Man, I love this site. I get answers to questions I didn't even know I had! I have one of those in an area I haven't tamed yet. Thanks for identifying it, now I won't pull it out. Thanks!

Now I'm puzzled as to why you asked for an id (using a photo dated 2015) if you already knew its name and had posted about it before? Your plant hasn't got typical Ixora foliage which is why I didn't recognise it. I'm thinking it could be Ixora 'Siam Ribbon' which has curled and distorted foliage. Reading up on its requirements I doubt it will flower in a terrarium as it likes full sun. Siam Ribbon

It is not knotweed......and it does look like a tree seedling (root shoot), what type I am not sure. But you will get faster and accurate ID's if you use the forum - Name That Plant - that is intended for plant identification. This is not the right place :-)

Looks like a California poppy seedling to me, Eschscholtzia californica.