The lima bean, Phaseolus lunatus, is a member of the legume family (Fabaceae) cultivated globally for its protein-rich, edible seeds. Although some varieties can behave as perennials, the lima bean is typically grown as a warm-season annual plant, completing its entire life cycle from seed to mature seed within a single growing season.
From Seed to Sprout: Germination
The life cycle begins when the seed absorbs moisture (imbibition), softening the protective seed coat. Germination requires sufficient warmth, ideally a soil temperature above 60°F, along with oxygen and consistent moisture. After seven to eighteen days, the embryo emerges.
The first structure is the radicle, the embryonic root, which grows downward to anchor the plant. Next, the shoot emerges, driven by the elongation of the hypocotyl, pulling the two large seed leaves, or cotyledons, out of the soil (epigeous germination). These cotyledons provide stored starches and proteins, fueling the seedling until its true leaves begin photosynthesis.
Developing Structure: Vegetative Growth
Once the cotyledons are above ground, the seedling begins its vegetative growth phase, establishing a robust structure of stems, leaves, and roots. The first true leaves expand and begin to harness sunlight, converting light energy into chemical energy through photosynthesis. This rapid growth period is characterized by the development of trifoliate leaves, which are compound leaves composed of three leaflets.
A distinguishing feature of legumes is biological nitrogen fixation, carried out by specialized soil bacteria, primarily Bradyrhizobium. These microbes colonize the roots, forming nodules where they convert atmospheric nitrogen gas into ammonia. This symbiotic relationship provides the plant with a continuous supply of nitrogen, supporting vigorous growth.
Flowering and Pod Formation: Reproduction
The plant transitions from vegetative to reproductive growth as it reaches maturity, beginning to flower. Small, delicate flowers, often white or violet, appear in loose clusters along the stem. Lima beans are highly self-pollinating; fertilization occurs within the same flower before it fully opens, minimizing the need for external pollinators.
Successful self-fertilization triggers the development of the ovary, which forms the characteristic flat, slightly curved pod. The ovules inside develop into the seeds, typically between two and four per pod. This phase is sensitive to environmental stress and requires consistent moisture; drought conditions can cause the plant to drop flowers or young pods, impacting the final yield.
Maturity and Seed Saving
The final stage of the life cycle is the maturation of the pods and seeds. Harvesting can occur at two distinct points: the shelling stage or the dry stage. For fresh consumption, pods are harvested when they have changed color and the beans inside are plump, green, and tender.
If the goal is to save seed for the next season, the pods must remain on the plant until they reach the dry stage. At this point, the pods become brittle and pale, and the seeds inside are hard with significantly reduced moisture content. Collecting these fully mature, dry seeds completes the reproductive cycle.