Answer: If you started with small plants from the greenhouse and took very good care of them, you might be able to grow spider plants fast enough to satisfy the requirement. (These are commonly available and a nursery or friendly house plant enthusiast might be willing to let you have babies from a mother plant.) Philodendron is also a fairly fast grower.
If you need to start plants from seed in 6 to 8 weeks, marigolds grow very quickly as do lettuce plants, and both could be grown easily under fluorescent lights, but neither is strictly a houseplant.
Coleus grows nicely from seed started at a warm temperature and is sometimes grown as a houseplant, so that might be a good choice, too. Good luck with your project!
Q&A Library Searching Tips