Hello from an amaranthus junkie,

I think your amaranthus will be fine planted by the roots. Amaranthus are very easy to grow and very hardy but Ive found they self seed tremendously. I would suggest not putting them anywhere that you didnt want a bunch the following year. I was very interested to learn amaranthus leaves are edible and the seed can be used as a grain although it is said to be much grittier than other grains.
I personally love the different varieties of foilage.

Enjoy and happy gardening.

Lookingglassgardengirl