I see most descriptions of CrHe spells make plants grow from wood, fertile soil or other plants. So, can't you Conjure the Sturdy Vine or create a Wall of Living Wood out of nothing? Didn't find anything on this topic at the Herbam sections of the book or searching forums, etc.
I see Bridge of Wood doesn't mention anything about fertile earth, so I guess it's not a requirement, but I'm asking just in case I missed something.
Thank you in advance for any answer/clarification.
I believe the difference is whether you are talking living plants or dead. Living plants require soil or a base plant. Dead plant material does not.
Wall of Living Wood is made of living plants, but maybe they forgot to state the "fertile soil" thing; all the other spells have that line.
Anyway I'm not sure I like this being a requisite (if it really is) for creating living plants, since you can create fishes out of water or light from nowhere, etc. I may be convinced otherwise, since it's actually a cool requisite in terms of setting.
My instinctive reply is to point at Auram. Creating weather effects (like lightning) gets a lot more difficult when doing it in unnatural circumstances (like indoors). But my book isn't with me, else I'd probably tangent into all sorts of ideas from there.
And here is my "I may be convinced". 
the thing is fish can be alive out of water. Not for long, admittedly, with a few exceptions, but it can happen. While plants can in fact live outside of dirt aereoponic technology was hardly common knowledge in the 13th century.
Well, like fish out of water, plants can live out of dirt for a short time In fact, for a rather longer time than fish!
This has been well known to farmers throughout the world for many millennia; forget aeroponics.
Would a field be enough of an "item" to be opened for enchantment?
I know a plant can survive outside of dirt, how well this is known to farmers in the 13th century I am not so certain about- transplanting a plant generally involves digging up a portion of the soil they are in, and most plant products are considered dead once cut or removed. Removing a plant from soil and replanting it wasn't something people generally tried doing... the world being round was probably more general knowledge than this.
Actually, not only removing plants from soil and replanting them was done by farmers a lot (e.g. with young trees) since antiquity, but anyone with any experience with fruit plants would know you can cut a "limb" from a plant and graft it to another plant so that it then lives and bears fruit. This was such commonly held knowledge throughout the world that it's used as a metaphor even in the Bible (see e.g. Romans 11:17), hardly a manual on agriculture.
Cool. Didn't know that one. There is an earlier book focused more on agriculture: Historia Plantarum by Theophrastus around 300 BC. I don't know much about it other than transplanting trees was sometimes fairly regular. It does mention some trees growing from roots. I would suppose the book would be more accurate in Mythic Europe than in Medieval Europe.
Definitely, considering that the laws of nature in Mythic Europe are "Aristotelian", and that Tyrtamus was probably the favourite student of Aristotle - who nicknamed him Teophrastos in homage to his eloquence (Communication +4 and Good Teacher?) and in his will named him his successor at the Lyceum (and guardian of his children, and heir to his library). And plants were Teophrastos' favourite research topic, almost an obsession according to historians; the great Linnaeus himself named him "the father of botany".
It's interesting to note that in 1220 his two great works on plants, "Plant history" (in the same sense of "natural history") in ten books, and "The causes of plants" in eight are still only known in greek, and will not be translated into latin until the end of the 15th century. But perhaps they are the source of Bonisagus' understanding of Herbam?