My wife and I have been playing Sims 3 for quite some time and one of the only challenges we have yet to be able to complete is to have a perfect quality omni plant. The best we've managed has been an excellent one and even that seemed to be random. Ordinarily to accomplish this we would simply plant something harvested from the plant in question. Since a plant will usually produce some harvestables that are one quality level higher than the plant itself and a plant grown from a harvestable often grows into a plant that is yet another level higher, it usually only takes about 3 to 4 generations to get a perfect plant. Unfortunately, because of the nature of the omni plant, it is the only one that you can't do this with, because you can't plant anything harvested from the omni plant and have it grow into another omni plant. Can anyone tell me how you can get or work your way up to a perfect quality omni plant?
From various sources, it seems that it is impossible to improve the quality of the plant.
However, it is possible to receive seeds of "outstanding" quality when completing the Opportunity The Omnificent Plant. That would be the only way to get a perfect Omniplant.
A green thumb sim who talks to a plant has a tiny chance of boosting it's quality a level per conversation. Also Super Green Thumb LTR automatically raises the quality of any seed you plant (so an outstanding Omniplant seed would result in a perfect plant automatically).
Also if you have the premium content Greenhouse set, the growing station thing can also let any sim talk to the plants in it's area of effect and it's a very brief and easy to spam action, making it far easier to get the quality boost, where as the Green Thumb trait conversation takes nearly an hour per attempt.