Rego Herbam Level 10 states that one can "make a plant or thing made of plant products move with purpose and intelligence, without requiring your constant control"
However, I don't know exactly how much "intelligence" that it moves with. There are no sample spells in the core book that interpret the guideline. If I animate a tree with this affect with duration sun, can I direct it to guard a house against anyone who was not present at the casting of the spell and have it do so? What are the limits of the animated plant's senses? What can it recognize? Can it make decisions? Can it show any problem solving, like knocking a sword from the hand of a person it is directed to attack?
Very little intelligence. It can follow a single instruction.
An example of this, in the core book, is The Treacherous Spear. For the duration of the spell, the target object attacks its holder until it is destroyed, the holder is dead (although how would the object knows that is beyond me) or the spell expires.
There is a spell in another sourcebook that uses the same guideline but which stretches it a bit farther. It is Carved Assassin from Tales of Mythic Europe. That spell allows the caster of the spell to direct a wooden statue to perform simple tasks without supervision, such as carrying stuff and guarding doors.
There is no official answer, to my knowledge. You can decide whatever you want.
Arthur's advice is sound - err on the side of caution, and grant the plant weak intelligence. I can see specific, Ritual, spells or Mystery rituals creating more intelligent plant-creatures, but that's another matter.
Edit: I'd generally consider a plant-spirit to have Cunning rather than Intelligence, and would advise to treat the plant as such: enough "intelligence" to understand basic commands, much like a trained-dog, but not more than that.
In regards to its perception, I'd try to come up with what a plant would "feel", which would be very different from what a person would see. I'd allow it to sense clearly the wind and movement on the ground, using metaphors and analogies from the seasons and plant's growth and disease and life cycle when describing it to the player. Its sense of sight would be much more restricted, probably only "seeing" what's close to it and not understanding concepts such as "armor".
Thank you very much to both of you for addressing my question.
I overlooked the Treacherous Spear as an example of this base affect. What I eventually had in mind was to create wooden servitors who would guard a forest. I don't think the kind of limited intellect that the weapons demonstrate is enough to fulfill what I had in mind.
All that you say seems very sound. The only part that I don't grasp is the matter of wooden perceptions. The treacherous spears created by eponymous spell would not have eyes or ears, but seems to be able to fight its original wielder even though it lost direct contact with the wielder as though the weapon had eyes and could see them.
Finally, could one add a magnitude or research a stronger version of this base affect or something to achieve a wooden servant with a more "complex A.I.", if you will?
Whether you could, or not, would really be a saga-dependent answer. As would be the ways this could be done.
Some might simply require a higher Base (+1 magnitude or more), like you suggest. This could be explained as summoning a spirit of Herbam to inhabit the wooden servant for the duration of the spell.
Some might simply require a Mentem requisite, to represent the fact that the wooden servant has a limited form of human intellect to guide it. This would of course also add one or more magnitudes.
Some might suggest that a seperate spell is needed to bind a spirit to the wooden servant. And that such a spell runs the risk of letting a dangerous spirit take control of the wooden servant (much like what can happen with the CrCo spell The Shadow of Life Renewed).
Some might require the effect you describe to be a ritual spell.
Some might say that some or any of the above methods could be valid, but that the magus must first do some Original Research using the rules from HoH:TL. Meaning many seasons must be spent in the lab. Or a number of seasons must be spent trying to track down the result of such research by another magus.
It is a tricky question which test the limit of the guidelines.
In previous editions, there was no real guidelines, so you would use other similar spell to gage the level of a new spells. Now with guidelines and regular reference of the state of science/knowledge of 13th century to fit within the paradigm, some spells are getting streched.
As you pointed out, a spell granting even just some limited ability to an inanimated objet grant it by the same token some form of perception and some limited intelligence. Which normally should be handle by Imaginem and Mentem.
To come back to your initial question, maybe you can use ReCo animation spell as guideline to what degrees of autonomy you can expect. There are a few reanimation spells which can be use as reference.
Or "The Shadow of Human Life" (also CrIm(Me) 40, with a +6 Mentem requisite) from the Ars Magica corebook, p.144, for those who don't have MoH and/or who consider its magical effects less "authoritative" because of the many errors and inconsistencies in the book.