>

This is actually really thought provoking. The answer is that we will never know, because we will never experience planthood first hand. We also can’t communicate with them either. Without brains they aren’t likely to be equipped with the same gear that MAKES us sentient, but it’s still an interesting thought.
There is a book I once read called out of my mind, about a girl who is disabled and cannot speak, yet is highly intelligent. At some point she was given a computer thing so she was able to communicate and people could finally realize what a prodigy she is, but it’s amazing how these people then recognized all the times they misunderstood and assumed she was dumb because she couldn’t communicate. Without the ability to communicate, we can’t prove that someone or something is intelligent. However, without firsthand experience we cannot prove that they aren’t intelligent/sentient.
How the fuck can an organism be sentient if it doesn't even have a brain? >

Um they have brains >

I mean, this is either factually true or not, I dont see how this can be a matter of opinion. >

As long as it's neither proven nor disproven, it's a matter of opinion. 
It is a wrong view on science to think that it's only about facts. It's about interpretation of facts. Some facts might suggest one interpretation and some another. 
There might be facts in OPs favor but the majority is against it. Still, it's not proven wrong just very unlikely and therefore an unpopular opinion. >


The same amount of evidence exists to suggest that humans are sentient as there exists to show that plants are

Do you want to provide any of this evidence? >

I didn't know they had feelings! I need to eat more vegetables now. >

You could probably make an argument that plants meet the standard for sentience by some measure.  But I’ve never seen it demonstrated that plants have emotions. Are they afraid of death or being hurt?  Possibly. But if so, they have not communicated that in any way which humans can understand.