You need to know more about your plants than whether they grow best in sun or shade. When a soil test shows that your soil has a pH level between 7.2 and 7.5 it is slightly alkaline, and a reading between 7.5 and 9 is moderately alkaline. Since most perennials prefer soil with a pH value between 6 and 7.5, you'll need to choose alkaline-friendly perennials for best results. If you want to reduce the alkalinity in small garden spaces, try working 1 to 2 inches of sphagnum peat into the soil.
Fragrant, Butterfly-Attracting Flowers
Certain perennials thrive in alkaline soils, perform best in full sun and bear fragrant, butterfly-attracting flowers in U.S. Department of Agriculture plant hardiness zones 5 through 8. Red valerian (Centranthus ruberi) is a bushy perennial that puts out red blossoms in spring, even in poor soil. Good for even very alkaline soils, it controls soil erosion and grows 1 1/2 to 3 feet tall, but might be an invasive species in your area. English lavender “Munstead” (Lavandula angustifolia “Munstead”) puts out lavender-blue flowers in summer and easily grows in dry, shallow, alkaline soil. This drought-tolerant plant grows 1 to 1 1/2 feet tall.
"Absolutely Amethyst” candytuft (Iberis "IB2401") and maiden pink (Dianthus deltoides) are evergreen, alkaline-friendly ground covers for full sun that tolerate deer. Plant “Absolutely Amethyst” in gravely to sandy soil and enjoy its purple blossoms in spring. Also tolerant of rabbits and drought, this perennial grows in USDA zones 4 through 8. Plant maiden pink in gritty, but fertile, slightly alkaline soil and enjoy its fragrant, deep-pink blossoms in spring and summer. This evergreen grows in USDA zones 3 through 8. Both flowering ground covers grow 6 inches to 1 foot tall.
In the Shade
Some perennials are just the right choice for shady places with slightly alkaline soil. The best site for forked aster (Eurybia furcata) features a sandy loam and partial shade, although it tolerates full sun. A beautiful woodland plant, its white flowers come out in summer and it grows 1 to 3 feet tall in USDA zones 4 through 8. The best site for hellbore “Royal Heritage” (Helleborus “Royal Heritage”) has organically rich soil and moderate shade, but it tolerates full shade, and sits in USDA zones 4 through 9. Outstanding under trees or shrubs, this evergreen’s flowers come out from winter to spring in a variety of color combinations, including greens and pinks, and it grows 1 1/2 to 2 feet tall.
Sword-Shape Foliage for Texture
False aloe (Manfreda virginica) and rush “Blue Mowhawk” (Juncus inflexus “Blue Mowhawk”) prefer slightly alkaline soil and add texture to your landscape with sword-shape foliage. False aloe puts up with shallow, rocky and dry soil, as well as drought. This long-blooming perennial puts out fragrant yellow-green flowers from July to October, on stems up to 6 feet tall in USDA zones 6 through 9. “Blue Mowhawk” prefers to sit in water up to 3 inches deep, but can also grow in moist soil. This ornamental grass has narrow, blue-green foliage and grows 2 to 3 feet tall in USDA zones 5 through 9.