Grows fast plus fits in a terrarium? Anything that grows fast will likely outgrow a terrarium.

How about something like plain ol' grass or wheat (if you purchase wheat straw at the hardware store it usually has viable seed in it)? Things that grow in woodlands aren't necessarily fast growing things (except for weeds!).

This time of year you can get spring bulbs (crocus, daffodils). Given the warmth/moisture of the terrarium they should sprout and grow.

Anyway, just some ideas.

Oh sorry, when I said "start growing pretty fast after planting," I meant that it wouldn't take too long to sprout. Outgrowing the terrarium is not too much of a problem because I will measure the growth in a specific amount of time, for example three weeks, so by then it should not have grown too much.

My project necessitates the need for plants of the Woodland or the Prairie biome, because these are supposedly the most drastically affected by CO2. I will definitely look into your suggestions, though, and do some research. Prairie grasses should do the trick.

Thank you!

Relatively fast growing trees include sassafras trees and tulip trees.

If you cold stratify the prairie grass seed for 60 days, it will sprout within days of being in a warm moist environment.

You can probably buy prairie grass seed already cold stratified from a reputable native plant nursery like "Prairie Nursery" or "Ion Exchange".

Really good suggestion, thank you