Answer: In my experience a layer of organic mulch is almost always preferable to a mulch of stones as far as the plants are concerned. The organic mulch breaks down slowly over time and helps feed the soil that way. This helps mimic the natural processes of decay and renewal to repelenish organic matter and keep the soil healthy. Healthy soil means healthier plants.
Stones tend to retain too much heat both summer and winter, thus causing stress on the plants. Stones must also be raked periodically to remove or disturb weeds and weed seeds that have blown in. And of course stones add nothing to the soil. Bulbs can come up just fine through a thin stone layer, though.
A rotted down mulch layer is a great environment for germinating seeds -- many plants will self sow with the seeds germinating the next spring if left fallen and undisturbed on old mulch. In spring you will need to weed until the desired plants begin to grow -- Celosia germinates late because it likes warm soil. You may need to thin the seedlings and you will also need to refresh the mulch taking care not to smother the seedlings you wish to keep. I should point out that your celosia may or may not self seed, some hybrids are sterile and others, although able to set viable seed, will not necessarily look like the parent. If you are hoping for a uniform planting to repeat the effect you have this year, you might be happier using purchased transplants.
I hope this helps.
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