Answer: Not surprisingly, lavenders top the list for heat tolerance. One good one is Lavandula 'Goodwin Creek Grey' which has deep violet-blue flowers that appear in spring and go all the way into late fall. It is a bit larger than most lavenders and has very dense foliage. 'Munstead' is an English lavender that has a very long bloom period, is a heat lover and makes a wonderful low hedge. Another particularly good heat-loving lavender is one of the Spanish ones ? Lavandula stoechas 'Otto Quast' which tends to be very long-lived.
Teucrium chamaedrys or Germander is a tough plant that holds up well in hot weather. I have this in my own garden where it gets full sun exposure all day and is flourishing with very sporadic watering; in fact, it cannot tolerate wet soil.
Origanum laevigatum 'Hopley?s' is an oregano that bears dense heads of purple-pink flowers and self-sows freely. It's a native of Turkey, so that should speak volumes for its ability to survive our summer heat. Another oregano that does well is the 'Betty Rollins' cultivar which spreads in a low-growing mat to make a wonderful groundcover.
Phlomis fruticosa, commonly known as Jerusalem Sage, is another Mediterranean native that is an easy perennial shrub to grow. It grows to about 4' tall and wide and has deep golden yellow flowers that appear in ball-shaped whorls. This perennial looks wonderful when paired with lavenders.
Nepeta x faassenii, or Catmint, is a low-growing, bushy perennial that can be mowed down to the ground periodically to keep it looking good. It makes a great border or edging plant.
One of the salvias ? Salvia officinalis 'Berggarten' ? is a particularly good choice for hot, dry climates. It is more compact, has denser growth and tends to be longer-lived than some of the other salvias.
If you like ornamental grasses, Calamagrostis x acutiflora 'Karl Foerster' is a true workhorse in our hot, dry summers. It is a sturdy, clumping-type grass with feathery, buff-colored flower plumes that persist into winter. This grass is beautiful when mass planted.
Some perennials to consider include:
Artemisia (Artemisia); Asters (Aster spp.); Baby's breath (Gypsophila paniculata);
Baptisia (Baptisia australis); Beebalm (Monarda didyma); Black-eyed Susan (Rudbeckia hirta); Butterfly weed (Asclepias tuberosa); Candytuft, (Evergreen Iberis sempervirens); Chrysanthemum, Mum (Chrysanthemum x morifolium); Columbine (Aquilegia spp.); Coneflower (Purple Echinacea purpurea);Coralbells (Heuchera americana);
Coreopsis (Coreopsis lanceolata, C. verticillata, C. tinctoria); Daylily (Hemerocallis spp.); Gaillardia, Blanket flower (Gaillardia x grandiflora);
Gaura (Gaura lindheimeri); Gazania (Gazania); Gerbera daisy (Gerbera jamesonii);
Goldenrod (Solidago spp. and hybrids); Hardy ice plant (Delosperma cooperi);
Helleborus, Lenten rose (Helleborus orientalis); Iris (Iris spp.); Lamb's ears (Stachys byzantina); Liatris (Liatris spicata); Lily of the Nile (Agapanthus);
Mexican sunflower (Tithonia rotundifolia); Peony (Paeonia lactiflora); Perovskia russian sage (Perovskia); Phlox, thrift (Phlox paniculata, P. divaricata, P. subulata); Red hot poker (Kniphofia uvaria); Salvia (Salvia farinacea, S. spp.);
Sedum (Sedum spectabile).
All the above suggestions will do well under hot, dry conditions as you describe the planting bed.
Best wishes with your new plants!
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