one attack.
2. E then IMMEDIATELY press W SLIGHTLY AHEAD of the DIRECTION of the E. The E takes time to travel (Appearance Time), but if you time it right the snare will hit and then the plant will attack, applying a slow.
Her burst is any combination of the above. Add Ult for more burst as needed.
Mid: Use combo 1 to harass the enemy laner while farming, use combo 2 to escape or engage (with jungler)
Support: Use Combo 1 either ON TOP OF the enemy ADC or BEHIND the Melee minions (Caster minions is better, but WAY risker). Use combo 2 to engage; use Ult to Disengage. (Combo 2 can be used to disengage as well)