Answer: No matter what you plant, you need to work in ample amounts ( a six-inch or even twelve-inch deep layer would not be too much!) of organic matter to improve the soil structure and help it hold moisture. You could use compost, old rotted leaves, aged stable manure and bedding or any similar organic material. Also be sure to use an organic mulch such as shredded bark which will also add organic matter to the soil as it breaks down over time as well as help the soil stay cool and moist.
Some perennial flowers to consider would be: bearded iris, thyme, artemesia "Powis Castle", any type of sedum, Dusty Miller, perovskia or Russian Sage, lychnis coronaria, dianthus, purple coneflowers, gaillardia, Black-eyed Susan and lance-leafed coreopsis such as "Early Sunrise". Yellow annuals such as zinnias, marigolds, tithonia "Goldfinch", lantana, and gladiolus should also do well for you. Fall planted spring bulbs such as crocus, tulips and daffodils should also do well for you -- and are all available in yellow. Burpee (http://www.burpee.com) or 1-800-888-1447 offers many if not all of these.
Keep in mind, however, that all plants will grow their best with adequate moisture, so you may need to water when the weather is dry. The rule of thumb is an inch a week, but on sandy soil you may need to water slightly more than that. It is also best to water deeply and less often than to water lightly every day.
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