Weedweave
This is the High Fantasy equivalent of plastic; its name is fairly literal, as the first step in the weedweaving process does involve weaving together plant stalks into a hollow structure of the desired shape. The mage then imbues the structure with magical energy, which permanently transforms it into something reasonably light, flexible, waterproof, fireproof, and non-conductive. Assume that a weedweave item is about as strong as hard plastic, and that a weedweave garment is about as supple as thick cloth.
The advantages to weedweave are obvious, and include the fact that a mage can repair damage done to a weedweave item by patching breaks or tears with more weeds and magical energy. By the same token: weedweave is peculiarly vulnerable to spellcraft. This very much includes plant-based magic; plant spirits in particular find it almost trivially easy to possess weedweave items, and once they’re in one it can be decidedly difficult to yank them out later.
There are many ways to deal with this problem, starting with not treating it as a problem at all as long as the spirit is friendly, or at least neutral. Some forest mages even try to entice particular spirits into their weedweave items; officially, the mages attract spirits with natures complementary to the item in question, but they’ll all happily settle for ‘placid and sleepy’ instead. And as for those annoying, but not-quite-intolerably dangerous, spirits? …Well. There’s always wandering adventuring parties with more money than sense, yes?