We have offshoots from a former cotoneaster bush that are growing very well. Whenever we try transplanting an offshoot, they don't seem to thrive. What can we do to help them thrive?
I don't have any specific knowledge of cotoneaster, but with other rosids the reason for failure of transplanting shoots is dessication and transplant shock (roots are too small to support the new growth). Basically you are taking a branch that is connected to a massive underground network of roots and cutting it off, leaving the shoot to take up all of its water through its puny little roots. To make matters worse the old shoot was grown in the shade of the parent plant.
Here is what I would try: take some peat moss and bury the botton 8-12" (kind of poor mans air layering) water that normally, figure out some way to keep the peat there: maybe a pot with the bottom cut out. Then this winter maybe January, dig it up and bury so that the top most roots are just below the soil.