Your plants look awesome.....
They are still small though ....plant the small corm a couple inches below grade in well amended and drained soil.
Here is a thread which you might find of interest
.
Hibernating a basjoo
This is just another option that you can use overwinter your plants, but do what you are comfortable with.
Generally you cannot go wrong; I am in zone 5 and many gardeners/professionals do this, however if you read the thread it's your choice.
The real key point is to plant the plant in well drained amended/with organics, soil... a million dollar hole for a ten dollar plant.
In the springtime feed the plant during heavy growth.
Use a fert that you can afford; I use whatever I have on hand.
16-16-16-....19-19-19... Miracle Grow, you are growing this plant for size
.
Amend the soil with anything organic .....lots of it..I use a lot of byproduct from a couple of tree services........wood chips, leaves.
Untitled by
Hostafarian,
on Flickr
Spring Basjoo growth by
Hostafarian,
on Flickr