Answer: The best way to achieve that very full look in our climate is to mix perennials and annuals and bulbs and vines as well as flowering shrubs at minimal spacing. In order for the plants to do their best when planted so densely, you also need to prepare the soil thoroughly ahead of time by incorporating ample amounts of organic matter and any other amendments as indicated by basic soil tests results. The soil preparation, along with an organic mulch, will help the plants withstand the heat and possibly dry summer conditions.
Start with some anchoring perennials for each bloom period from early to late, and then add annuals to complement them with their ongoing steady display. Many of the prairie perennials (such as purple coneflower, black-eyed Susan, liatris, and coreopsis) perform very well during the heat of summer, as do a number of decorative herbs (such as assorted thymes, chives, catmint, bronze fennel, and Russian sage).
By adding in some summer-blooming shrubs, such as Caryopteris, japanese spireas, and butterfly bushes, you will greatly extend the variety of both color and height. Careful use of spring, summer, and fall bulbs can also add an extra fullness to the bed, as can flowering vines, such as morning glories, moon flowers, clematis, and the native honeysuckles.
Finally, allow annuals to self sow freely to fill in any gaps. Some good ones to try are cleome, flowering tobacco, California poppies, cosmos, Johnny jump-ups, bachelor's buttons, and larkspur.
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