"spores casing"
Spore casings (with "spore" in the singular). Strictly speaking, "capsules", "packets", or even "cases" would be a better word. I figured the spore packets of bird's nest fungi would be the best comparison for structures that were specifically loaded with spores, rather than seeds that rattled around inside something else.
Keep in mind that spores aren't simply seeds. They are much smaller.
100 micrometers is apparently on the larger end for a fern spore, and that's 0.1 millimeters. Meanwhile, a quick check suggests the
]smallest plant seed is 0.05 mm, which is five times bigger.
Therefore, you'd have to specify it's the packets that are as big as a mustard seed (1 mm), not the spores themselves.
The shoot doesn't necessarily need to detach once nutrients are depleted. When grass or other spreading plants like strawberries colonize new spots, they don't necessarily discard the part of itself in the previous spot, even if it depleted nutrients there. Why would they? The new growth can send nutrients to the old growth, and the old growth can continue to photosynthesize. I figure you have it discard shoots rapidly as it grows underground to suggest the plant is (over long time scales) not simply growing but "marching" to more fertile areas. A compromise is making the organism as a whole perennial, but the individual shoots annuals, and unusually short-lived annuals at that. (e.g., dying half a month before most other large flora in its environment dies or goes dormant) As the shoots die, the nutrients would be redistributed to the tuber, and the shoot becomes so dried-up and fragile as to very easily detach, even in a moderately-strong breeze. On a year-by-year basis, it would appear to "march" in a particular direction.
It doesn't make sense for the shoot to detach if it gets eaten, unless its defense mechanism is, in fact, lodging in a would-be predator's mouth, and detaching most of the shoot makes this defense more effective.
For future flora submissions, I recommend starting with a concept that's less ambitious or alien, so you can practice non-conceptual aspects of organism submissions. You could adapt an obscure real-life plant, for example, or try to merge multiple real-life plants with similar niches into one concept. To learn about interesting plants, I recommend the online resource
Wayne's Word, (excuse the circa 2004-esque look), or David Attenborough's
The Private Life of Plants.
Conveniently, The Private Life of Plants is available to digitally borrow on the Internet Archive.
P.S. Remember: put the quote you're responding to at the top, not the bottom. That reminds people of what, exactly, you are responding to before you respond to it.