I remember a streaming service called curiosity stream being the only one that offered Invisible nature, which is a series about plants. It's 3 episodes if memory serves me right, albeit a bit old and poor quality. Oak tree: natures greatest survivor contains some pretty neat stats about how much foliage a large tree produces, how branches develop and stuff like that, but it's superficial. Secret Life underground is not that good, it too scratches the surface but that's about it.
Most of the documentaries I've found are terribly outdated.
Even BBC Life episode 9, titled Plants, was kind of a let down. Especially if you watch the making of; it shows why all those plants look so photoshopped.. It's because they are..
It's weird when you think about it, there are at least 90 film crews in the African Savannah at all times, to capture big cats that are played back to back on natgeo and discovery (if they even broadcast nature shows). There's nothing they haven't taped, there's nothing they haven't seen, there's nothing about those animals that isn't documented. Yet they're still there, full time.
But filming plants, basically setting up a camera and checking it weekly, is almost too much to ask. Even for the BBC.
If every documentary covers nephentes, venus fly traps, drosera, baobabs, cacti, the trembling giant / pando and some bristlecones, then you've seen it all after a single watch.
Sorry man, we're just fans of organisms everybody likes to watch but nobody wants to film.