WELCOME BACK FOR THIS third round of Cultivate the Rainbow, a seasonal series exploring color in the garden. This time we’re looking at blue and purple.

From gemstones to denim, blue can be highbrow or humble. The same is true in the garden where blues range from finicky perennials to easy-to-grow annuals.

With dramatic flowers that range from sky blue to cobalt and ultramarine, delphiniums promise true-blue blooms. A diva in the summer garden, the plant prefers long days, cool summers, rich soil and consistent water. And if you’re growing one of the stately hybrids, like Delphinium elatum ‘Paramo Azul,’ which has beautiful spires that reach 4 to 5 feet tall, you can add staking to the plant’s list of demands.

Less needy Chinese delphiniums (Delphinium grandiflorum) deliver deep blue blooms on relatively carefree plants. Low-growing mounds of lacy foliage generously bear loose sprays of flowers on 18-inch stems in full sun. While Chinese delphinium is a short-lived perennial, the plant is easy to grow from seed.

An even easier annual, larkspur (Consolida regalis), continually produces 3-foot flowering spires as long as the plants are kept picked — the most delightful maintenance task of all. Larkspur is available in a variety of colors, including white, pink, lavender and peach; search online or at your neighborhood nursery for light and dark blue seed mixes. Plant larkspur in full sun and feed once a month to support growth.

Growing Himalayan blue poppy (Meconopsis spp.), a famously fussy perennial with transcendent blooms, is an elusive achievement for most home gardeners. Thankfully, the folks at the Rhododendron Species Botanical Garden (rhodygarden.org) in Federal Way are blue poppy pros that cultivate these plants — swing by next May to catch the blossoms in peak bloom.

Borage (Borago officinalis) flowers look like baby blue shooting stars, delivering a casual transcendence that anyone can grow at home. In full sun to part shade, spring seedlings quickly form 2½-foot mounds of foliage covered in prickly hairs. Succulent stems produce constellations of pollinator friendly blooms in summer that readily reseed all on their own for future growing seasons.

While fewer than 10% of all flowering plants produce truly blue blooms, when it comes to purple flowers the selection is vast, though they aren’t purple exactly. Perhaps more than any other color, purple appears in a multidimensional spectrum that includes mauve, lavender, plum and wine. True purple, like the color of an eggplant, the intense flowers of ‘Hidcote’ lavender, or any of the following plants listed below lend a regal touch to perennial plantings.

Cultivars of perennial meadow sage (Salvia nemorosa) bloom in various shades of lavender, but ‘Caradonna’ is especially striking with rich violet blue flowers on distinctive inky purple stems in summer. Flowering spires rise above compact mounds of foliage, attracting butterflies.

Tried and true Clematis ‘Jackmanii’ is popular for good reason. Vigorous 8- to 12-foot vines produce large velvety purple blooms with creamy centers. Flowers appear in late summer on new growth, which simplifies pruning: Cut back stems in late winter or early spring to a pair of strong buds 1 foot above soil level — or, if training the vine through a shrub or on a trellis, trim stems back to the point where you want new growth and blooms to appear. Clematis prefer to grow in full sun to part shade, with roots in the shade of other plants or use rocks to keep the soil cool.

Common monkshood (Aconitum napellus) grows up to 4 feet tall with densely packed spikes of deep purple blooms that resemble the hoods worn by medieval monks. The upright perennial thrives in full sun to part shade in rich soil with even moisture. Plant with caution, however: Every part of monkshood is deadly poisonous to dogs, cats and humans.

Purple spiderwort (Tradescantia pallida ‘Purple Heart’) is prized for its royal purple foliage than the plant’s rather inconspicuous tiny pink flowers. The trailing vine adds drama to container plantings or introduces eye-catching contrast when tucked in among groundcovers. The narrow leaves have the best color when grown in full sun. A tender perennial, hardy in zones 8-11, the plant will die back in all but the mildest winter or can be brought indoors and grown as a houseplant.