In addition to the other posts this is what I will add to help understand what's going on.
First lets approach the plant that has been in that container way too long.
The plant will need to be repotted with new/refreshed soilless mix and lowered in the pot about 3-5 inches to cover the corm once again.
When a plant is container grown the rhizome will rise/enlarge until it moves above the soil line causing potential issues over time......like now.
At this time you may also split the plant and repot the other in a matching pot
Lower your corm in the soilless mix to fully cover the rhizome, by doing this new viable roots/pups will soon form and improve the health of your plant.
You will need to do this every few years......looks like that has not been done for at least three years
Here is a plant of mine that needed lowering.
The rhizome was clearly above the soil line hindering the plants potential.
Repot and maintenence
by
Hostafarian,
on Flickr
Here is the plant lowered in the landscape container.
This will have to be repeated in three years or so.
Repot and maintenence
by
Hostafarian,
on Flickr
Now those Basjoo beasts.
As another post said you will need to refresh/amend the soil in the immediate area......don't be shy dig that heavy clay soil out of that devil strip heat trap and mix some amendments in there and replant your plant.
Try not to disturb the root structure too bad because it is late in the season and you want the plant to recover.
This is why your plant has gotten where it is today. Short answer
Remember this ....basjoo only get larger on LAST years corm size and growth.
That means the longer the corm survives the taller and bigger the beast will get.
So if you mulch and protect your corm from the winter elements and cold you will have a larger plant come spring..... that is if your corm did not perish over winter.
If you have a winter kill of large corm then your plant "starts over" each spring with little pups as in your case.
An easy way to protect your plant is to get a bag of mulch and simply lay it on top on your dormant plants after a hard freeze and you should be good to go.
Strips between the home and sidewalk are usually difficult for anything to grow because of poor soil
and radiant heat from the structure.
With basjoo I will always tell the homeowner this.
The first year it sleeps .
The second year it creeps.
The third year it leaps.