Is there a standard way of handling this sort of thing that I missed somewhere?
I think generally, it's the core object that gets the designation. Offshoots like the -1 sub-designations you've got here, if they're destroyed soon after manifestation or otherwise not kept in containment, don't factor into object class.
- Containment procedures are too long—you've got a ton of material that isn't related to containment at all, but rather is applicable to experimental testing (which is not "how to keep thing locked up").
- I recommend putting that stuff in another section, and also trimming it down for better readability. Instead of going on about "talk to [x person] to ensure that [x] is in place", you can probably just say "follow Protocol SCP-XXXX-##" or something for the fluff text. People assume that the Foundation personnel running the experiments know to first get proper permissions/authorizations, excessive detail about that stuff is probably going to bore a reader.
- Hyphen between Site and 77. So Site-77.
- You've got some casual language that doesn't really fit clinical tone. Something like "take SCP-XXXX out for testing" is more colloquial than professional.
- Last two paragraphs of containment have sentence structure that is repetitive to the point of boring. (Reminiscent of storytelling that's just "and then… and then… and then…") Consider putting all that stuff in an addendum as a test log.
- Why are all those measurements so exact? Will something cataclysmic happen if the measurements are a little off? If not, no need to specify them.
- Description is a little clunky in places, wording-wise. Remember, scientific reports should be succinct and verifiable using evidence collected from experimentation and observation.
- "If SCP-XXXX is used to plow a field, whatever seeds are sown" —> what if no seeds are sown? What if it's just used to plow a small plot of land? What if it's not a field that's being plowed, but a hill with terraces?
- kudzu-like plant —> might want to give a species/Latin name for this comparison.
- "Each pod takes up roughly 1 square meter of SCP-XXXX-1's surface area." seems like an odd detail. Did someone actually measure the surface area to determine this? Are the pods fused to the climbing parts?
- "Flesh," in this instance, refers to muscle tissue and internal organs; skin, bone, and cartilage are not digested. —> can be put in footnote. Seems to break the flow as it is currently.
- "If it is attacked or has not consumed living flesh in the last 4 to 6 hours, SCP-XXXX-2 will enter its agitated state" —> EHHHHH. The whole "RAEG STATE" effect was outplayed a couple years ago, and tends to be something people laugh at more than find interesting nowadays. Besides, the whole "but if you make it mad/angry/scared/sad, it messes you up" addition tends to be seen as tacked-on for danger aspect if it doesn't really relate too much to the concept. SCPs don't need to be dangerous to be successful.
Overall… it's not too interesting. It's a plow that when used to plow, seeds planted in the furrows grow into plant-men that eat flesh and want to kill things under certain circumstances. Reminds me vaguely of Jason and the dragon's teeth, but like… without the same sort of overarching narrative. Why this plow? Why venus flytrap humanoids? What purpose were they supposed to serve, if any at all? Why does this thing exist?
I feel like there could be something cool here, if the reader is given more than just the the "it makes monsters" approach.