Answer: It's fine to leave the leaves there as a partial mulch as long as they are not matting down and staying wet and heavy on top of the plants. For example, a thick layer of maple leaves will mat, but oak leaves or a mixture of leaves will not. You do not need to wait until the cold weather hits. In general you can keep a layer of several inches of mulch year round -- it helps to feed the soil as it decays, moderates the soil temperature and also helps maintain soil moisture. Put the mulch around but not over the visible crowns of plants. In winter you may find that the snow acts as additional insulation andhelps protect your plants. It sounds like most of the plants you have are quite hardy but if you tend to have "brown" periods of intense cold without snow then a fluffy mulch such as straw or evergreen boughs or chopped leaves can be layered over the beds for the winter but it must be pulled aside in early spring to avoid smothering the plants. One possible exception is the butterfly bush -- it will in all probability die to the ground and may freeze out altogether no matter what you do because it is usually only root hardy into zone 5.
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