There are so many blue flowers for spring, but what blue perennials can you grow in the heat of the summer? There are quite a few perennials listed as "blue" but they look like purple or lavender to me. However, there are a few beautiful blues among the choices for deer resistant, full sun, zone 7 gardens.
A few weeks ago, I fell in love with a Blue Satin® hibiscus syriacus, which is commonly called a Rose of Sharon. I had to bring this plant home! The foliage is a beautiful dark green that sets off the incredible, large blue flowers. The blooms close up at night and open each morning. I've planted this new addition along the path in the front deer resistant garden. This is a deciduous shrub that can be trained into a small tree and is suitable for zones 5-8. The hybiscus is listed as deer resistant, so I'm putting it to the test! I tucked it in behind a mass planting of perennial blue ageratum, eupatorium colestrum 'Wayside'.
A most unusual azure blue is perennial bog sage (salvia uliginosa) that will grow in water as well as boggy or somewhat dry soil. It is deer resistant and is favored by bees and butterflies. The blue and white flowers seem to disappear as the day goes by, but are blooming again each morning. I've never had to cut mine back for additional blooms, but that's the suggestion for maintenance of this salvia. Bog sage is rated for zones 6-9 and spreads rapidly by stolons that are easy to pull should you find yourself with too much. Like the perennial ageratum, you only need one of these plants to create a mass planting within a year or two. 
A hummingbird favorite is salvia guaranitica 'Black & Blue'. I am working on the creation of several mass plantings throughout the gardens. This deer resistant salvia may be sampled by deer, but rarely will I be missing more than one bloom stalk. Since the salvia is fine with being pinched back, there is no harm done. The foliage is very lush and a light green color, making it a nice backdrop for bog sage. If you like butter yellow and cobalt blue color combinations, this is the salvia to use. I'm trying out gaillardia 'Golden Goblin' this year for the first time. If it grows well and is truly deer resistant, then I will add more of this yellow flower around the salvia 'Black & Blue'. 
Zones 3-8 with hot, dry conditions may like echinops 'Ritro' (globe thistle). This prickly plant is a beauty in bloom. I like it with burgundy foliage plants such as crepe myrtle 'White Chocolate' as well as rose violet and purple blooms of cleome (a reseeding annual) and agastache 'Golden Jubilee'. Although I've never had this plant munched by deer while it is in bloom, they have eaten the foliage to the ground after I deadheaded the plant. It comes back every year, so this isn't catastrophic. In fact, the foliage gets a bit ugly here in late summer, so the deer do a nice clean up task for me! I'd like to have more globe thistle, but I've had trouble getting this plant established when using small plants. The hardiest plant was purchased in a 6" pot two years ago from a local nursery. 
I went overboard and into the deep blue when I brought home six salvia 'Mystic Spires Blue' this year! Once I got these plants home in my garden, they appeared so dark. I still planted three together in the cottage garden, but decided to place the other three far apart along the top edge of my deer resistant garden where I could try to place them with lighter colors such as the yellow/green foliage of osmanthus 'Goshiki' and with yellow blooms of gaillardia 'Golden Goblin' and coreopsis 'Creme Brulee'. I'll have to experiment a bit with this deep blue color. This is supposed to be a shorter version of 'Indigo Spires' (I have one of those in my deer resistant garden, too) and should provide a long bloom season in zones 7-9. I also grow two varieties of caryopteris that will have blue blooms in late summer. 'Longwood Blue' has silver foliage and a nice, mounding habit and is a great deer resistant, drought tolerant shrubby plant for zones 5-8. I am seeing buds forming on the three caryopteris in my garden. For several years, I've had 'Snow Fairy' in the garden. This one hasn't bloomed very well for me since the first year, but the variegated green and white foliage is nice when used with other plants with dark blooms in blue, orange or red. I am using three 'Longwood Blue' with a mix of agastache that bloom pink, purple and salmon. I have another beside an 'Adonis Blue' buddleia and a salvia 'Mystic Spires Blue' that are behind a mass planting of pale yellow coreopsis 'Creme Brulee'.
On a hot summer day, cool blue is a refreshing color for the garden!
Photos and words by Freda Cameron; Location: home garden; July 2009