It's very hard to find flowers in shades of blue. Most lean more toward lavender or purple, than a true blue. Sunlight also plays tricks on blue coloring and the photos you see in seed catalogs are often not the colors that wind up in your garden. Blue flowers may remain illusive, the the plants highlighted here come darn close, if not right on the money. Keep in mind that when plants get hybridized, they can often change color ever so subtly. So 'Longwood Blue' blue mist shrub might not be as blue as 'Blue Myth'. If you plan on investing in several perennial plants or shrubs, you should try to see them in bloom, before you purchase them. You can do more experimenting with annuals, since you can usually start them from seed.
1. Agapanthus africanus
The genus Agapanthus contains about 10 species, all with large clusters of flowers in shades of blue or white. Agapanthus africanus is known as the Lily of the Nile, the African Blue Lily or just African Lily. The flowers are somewhat lily-like. It is a tender perennial, only reliably hardy down to USDA Zone 8. It's grown from a fleshy rhizome and can be dug and stored for the winter, in cooler areas. The umbels of blooms are held above the grass-like foliage and bloom throughout most of the summer.
2. Amsonia sp.
There are enough species of Amsonia for everyone to find their favorite shade of blue. The fluffy clusters of blooms are comprised of small, star-shaped flowers, giving Amsonia is common name, Blue Star. The flowers stay lovely for weeks and even the seed pods are nice. Arkansas Amsonia (Amsonia hubrichtii) was named the 2011 Perennial Plant of the Year. It has deep blue flowers and narrow, lance-shaped leaves that burst into gold, in the fall.
A close look at the leaves and flowers of Baptisia australis should tell you that it's in the pea family. This is a native American plant that was very popular for making blue dye. It's common name is blue false indigo. I've never tried making dye from it, but the flowers are so rich looking, I'm sure it would be wonderful. Baptisia can be slow to get established, but once it is, it sends down a deep tap root and doesn't like to be budged. Don't worry, you'll love it and want to leave it be.
Borago officinalis is an underused plant, which is ironic since it's a member of the Forget-Me-Not family. It's considered an herb with medicinal qualities that include being a diuretic and an emollient. It's also a culinary herb, with a scent and flavor similar to cucumbers. The leaves get prickly as they mature, but you will want to let some of them grow to get the lively blue blossoms - which also have a mild cucumber flavor. Borage is an easy to grow annual that will self-sow.
Caryopteris x clandonensis, the Blue Mist Shrub, really does create a hazy of blue, in late summer. When in bloom, it will be covered with buzzing bees, that love its nectar rich flowers. Caryopteris is often considered a sub-shrub, meaning it has woody stems, but the plant blooms on new wood and is cut to within inches of the ground each spring. Other than that, it is maintenance free. There are several cultivars available, in various shades of blue. 'Grand Bleu' is a rich dark blue, 'Longwood Blue', pictured here, is a bright, pale blue and 'Sunshine Blue' has lavender-blue flowers on bright gold foliage.
6. Centaurea cyanus
Whether you call them Cornflowers or Bachelor's Button, these are one charming wildflower. They got the name cornflower because they grew wild in European corn fields. And story has it that bachelor's would where one in their lapel, when they went courting. The abundant flowers are spiky disks, somewhat resembling thistles, without the thorns. Centaurea cyanus is an annual that will self-sow. Centaurea montana, the mountain bluet, is considered a perennial cornflower.
7. Gentians sp.
There are over 200 species of Gentians and not all of them are blue, but so many of them are that it's impossible to talk about blue flowers and not include them. Naturally there is a good deal of diversity among the 200 species, but most are alpine or woodland plants that prefer the cooler, wet seasons. Gentiana dahurica, pictured here, is a late season bloomer and tends to sprawl a bit more than other varieties. It has the familiar 5 petal slightly tubular shape of most gentians. Some of the spring blooming species, like Gentian alpina and Gentian angustifolia are more mat forming, with tubular flowers.
8. Meconopsis betonicifolia
The Himalayan Blue Poppy (Meconopsis betonicifolia) is a gardening legend. Some even consider it the test of a gardening master. It can be difficult to grow because it is native to the mountains of south-eastern Tibet, where it grows in shady, moist shelter. Not many of us can recreate the climate of Tibet in our gardens, but plenty of gardeners have had success growing this plant. I've only grown it once, and it is stunning. It can perennialize and spread, under good conditions. (Mine did not.) But even growing them for a single season is worth the treat. The seed packets usually come with detailed directions for sowing, but several people have told me they've had great success winter sowing these seeds. They all agreed that you have to keep the plants moist. They don't like dry soil.
Nigella damascena got the name Love in a Mist because of the airy dill-like foliage that surrounds the flowers. Its a freely seeding annual flower that will tuck itself throughout your garden. The plants do not like to be moved, but you can scatter seeds with little effort and they will make themselves at home. Nigella will flower throughout the season and makes a great cut flower.
It's one of the first plants to perk up in the spring and although not all varieties bloom blue, those that do are resplendent. You may be familiar with the variegated leaf varieties. Some are dotted with white and others are washed with silver. These look good all season, however many the plain green-leaved varieties have the most brilliant blue flowers. Pulmonarias are shade lovers and look lovely in woodland settings.
11. Salvia guaranitica 'Black and Blue'
The striking combination of vivid blue flowers and black sepals on Salvia guaranitica 'Black and Blue' are reason enough to try growing it, but it is also a hummingbird magnet. You may see it listed as Brazilian Sage or Blue Anise Sage. It's perennial to USDA Hardiness Zone 7 and can become aggressive in some areas. In cooler zones, 'Black and Blue' is popular as an annual. It's taller than most of the annual salvias and unique in form and color.
12. Scilla sibirica
They maybe called Siberian squill, but these tiny bulbs will naturalize in USDA Zones 2-8. All they need is a little bit of of chill, to take a winter rest. They'll build up enough energy to reproduce and bloom very early in the spring. They are used to best effect when allowed to spread far and wide. They look like a sea of blue, when naturalized in a lawn. As with most early bulbs, they have few pests or problems. Their own peccadillo is a need for moisture while they're growing and drier conditions while they are dormant.