From our perspective as Lupinus species, often called lupines, we are not just a pretty floral display. We are powerful allies in building soil health, and using us as a green manure is one of the most beneficial roles we can play in your garden ecosystem. Here is how you can partner with us to achieve this.
We thrive in conditions that many other plants find challenging. We prefer well-drained soil and a sunny location. While we can tolerate poorer soils, our true potential is unlocked there. You should sow our seeds directly into your garden bed in the early spring or, in milder climates, in the autumn. Scatter the seeds and rake them in lightly. We appreciate being watered in, but once established, we are quite drought-tolerant. It is crucial to remember that we form a symbiotic relationship with specific soil bacteria (Rhizobia). To ensure this partnership begins successfully, you can dust our seeds with a legume inoculant before planting. This simple step is the key to our nitrogen-fixing power.
As we grow, our primary mission unfolds beneath the soil surface. Our roots seek out the specific Rhizobia bacteria. Once we form a partnership, we develop small, white nodules on our root systems. Inside these nodules, a miraculous exchange occurs: we provide the bacteria with carbohydrates, and in return, they capture atmospheric nitrogen—a nutrient most plants cannot access—and convert it into a form we can use, ammonium. This process is called nitrogen fixation. Above ground, our deep taproots act as biological tillers, breaking up compacted soil layers, improving aeration, and bringing up nutrients from the subsoil that are unavailable to shallow-rooted plants.
The most critical decision you will make is when to end our growth cycle. The ideal moment is just as we begin to flower. At this stage, we have accumulated the maximum amount of vegetative growth and, consequently, the maximum amount of nitrogen in our tissues. If you wait until we form hard seeds, we will have redirected much of that valuable nitrogen into seed production, making it less available to the soil. To terminate us, simply cut or mow us down at the soil line. Alternatively, you can use a technique called "crimping," where you break our stems without severing them, which speeds up decomposition.
After termination, our foliage, known as biomass, should be left on the soil surface as a mulch or lightly turned into the top few inches of soil. Do not bury us deeply, as this can slow down decomposition. Once incorporated, soil microorganisms, earthworms, and other decomposers begin to break down our plant matter. This process releases the stored nitrogen and other nutrients back into the soil, creating a rich, slow-release fertilizer for your next crop. Our deep roots also decompose in place, adding valuable organic matter deep within the soil profile, which improves its structure and water-holding capacity for seasons to come.
