Few plants are so beloved in their native territories as the Texas bluebonnet (Lupinus texensis), a wild blue lupine that blankets open meadows and hillsides throughout that state. Classified as a hardy annual, the plant dies back each winter with the arrival of frost, but new plants sprout from the previous season’s seed. Bluebonnets in the garden can be grown from transplants or from treated seed.
The subject of fond legend and lore, Texas bluebonnets are one of five species of lupine native to Texas. Bluebonnets grow throughout U.S. Department of Agriculture plant hardiness zones 6 to 8. The characteristic bright blue flowers show up from February through April on spikes 12 to 15 inches in height from a basal leaf rosette. White and pink strains show up occasionally in the wild, but the nursery industry has developed reliable cultivars in pink, white, varying shades of blue, and “true Aggie red,” a maroon shade that honors Texas A&M University.
Current-season bluebonnets will be killed back by winter frosts, technically making this lupine an annual plant. The term “hardy annual” refers to its habit of producing new plants in the same year from seeds produced during the current year of growth, which transform from tiny seedlings in late summer or autumn into large, ground-hugging rosettes by winter’s arrival. The lower leaves of this flattened plant may turn reddish during the winter, but once spring temperatures arrive, bluebonnets put on a flush of new foliage followed by the tall, characteristic flower spikes. Though seeds may sprout in areas below USDA zone 6, the harder winter freezes in these colder zones will kill off the plant before spring.
The exceptionally hard coating of a bluebonnet seed is an adaptation that allows seeds to persist through adverse seasonal conditions such as drought or extreme cold. Bluebonnets’ seed pods shatter and explode, which serves to cast the plant’s own seeds at some distance from the parent plant. In nature, seed germination is sporadic, taking place over time as the elements and fluctuations in temperature weaken the tough seed coat. Commercially available seed has been scarified, or pre-treated with an acid or chemical bath to artificially soften the seed coat, which improves germination rates for a better spring flower display. Seed should be planted from October to mid-November in most areas, and it can take as few as 15 days to germinate for treated seed, or as many as 75 days for untreated seed.
Texas bluebonnets need abundant sunshine, between eight and 10 hours daily, and they prefer to grow on well-drained slopes or sandy meadows of moderate alkalinity. They will not tolerate clay soils and should be grown in raised beds amended with compost and sand in areas with heavy soils. As lupines are drought-adapted, do not overwater them; this can lead to fungal diseases or drowning of the plant.