We recommend you add some Perlite, to loosen the soil. Perlite is a very absorbent media and will release the water only when soil is dry. It also improves aeration and drainage. Our Perlite has a neutral PH, is sterile, and can absorb up to 4 times its volume.
Make sure you also plant some water-loving plants like Bearded Iris (Iris germanica), Butterfly Bush (Buddleia davidii), or Daylily (Hemerocallis).