Cool-color gardens create a restrained look with the use of different types of blues like soft lavender-blue to bright sky blue to cobalt blue. To achieve the desired look of a blue garden, you must have a good mix and balance of plant types. Use blue-flowering annuals, perennials and shrubs. Some plants even have bluish foliage. Plant the taller plants in the background and the shorter ones up front. Keep the ones that are going to need replanting each year within easy reach.
Shrubs are the backbone of any garden. They are a long-lived addition and should be placed carefully. Plan their placement according to their mature size. They tend to be the tallest of your plants in a blue garden so they should be planted near the back of the garden. Some shrubs to consider are Caryopteris x clandonensis (bluebeard) and Vaccinium spp. (huckleberry).
Bluish foliage can maintain a blue cast to your garden even when flowers are not blooming. Some plants with blue tones to their leaves are Eryngium amethystinum (amethyst sea holly), Baptisia australis (blue false indigo), Ruta graveolens (Rue) and Festuca cinerea (blue fescue).
Perennials will last for years. They are a long-term addition to the garden. These should be spread out in your garden so they can cover the areas where annuals and bulbs leave bare after they die back. Pinch off dying blossoms to encourage perennials to bloom all summer long. Some blue-blossoming perennials include delphiniums, Iris sibirica (Siberian iris), Campanula spp. (bellflowers), Scabiosa caucasica (pincushion flower), Cichorium intybus (chicory), Viola adnuca (blue violet) and Penstemon acuminatus (sanddune penstemon).
Annuals are flowers that die back each year. They should be in areas that are easy to cultivate and care for. Deadhead dying blooms on annuals to encourage the plants to produce blooms all season. Some blue-flowering annuals to try out in your blue garden are Ageratum houstonianum (ageratum), Brachycome iberidifolia (Swan River daisy), Browallia speciosa (browallia), Centaurea cyanus (cornflower), Consolida ambigua (rocket larkspur), Convolvulus tricolor (dwarf morning glory), Ipomoea tricolor (morning glory), Lobelia erinus (edging lobelia), Myosotis sylvatica (forget-me-not), Nemophila menziesii (baby-blue-eyes), Nigella damascena (love-in-a-mist) and Salvia farinacea (mealy-cup sage).
Bulbs are used for spring color. They look best when planted in clumps of 1 or 2 dozen. This will help make a dramatic look. Some blue-blooming bulbs are Hyacinthoides hispanicus (Spanish bluebells), Scilla sibirica (Siberian squills) and Camassia quamash (camas).
Groundcovers sprawl along the ground and fill in any bare spots left by annuals and bulbs. They also make excellent borders because of their dense growth. Some blue-flowering groundcovers are Ajuga reptans (ajuga), Veronica prostrata (rock speedwell) and Clematis columbiana (blue clematis).