Perennial Soybean: A Hardy Ground Cover for Tropical Gardens
If you’re looking for a tough, low-maintenance ground cover that can handle challenging conditions while actually improving your soil, perennial soybean (Neonotonia wightii) might just be the unsung hero your garden needs. This resilient legume brings practical benefits to the landscape, even if it won’t win any beauty contests.
What Is Perennial Soybean?
Perennial soybean is a hardy herbaceous perennial that grows as a sprawling ground cover, reaching about 2.5 feet in height. Unlike its annual cousin that we know from dinner plates, this perennial variety is all about longevity and function. It features the characteristic three-leaflet compound leaves typical of the legume family and produces small, inconspicuous white flowers followed by small black seeds.
Native Status and Geographic Distribution
Originally hailing from tropical Africa, perennial soybean is a non-native species that has naturalized in Hawaii, where it reproduces spontaneously in the wild. While it’s established itself quite successfully in the Hawaiian Islands, it’s worth noting that this adaptable plant has found its niche without causing major ecological disruptions.
Why Consider (or Not Consider) Perennial Soybean
The Good:
- Excellent nitrogen-fixing abilities that actually improve soil fertility
- Outstanding drought tolerance once established
- Rapid regrowth after cutting or grazing
- Thrives in various soil types from sandy to clay
- Low maintenance once established
- Effective for erosion control on slopes
The Not-So-Good:
- Limited ornamental value – it’s more function than form
- Can be aggressive in spreading (though not classified as invasive)
- Requires consistently warm temperatures (minimum 50°F)
- May not suit formal garden designs
Garden Role and Landscape Use
Think of perennial soybean as your garden’s hardworking intern – it may not be flashy, but it gets the job done. It excels as:
- Ground cover for large, informal areas
- Erosion control on slopes and banks
- Living mulch in food forests or permaculture designs
- Soil improvement cover crop
- Low-maintenance option for challenging sites
Growing Conditions and Care
Perennial soybean is refreshingly unfussy about most growing conditions. Here’s what it prefers:
Climate: USDA zones 9-11, requiring at least 350 frost-free days per year
Soil: Adaptable to coarse, medium, or fine-textured soils with pH between 5.5-7.8. Moderate fertility requirements and medium calcium carbonate tolerance.
Water: Medium water needs with excellent drought tolerance. Can handle annual precipitation ranging from 15-80 inches.
Light: Intermediate shade tolerance, but performs best in full to partial sun.
Planting and Establishment
Getting perennial soybean started is straightforward:
- Plant from seed (about 58,000 seeds per pound) or container plants
- Seeding rates range from 1,000-40,000 plants per acre depending on desired coverage
- Spring planting typically works best
- Seeds have good persistence and medium abundance
- Expect moderate growth rates with medium seedling vigor
Wildlife and Pollinator Benefits
While perennial soybean’s small white flowers aren’t particularly showy, they can provide some nectar for small pollinators. However, if supporting pollinators is your main goal, you’d be better served by more specialized native flowering plants.
Consider Native Alternatives
If you’re in Hawaii and want to support native ecosystems, consider these indigenous alternatives for similar functions:
- Native Hawaiian legumes for nitrogen fixation
- Indigenous ground covers for erosion control
- Native plants suited to your specific island and elevation
The Bottom Line
Perennial soybean is like that reliable friend who always shows up when you need help moving – not the life of the party, but incredibly useful to have around. If you need a no-fuss ground cover that improves soil while handling tough conditions, and you’re in a tropical climate, this plant delivers. Just remember that with its non-native status, it’s worth exploring native alternatives that might provide similar benefits while supporting local ecosystems.
For ornamental appeal, look elsewhere. For practical, soil-improving ground cover that won’t quit, perennial soybean might just earn a spot in your landscape toolkit.
