As most gardeners know, clearing an area of weeds without installing a new plant is an open invitation to more unwanted plants. Planting an area of open soil with a fast-growing ground cover that draws nitrogen from the air to feed itself and the surrounding plants is an ideal alternative to leaving the soil bare. Several different plants can fill this requirement. One of the fastest growing plants for this use is a native Hawaiian plant called nanea, or beach pea.
Known botanically as Vigna marina, this flowering ground cover is an ideal choice for sites at lower elevations, even in sandy soil near to or on the beach. Though it will grow well up to 1,500 feet, the non-native perennial peanut has similar characteristics and might be a better choice in areas that lack the dry heat and good soil drainage found below 500 feet elevation.
The genus Vigna includes about 150 species native to tropical locales. The genus was named to honor the doctor and horticulturalist Dominico Vigna who was a botany professor at the University of Pisa in Italy and died in 1647. All Vigna species are in the Fabaceae (bean) family and several are important food crops. Both adzuki and mung beans are close relatives of Vigna marina or nanea but are not usually grown commercially in Hawaii. Other close relatives include the black-eyed pea (V.unguiculata) and the yard-long bean (V. sespuipedalis), which are often grown here.
Unlike its close relatives, nanea (V. marina) is well-suited to coastal or lower elevation landscapes. Though two other Vigna species are native to Hawaii, one is apparently extinct and the other is rare. The Latin species name for nanea is marina, referring to its seaside preference. The plant serves well as a fast-growing ground cover that gathers nitrogen from the air and sequesters it in root nodules. The stored nitrogen provides nanea with nourishment and releases some nitrogen into the soil to feed nearby plants. Though nanea produces pods that contain beans, they are not usually eaten.
The beach pea is indigenous to Hawaii but is also common in coastal vegetation throughout the tropics. It can sprawl over rocks or sand along the coast or cover cliffs at slightly higher elevations. Anywhere in the lowlands, nanea can be grown on trellises or fences, can help control erosion on hillsides or simply can be used as a ground cover or accent plant. The plant is drought and salt tolerant, making it easy to care for at a low elevation.
Nanea is a short-lived herbaceous plant that has a vining habit and will usually thrive in appropriate locations for two to four years. During that time, the original plant will probably re-seed itself or you can easily grow new plants from the seeds it produces.
Among the trifoliate leaves, nanea produces flower spikes with clusters of bright yellow flowers about 1-inch across. Following flowering, the plant produces 2- to 3-inch seed pods containing many small brown seeds. The green pods dry to brown or black and open, dropping mature seeds that germinate easily.
Nanea can be propagated from cuttings quickly and easily by placing them in a medium that drains easily while retaining moisture. Dipping the cuttings in a rooting hormone and placing them in a 50 percent combination of perlite and vermiculite should work well. The seeds can also be used to produce new plants. To propagate from the seeds, remove them from the plant as soon as the pods are dry and brown and before seed weevils can attack them. The seeds can be stored in a dry location for quite a while and will remain viable for a few years if refrigerated. For germination within a week, soak the seeds for 24 hours or scarify the seed coat prior to planting. Maintain a moist environment in a mix of perlite and potting mix by watering lightly every second day. Avoid overwatering or heavy rains as the seeds and young plants are prone to rot in overly wet environments.
Within three months nanea can be out-planted. Choose a site that is hot and sunny with soil that contains sand or drains very well. Since the plants are drought tolerant, you only need to water them well at transplanting and then water weekly if the weather is dry. The nanea vine grows well in open, sunny areas and, given room to spread, can quickly cover an area.
This plant is relatively care-free. Light fertilizing every six months or foliar feeding every six weeks will encourage flowering. Very few pests are attracted to nanea. If ants arrive, use ant stakes to control them. You can prevent seed weevil damage by collecting the seeds as soon as they are dry. Red spider mites can infest a patch of nanea, especially during the cool, dry winter months. Ladybugs and lacewings are good bio-controls for these mites or you can speed up the process by spraying with a combination of insecticidal soap and horticultural oil.
Dropped nanea seeds will likely germinate and result in volunteer plants appearing in the surrounding area. Picking the seeds as soon as they are ready can prevent the plant from getting weedy or becoming invasive. By growing a controlled number of new plants you can maintain an ongoing bed that includes young as well as mature plants. Periodically pruning the plants may be required to control growth and limit its spreading into unwanted areas.
In addition to using nanea as an attractive, nitrogen-fixing ground cover or vine, some parts of the plant have a history of medicinal use by the Polynesians. The leaves, stalk, midrib and stems were often pounded until soft and applied to cure wounds, boils and ulcers on the skin.
Regardless of your purpose or purposes, you definitely want to consider a ground cover like nanea to fill in areas of bare soil. The soil will benefit and you will help prevent weeds from returning to the area.
Amy B.H. Greenwell Ethnobotanical Garden in Captain Cook usually carries flats of nanea. Call to see what they’ve got in stock. This is a perfect plant to cover the ground in a low elevation native Hawaiian garden.
Diana Duff is a local organic farmer, plant adviser and consultant.
