Answer: Annuals need to be planted each year. During the growing season they will bloom, set seed and then die. Common annuals would include marigolds, petunias, and zinnias. Perennials generally live for at least three seasons or more assuming they are planted in a spot which suits their needs.
Based ony our description, it is plausible that the myrtle died because the spot is too hot and dry and above all too sunny for myrtle. Instead, you might want to try some sun-loving flowering plants which are tolerant of drier situations.
Some common, easy care perennials for a hot slope in full sun would include daylilies (hemerocallis), creeping phlox, purple coneflower, yarrow, and assorted varieties of sedum. Although these are very tough plants, they will still need care and watering until they become established and again in times of drought. They will also appreciate the addition of some organic matter to the soil, (such as compost, rotted leaves, or aged manure and bedding) along with a few inches of mulch.
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