Only if there are already plants in the area of effect.
Plant growth doesn't create plants, it just amplifies the growth of existing, non-magical plants. As such, if cast on plants floating in the air, it would create the described "sphere of floating plants". This might be plants that are growing on an earthmote, plants growing in an air pocket in Wildspace, plants that generate lighter-than-air gasses somehow, etc. If you cast it on a point of space where these sorts of plants exist, you would get the effect you're asking for.
Alternately, you could cast it on a point in the air such that some ground-based plants were caught in the area of effect, so that they will grow and fill the 100-foot-radius sphere.
Once you have some suitable plants within the area of effect, they will grow to fill the entire area of effect:
A creature moving through the area [the 100-foot-radius sphere] must spend 4 feet of movement for every 1 foot it moves.
This movement penalty applies to the entire area, and it is caused by the overgrown plants.
As for the duration of the spell, it's listed as instantaneous - the plants will grow to fill the space, and then nature will take its course, depending on the nature of the plants. If you create a two-hundred-foot-diameter aerial sphere that grew from a five-foot patch of grass where it touched the ground, it's probably going to promptly collapse to the ground and die.