You only need to have seen the plant
The description for Scrying reads
You can see and hear a particular creature you choose that is on the same plane of existence as you [...] the spell creates an invisible sensor within 10 feet of the target. You can see and hear through the sensor as if you were there
There is no difference built into this or other features that allow different sight between in-person seeing and the sight granted by the spell/effect
Transport via Plants reads:
You must have seen or touched the destination plant at least once before.
Again there is no distinction drawn between in-person seeing and seeing by another means.
The Scrying spell allows you to see the plant and that is all that is required for Transport via Plants to work. This is a good example of how powerful Scrying can be and how spells can be used creatively.