1. The amount of health they have rounds up. 4 + 10 / 20 / 30 / 40 / 50% equates to 5 / 5 / 6 / 6 / 6. There is no reason to improve the ability after rank 3.
2. The passive is worded in an unreadable way. Can't it just say "Increases the health of Zyra's plant by 1 / 1 / 2 / 2 / 2, for a total of 5 / 5 / 6 / 6 / 6"?
3. The full value of the passive is achieved at rank 1, with only an increase versus minions at rank 3 and nothing else at any other rank. Plants take 1 / 2 / 3 / 4 damage (minions/monsters; champion attacks; champion AOE; champion non-AOE and Dragon); meaning at 4 health it takes 2 hits from champion attacks/aoe and 1 hit from dragon/single target spells. The increase to 5 health at rank 1 increases this to 3 champion basic attacks and 2 dragon/single target. The increase to 6 at rank 3 only helps versus minions.
* Change the base health to 3 and make the passive grant + 1 / 1 / 2 / 2 / 3. This solves the max-rank incentive, and also nerfs the amount of health she has available at level 1. It also requires rank 3 to for her plants to start tanking multiple dragon attacks. (Did you intend for her to be jungle so easily?) This does not address the fact the passive is only useful versus minions.
* Change their health to 3 + 1 / 2 / 3 / 4 / 5 or 4 + 0 / 1 / 2 / 3 / 4 (she probably doesn't need the rank-1 improvement given it already grants her on-demand seeds), for a maximum of 8 health at rank 5. This solves the max-rank incentive, as well as the incentive versus champions. 2 / 2 / 3 / 3 / 4 / 4 attacks or 2 / 2 / 2 / 2 / 3 / 3 abilities to kill. There's no max-rank incentive versus champions, but it's an improvement to the current passive.
* Change the base health to 3 and make Stranglethorns passively grants + 1 / 2 / 3 bonus plant health, for a maximum of 6. For Rampant Growth, it now grants the passive: "Zyra's plants last for an additional 1 / 2 / 3 / 4 / 5 seconds.", or whatever scaling you like, and adjust the base duration to your taste. The passive could also reduce the cooldown on Garden of Thorns, instead of it scaling with level. That said, tying an increase in plant health that only really impacts minions to her ultimate seems weird, the above solution is probably better.
* Flatten her plant's health or have it scale with ranks in Stranglethorns. The damage of Zyra's plants now scales with ranks in W. This allows Zyra players to choose if they would like higher burst or higher DPS, at least early-mid game. As with Cassiopeia, you could keep the per-level and have ranks in W simply add a bonus value.
* If W grants bonus damage, Stranglethorns could instead empower plants with invulnerability.
* Stranglethorns could leave behind a thicket for an extended duration beyond the 2 seconds, retaining Zyra's zone control power by giving her a persistent zone that she's strong in for a while. (Her innate and W promote far too much of a strong pusher - constantly moving her pieces forward, which isn't a bad theme, but her theme has always been "this land is mine".)
P.S. As a complete aside, can you disable her ability to take W at level 1? Now that her seeds do not grant sight, there is no reason for her to have access at level 1.