Purple Bushbean: A Fast-Growing Legume for Warm Climate Gardens
If you’re looking for a quick-growing ground cover that can handle tough conditions while adding a splash of purple to your landscape, purple bushbean (Macroptilium atropurpureum) might catch your attention. This vigorous legume has quite the personality – it’s a climber, a spreader, and a nitrogen-fixer all rolled into one herbaceous package.
What Exactly Is Purple Bushbean?
Purple bushbean is a perennial forb that behaves more like an enthusiastic friend who never knows when to stop talking. This climbing plant can reach up to 4 feet in height and forms dense thickets with its sprawling growth habit. Despite its common name, you might also see it listed under its former scientific name, Phaseolus atropurpureus, in older gardening references.
The plant produces fine-textured, dark green foliage that creates a lush backdrop for its conspicuous purple flowers. These blooms appear during spring and summer, followed by brown seeds that add another layer of visual interest to your garden.
Where Does It Grow Naturally?
Purple bushbean has a somewhat complex native status. It’s considered native to parts of the lower 48 states, Puerto Rico, and other tropical regions, but it’s also established as a non-native species in Hawaii where it reproduces freely in the wild. Currently, you can find it growing in Arizona, Florida, Hawaii, Texas, and Puerto Rico.
The Good, The Bad, and The Purple
Let’s start with the positives – and there are plenty! Purple bushbean is a nitrogen-fixing legume, which means it actually improves your soil while it grows. It’s also remarkably adaptable, tolerating various soil types from coarse to fine textures and handling pH levels anywhere from 5.0 to 8.0. The plant shows medium drought tolerance and can thrive with relatively low water use once established.
The rapid growth rate (we’re talking about a plant that can go from zero to full coverage pretty quickly) makes it excellent for erosion control on slopes or as a living mulch in permaculture systems. Plus, those purple flowers aren’t just pretty – they attract bees and other beneficial pollinators to your garden.
However, this enthusiasm for growth can be a double-edged sword. Purple bushbean’s thicket-forming habit means it can potentially crowd out other plants if not managed properly. While its invasive status isn’t clearly documented, its vigorous nature suggests you’ll want to keep an eye on it and possibly do some periodic trimming to keep it in bounds.
Growing Conditions and Care
Purple bushbean is surprisingly low-maintenance for such a vigorous grower. Here’s what it needs to thrive:
- Climate: Warm regions only – it needs a minimum of 365 frost-free days per year and can’t handle temperatures below 23°F
- Sunlight: Full sun to partial shade (intermediate shade tolerance)
- Soil: Adaptable to most soil types with medium fertility requirements
- Water: Low to moderate moisture needs once established
- USDA Zones: 9-11
Planting and Propagation
Getting purple bushbean started in your garden is relatively straightforward. The plant is routinely available commercially and can be propagated through containers or cuttings. Interestingly, despite being a legume, seed propagation isn’t the preferred method for this species.
When planting, space plants according to your intended use – anywhere from 1,200 to 1,700 plants per acre for large-scale plantings, though home gardeners will obviously need far fewer plants. The seedling vigor is high, so once established, expect rapid growth during the spring and summer active growing period.
Landscape Uses
Purple bushbean works well in informal landscape settings where you want quick coverage. Consider it for:
- Erosion control on slopes
- Ground cover in naturalized areas
- Living mulch in food forests or permaculture gardens
- Nitrogen-fixing companion plant in sustainable landscaping
Wetland Tolerance
One interesting characteristic of purple bushbean is its varying relationship with wetlands depending on location. In most regions (Arid West, Atlantic and Gulf Coastal Plain, and Great Plains), it’s classified as facultative upland, meaning it usually prefers non-wetland conditions but can tolerate some moisture. In the Caribbean, it’s obligate upland, while in Hawaii, it can handle both wetland and non-wetland conditions.
Should You Plant It?
Purple bushbean can be a useful addition to warm-climate gardens, especially if you need quick erosion control or want to add nitrogen to your soil naturally. However, given its vigorous growth habit and non-native status in some regions, it’s worth checking with your local extension office or native plant society before planting to ensure it’s appropriate for your area.
If you’re in a region where purple bushbean isn’t recommended, consider native alternatives that provide similar benefits – nitrogen-fixing legumes, ground covers, or erosion control plants that are indigenous to your specific area. Your local native plant society can point you toward species that will provide the same garden benefits while supporting local ecosystems.
Remember, the best gardens work in harmony with their local environment, so whatever you choose to plant, make sure it’s a good neighbor to the plants and wildlife already calling your area home!
