Answer: You would need to plant the annuals in the ground and then surround them with the mulch. (The mulch should not touch the stems of the plants.) Prior to planting you would want to remove the grass or weeds if there are any there, loosen the soil about six inches deep, then work in ample amounts of organic matter such as good quality compost, then plant, water each plant to eliminate air pockets at the roots, then mulch around them with a layer of organic mulch about three inches thick. Some gardeners will also place a few layers of damp newspaper over the soil just under the mulch layer. This helps exclude light and prevent weed seeds from germinating and deep rooted weeds from re-surfacing.
Some gardeners have had success planting through mulch. In this technique they cut the existing grass very short then cover the area with overlapped sheets of newspaper in a thick layer, top that with six inches or more of organic matter such as compost, old rotted leaves, and so on, and top that with a layer of organic mulch. For each plant, pull aside the mulch, dig a hole for the rootball, then plant. Water deeply, replace the mulch around the plant. In my experience this can work but it depends on the type of native soil you have and what you are planting into it; personally I think it works better for shrubs and larger plants than for annuals and is better with a good loam or sandy loam than with a clay based soil. If this is what you are envisioning, I would suggest experimenting with a smaller area and see how it works for you then if you like it, expand it next year.
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