Thought Experiment: Plant-Based Alien Intelligence

If there were sentient life on another planet in the universe, and said life were more akin to terrestrial plants than to terrestrial animals...

They might have several-hundred-year lifespans, like trees. This would make the notion of multi-year sublight voyages from world to world less off-putting.

They might have chlorophyll or a similar internal chemistry that requires little more than light and water to sustain life. This could greatly reduce the amount of supplies needed for long voyages, reducing the economic burden and environmental risks associated with space travel.

If all of the above were true, they'd be a bit more likely than us to travel the void and visit new worlds.

If so, they might pass near earth, and in the process, they might observe us or our EM-frequency broadcasts.

In their initial observations, they'd likely see that we eat things. As chlorophyll-filled alien plants themselves, this would seem really shocking. They don't eat things, but we do. It's even possible that there homeworld might not have any animal-equivalents, or just have a relatively small number of animals, such as pollinating insects. Their only experience with predation might involve diseases. If so, we'd be about as creepy to them as flesh-eating bacteria is to us.

And so they might want to learn more about us before they introduce themselves. It'd be in their own best interests to figure out how much of a threat we pose. They might want to know if the plants and animals we feed upon are sentient. If any of our current food sources are self-aware and capable of independent thought, they might conclude that we would only see the alien plant people as food, too.

That'd be an interesting explanation for crop-circles, within the context of the thought experiment. In this hypothetical scenario, the circles are not a natural earthly phenomenon, and not a hoax, and not an attempt by aliens to communicate with humanity. Crop circles could be the attempts of the alien plants to communicate with terrestrial plantlife. Or, at least the really intricate ones where the stalks are bent without being broken could be. (Or, they could be the earthly plants attempts to communicate back, but that's probably a greater stretch of the imagination.)

We humans rely on beef, and feed huge quantities of plantlife to the cows. Perhaps aliens were trying to talk to the plants, when a cow came along and ate the stalk of grain who was the leadership of the field? For a plant-based alien lifeform who saw all life as precious and/or sentient, this could be a barbaric act that deserved punishment. Perhaps that was the first cattle mutilation? Perhaps there's some sicko unhinged alien plant out there, mutilating cattle as a vigilante protest against our earthly system? Perhaps the other alien plants are right now trying to figure out what to do with him? This might result in further reluctance to make themselves known to us. Cattle ranchers have a history of lynching those who rustle or poach their herd, so it's not exactly the best context to initiate diplomatic relations under.

Perhaps then our space-plants also abduct humans. Those who claim to have experienced alien abduction often describe various probings and sexual extractions. While this is a huge affront to most humans, it wouldn't necessarily be distressing to a plant/alien. If you're used to being pollinated by insects, the idea of a stranger (of another species) milking your pistil might be comforting, not shocking.