The offshoots have been growing for 3 weeks and only grew 1 and half inches since then.

probably the cooler temps have them slowly awakening. I have several Basjoos in ground in three different groves and mine seem to be creeping along. As soon as some warmer temps comes along we should see them pick up the pace. My mature Basjoos are pushing out new leaves but kind of taking their time. I fertilized mine about three weeks ago. I am down in B'ham, Al so my temps are probably a bit warmer than yours. My Saba and Black Thia are still dormant and have shown no signs yet. My Yunan has already shot up a runner/pup in a spot I had not planned for bananas . Its about 2-3 inches tall and not showing very much activity. Again once mid 70's or so kick in they will pick up the pace.

Also, depending on what your preference is for your landscaping I would suggest spreading out some shredded pine mulch. The cheap, natural shredded kind, not the nuggets, colored mulch or cypress mulch. This will help attract and retain the suns heat to give them a boost and also breaks down very nicely from year to year as you add to it. I do this every late fall and spring for mine.

I am guessing that not all strains of musa basjoo are equal. Either there are genetically flawed strains floating around or some are infected with a virus that shows no overt symptoms other than slow growth. I have experienced this kind of thing with other species of musa. About 40 years ago, I bought a musa coccinea from California Jungle Gardens that just refused to grow and eventually died. I falsely concluded that coccinea was a delicate species. Later, I got an M. coccinea from a different source and it grew like crazy. Also, to a lesser extent, I have encountered variable vigor in musa basjoo from different sources.

If you have a bad strain, you are just screwed.