The water in the plant would immediately begin to boil (not because of heat, but because of the lack of air pressure in the vacuum of space), probably rupturing the cell walls of most cells I would think.
So to answer your question would probably be: The plant could continue to grow in space for a few fractions of a second.
The pot of petunias are dead on impact
Oh, not again...
Hard to say. For a small plant with low water content, I could imagine that it would be effectively cryogenically preserved, meaning indefinitely. An aloe or other succulent, for example, would freeze and die. But maybe a stem cutting or woody plant might survive. Or a moss or lichen (though lichen aren't plants). Assuming it doesn't get baked by unfiltered sunlight or destroyed by high-energy radiation.
Things don't freeze in space. It's actually very hard to lose heat in space because a vacuum is a very good insulator. If it's in direct sunlight it'll get hot.
Also to freeze you need moisture, which typically boils off in a vacuum.
It depends on the plant, moss apparently is pretty hardy. https://www.cell.com/iscience/fulltext/S2589-0042(25)02088-7
I don't have any real expertise here. But my bet is that it would die pretty much instantly as all the water in its cells boiled off and burst all the cell walls.
Cells, especially plant cells can withstand much more than 1 Bar of pressure difference. A plant will not start to boil in a vacuum, contrary to other comments. Every land organism has gone though a great deal of evolution to make sure it doesn't loose to much (or any) water though evaporation on its surface.
However plants require on pressure and evaporation to pull water from the roots to the leaves. It will loose its water through its stromata in the leaves a loot more than normal. If the roots are in water it will be ok, just dry out the ground realy quickly.
Most likely the leaves will die fairly quickly, followed by the roots. The cambium of a woody plant will probably not even notice the vacuum for a few months. Just like in winter it will basically hibernate beneath the bark and form new sprouts once it is in atmosphere again.
You will probably have to cut away some dead wood to expose the living cambium and initiate regrowth of roots and new sprouts but a tree will be able to regrow after being in a vacuum for a (relatively) long ass time. Think of a tree stump getting new sprouts around the outer edge a year after it was cut down.
No idea about radiation and heat though. If it is floating in direkt sunlight its probably fucked after a few days.
You left out the temperature,though. Regular space temperature would kill any cells very very fast and the temperature change would do the rest.
A vacuum is a great insulator, though. It may well cool slowly enough to go dormant and eventually vitrify, if it's something cold-adapted.
Somebody with a vacuum chamber please put a plant in it for science
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