Plants can be set to decorative mode so they don't need tending at stage nine. If the plant produces seeds or resources those take an additional eight days of growing time.
To set a plant to decorative mode, double-click the plant to get it's tending window, click the flower button at the top of the left side column of buttons to change the window, then click the upper right button on the new window and then click the check mark on the new window that appears. Your plant is now done growing and requires no tending. If you do this while it has seeds or resources on it, those are lost, so empty the plant of anything you want before setting it to deco.