Black and Blue salvia (Salvia guaranitica "Black and Blue") is an anise-scented sage cultivar which may grow up to 6 feet tall in ideal growing conditions, producing 15-inch spikes of deep blue, two-lipped flowers from mid-summer into fall. Effective as a border, bedding or container plant, Black and Blue salvia offers the added benefit of attracting hummingbirds and butterflies to the garden. The low-maintenance perennial requires little care to thrive.
Plant Where Hardy
Black and Blue salvia is hardy in U.S. Department of Agriculture plant hardiness zones 8 to 10, where it will grow in full sunlight to partial shade. In overly shady conditions, the plant may stretch out and fall over. In USDA zones 8 to 9a, Black and Blue salvia often dies down to the ground in winter, though it re-emerges in spring. With winter mulch, Black and Blue salvia may even survive the winters in USDA zone 7. Where not hardy, the plant may be grown as an annual.
Use Organic Soil
Black and Blue salvia prefers a well-draining, rich soil. Avoid heavy, poorly draining soils, as these can cause rot diseases. When there is no rainfall, water well and allow soil to slightly dry out between waterings. If the top 2 inches of the soil are dry, it's time to water again. You can increase soil drainage and fertility by mixing 2 to 3 inches of organic matter such as leaf compost or peat moss into the top 6 to 8 inches of the soil, using a tiller or garden shovel.
If planting in a container, use a well-draining, good quality potting mixture and a container with a hole in the bottom for drainage.
Fertilize monthly during the growing season with 2 tablespoons of a 20-20-20 water-soluble fertilizer per 1 gallon of water, at a rate of 1 quart of solution per 1 square foot of gardening space. Apply solution directly to the soil, keeping it from splashing the plant. Store unused fertilizer in a secure location away from children and pets.
Keep Plant Tidy
Deadhead spent flowers throughout the growing season to prolong bloom period and to keep the plant looking tidy. Keep the planting area free from weeds, which compete with the plant for water and nutrients in the soil. If the plant becomes unwieldy or too tall, cut back old stems down to the lowest bud on the base of the shrub in late spring, after new stems have appeared. Cut the entire shrub down again in autumn. Disinfect pruning equipment before and after pruning by soaking tools in a solution that is equal parts water and alcohol for 5 minutes. Rinse with water afterwards and air dry.
Prevent Pests and Diseases
Black and Blue salvia is usually not bothered by serious pests and diseases, though fungal diseases such as powdery mildew, which causes powdery white or grayish fungal spores on foliage and sometimes flowers, may become a problem when plants are overcrowded. Provide at least 5 feet of space between plants to allow for ample air circulation. Take care not to splash the leaves when watering, as wet leaves encourage fungus. You can treat powdery mildew by mixing 2 to 5 tablespoons of horticultural oil with 1 gallon of water and spraying it over the entire plant, using a hose end or handheld sprayer, until wet. Store unused oil out of direct sunlight, away from food products.
Young plants and tender new growth may also attract slugs and snails, which chew holes in foliage and leave behind iridescent trails of slime. Slugs and snails can be removed in the evening, when they are most active, and crushed or drowned in soapy water.