For impressive plants with tall, showy flowers, few can top the lupines (Lupinus spp.) with their columnar flower spikes ablaze with color. These beauties only need the right conditions and basic care once established.
Although some wildflower lupines are annuals, cultivated lupines usually grow as perennials that die to the ground in winter and return in spring. Most perennial lupines need some winter chill. Examples include Texas bluebonnet (Lupinus texensis), which has bright blue flowers in spring and grows in U.S. Department of Agriculture plant hardiness zones 5 through 8, and the silvery lupine (Lupinus argenteus), which has silver-green leaves and violet flowers, and grows in USDA zones 4 through 8. Lupines do best in full to partial sun.
Lupines suffer in high heat and humidity. If your summers are hot, plant taller perennials or shrubs nearby to cast cooling shade during sunny afternoons. In these hotter areas, you can also treat lupines as annuals.
Soil and Watering
Some lupines, such as Texas bluebonnet and silver lupine, are native to dry, rocky areas and thrive in soil with modest or low fertility that tends to stay dry. Other types do best in fertile, well-draining soil and need consistent moisture. These include the big leaf lupine (Lupinus polyphyllus), which grows in USDA zones 5 through 9, and the hybrid lupines (Lupinus x hybrid), often just called lupines, which comes in many cultivars and grow in USDA zones 4 through 8. Adding 1 to 2 inches of compost around these plants each spring helps improve the soil's fertility. Mix it into the top few inches of soil, being careful not to injure the plants' roots.
Water big-leaf or hybrid lupines whenever the top 1 to 2 inches of soil feel dry to the touch, but use drip irrigation or a soaker hose to keep the leaves dry and help prevent fungal diseases. Spread a 3- to 4-inch layer of organic mulch under each plant to help conserve soil moisture, while also keeping down weeds. Keep mulch back 1 to 2 inches from the plant's base to prevent disease problems. If you grow native lupines that prefer dry soil, don't water them unless there's an extended drought.
Like most perennials, lupines don't need much fertilizer, but mixing a small handful -- about 1/4 cup -- of a low-nitrogen granular formula such as 5-10-5 into the soil around each plant can boost new growth in the spring. Repeating this treatment twice more, at six-week intervals, can help keep new flower buds coming, but keep the fertilizer off the leaves, because this might burn them. Don't exceed a total treatment of 4 or 5 pounds per 100 square feet of planted area for the entire season.
Some lupines become quite tall -- a big leaf lupine can grow up to 5 feet tall, while hybrid types are often 3 or 4 feet tall. To keep flowers upright and prevent leaning or drooping, drive a strong wooden or metal stake into the ground before planting. Once you've planted the lupine, tie each plant stem to the support with a soft tie.
Removing spent flowers from lupines -- called deadheading -- keeps the plant looking tidy and also helps encourage new flowers so the plants bloom for longer. Use sharp gardening shears when removing flower spikes, wiping your blades with a cloth soaked in rubbing alcohol between cuts to prevent spreading plant diseases.
Pests and Diseases
Lupines are susceptible to several fungal diseases, including mildew, which produces fluffy white growth on leaves, damping off, which can cause young plants to wilt and die. Other fungal problems cause spots on leaves. You can prevent most by watering at the plant's base, planting where air circulation is good and clearing away plant debris regularly.
The plants can also attract slugs or snails that feed on foliage. Hand pick these as needed. Aphids, small insects that cluster near the bases of leaves, can also damage lupines. Control a severe infestation of these by spraying the plants until they're dripping with a ready-to-use insecticidal soap. Wear gloves, long sleeves and long pants when spraying. Spray on a calm day when the temperature is below 90 degrees Fahrenheit.