Do you water plants in that area more? As its in full sun, it will dry out far quicker than anywhere else in the garden. Second question - when was the last time you gave it a good dig and incorporated composted manure or similar to it? Third, what's behind the border (next door, not in your garden? And fourth, are there any tree stumps or old shrub stumps anywhere nearby?

Place a mix of composted manure 1/2 +good garden soil1/2 and work into your border soil. It might be that your soil could have a heavy clay content. Then plant Tithonia rotundiflora ( mexican sunflower) seeds according to package directions. Let germinate, weed and water as recommended. Each plant is 4ft high and 3ft wide, flowers like crazy from july till first frost, just keep up with the deadheading. It is October the 1st and the hummingbirds are still fueling up on the flowers and the honey bees are at work too. ( yes all of a sudden the bees are back after being absent for many years in my area of the USA). This plant is an easy grow and gives great rewards. Another idea would be to line several half kegs with heavy plastic, set them along your border spaced appropriately, fill with water,refill as evaporation dictates and set in them water garden plants such as water hyacinth, lotus, water papyrus, water iris,...etc. These are the methods that I use in a border area of my garden with similar conditions as yours with great results.