Answer: In my experience it is always better to use an organic mulch in a flower bed. A finer shredded material or even chopped rotted leaves will work well. It will break down over time and help feed the soil, and when you dig and divide your plants or re-arrange them it is much easier to brush aside the organic mulches. The gravel will work down into the soil making it difficult to dig whereas a little of the organic mulch does no harm if it slips into the hole or is worked into the soil. And, as you mentioned, the gravel holds heat both summer and winter and can stress your plants as a result.
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