Aha! So the plastic bales are kept intact. I'm guessing some drainage holes were cut in the bottom. I'm not real impressed with the growth as far as comparing it to how they'd do with a potting mix of peat and sand with wetting agent. If straight peat was the most affective medium for acid lovers like blueberries, my hunch is that nurserymen wouldn't bother weighting it down with sand. Just a hunch.
[NAFEX] blueberries in peat bales,
Alan Haigh, 10/27/2008