Over wintering a bed; chipped bark or green manure

I would sow a green manure crop if you can get it started before it gets too cold. The fresh green material dug in next spring should give good nutrients to the flowering shrubs, where as chipped bark will take longer to break down. The green manure dug in properly would release nutrients at the time needed for the shrubs to get established. Chipped bark I have read has low levels of nitrates and may in fact lower some nutrient availability levels in the soil while microorganisms work to break down the wood fiber. I would be more inclined to use chipped bark on the surface in between the shrubs when they are planted to help to reduce the competition from weeds then.

If you know anyone with horses they might have old wood chip they have used for horse bedding and this would be better to use as the horses will have added extra nutrients. Even straw horse bedding stacked on the plot now should break down gradually over winter and would be ready to dig in to the plot adding nutrients when you are planting your shrubs. There can be some viable seeds in this material so It may need a bit more digging than the fresh wood chip.