Answer: Peonies generally grow to their mature size and stay that way, say about three feet across, so you can't make the vigorous ones spread bigger than they already are. Based on your description it's hard to tell why the three don't flower. However, one common cause is that they were planted too deeply. If they are a different type or color, it might also be that they are simply of a less vigorous variety. If they are all the same type and were planted approximately the same way, it might be that there is some difference in the soil, perhaps something underground you can't see such as a competing tree root or a different subsoil or perhaps even a different soil altogether (this can happen during the home construction process) or maybe even a difference in the surface runoff pattern. Finally, it is possible that they are suffering from some sort of root problem; you might wish to dig one up and see.
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