Note: This article is a follow-up to “Microbes in the Garden- biological nitrogen fixation” from the 1000 Islands Master Gardener website.

The most widely-studied group of nitrogen-‘fixing’ (NF) plants comes from the Fabaceae family. This group includes agriculturally significant crops like beans, peas, alfalfa, peanuts and soybeans, and many garden-worthy forbs. The plants themselves do not actually ‘fix’ the nitrogen. Species of Rhizobia bacteria in the soil do so, housed in nodules on the plants’ roots. In order for nitrogen fixation to take place, the appropriate species of Rhizobia bacteria must be present in the soil for the species of plant grown. Gardeners may choose to buy seed inoculated with the correct bacteria species (available for certain crops), or buy the inoculant to treat the seed themselves to assure biological nitrogen fixation. Once inoculated seed has been used, the bacteria will remain in the soil for several years after planting. 

Beyond the legumes, another group of mostly trees and shrubs (Actinorhizal plants) engage in nitrogen fixation through a similar symbiotic relationship with Frankia bacteria. Both leguminous plants and the primarily woody actinorhizal species can play a significant ecological role revitalizing poor, infertile, dry or wet soils. Most are sun-loving pioneer species used in the initial stages of site restoration. Over time, they improve the soil primarily through the slow release of nutrients from the decomposition of their leaves, roots, and stems or branches. 

In general, NF plants actually perform better, and appear to be more attractive to pollinators, when grown in poor soils. The amounts of nitrogen they ‘fix’ varies between plant species, with some plants like Alders (see photo below) at the medium to high range (All Nitrogen Fixers are Not Created Equal, Perennial Solutions). Nitrogen-fixation levels also vary depending on soil fertility levels (nodules may not form at all in nitrogen-rich soils) and on other adverse growing conditions. NF plants are often excellent pollinator plants, and NF native species may also be excellent larval host plants for butterflies and moths.

Nitrogen fixing cover crops 

Using a cover crop from the Fabaceae family like winter field peas or red clover is an excellent way to elevate soil nitrogen levels without chemical fertilizers as long as the crop is incorporated back into the soil before flowering. More information about cover crops is found in: Cover crops: Why plant a crop you won’t eat

A few garden-worthy nitrogen fixers from the Fabaceae family

Nitrogen-fixing plants provide shelter, habitat and food for wildlife; they are important nectar and/or pollen sources for our native bees, butterflies, moths and other beneficial insects. Many leguminous plants’ pollen is a favourite of bumble bees. Many are also important lepidopteran larval host plants. They are also valuable for more sustainable agriculture, forestry, land reclamation and agroforestry, increasing soil fertility naturally. The following are nitrogen fixers you may want to try in the right location.

The annual Chamaecrista fasciculata (Partridge Pea) is native to Eastern North America. It has showy yellow flowers from July-September and lovely fine, fern-like leaves that are sensitive to the touch. It’s an excellent species for disturbed sites, establishing easily, fixing nitrogen, self-sowing from one year to the next and ultimately disappearing when later successional perennial species move in. However, because it self-sows prolifically, it can become weedy. Like some other members of the Fabaceae family, it is buzz-pollinated, and is a popular bumble bee plant. It is an excellent nectar plant, unusual for its extrafloral nectaries located at the base of each leaf in small orange glands. Importantly, these attract beneficial insects.

Senna hebecarpa (Wild Senna) is a perennial with some similar characteristics to Partridge Pea. It is a much taller, architectural plant (1.5- 2m) with late-summer, yellow, flowers that offer a nutrient-rich pollen bumble bees prefer for their larvae. Like the Partridge Pea, Wild Senna has its nectaries on the leaf petioles. If growing in part shade, stake the plant, as the flowers are heavy.

These two Lespedeza species pictured above, like many other dry soil NF plants, are deep-rooted and perfect for soil stabilization and roadside restoration. L. capitata is an Ontario native, found in dry open woods, prairies and sand dunes. Given its strong, erect stems and brown seed heads, it shows well in the winter garden. It is an excellent pollinator plant, and larval host plant. L. virginiana exists in small populations in S.W. Ontario and was designated endangered in 1986. Both species are medium nitrogen fixers, and fairly long-lived (10-20 yrs).

Astragalus canadensis (Canada Milk Vetch) has a sprawling habit suitable for larger meadow gardens. It is often used for erosion control in dry, poor soils. It is an excellent food plant for wildlife, from bees to hummingbirds to deer, rabbits, and birds feeding on the seeds. 

Lupinus perennis (Wild Lupine) pictured below, an ‘at risk’ Ontario native, is the larval host plant of three extirpated butterflies: the Karner Blue, the Eastern Persius Duskywing, and the Frosted Elfin. It is  an important nectar source for native bees, butterflies and hummingbirds. Blooming in spring when bumble bee queens are emerging, Wild Lupine has in the past been one of their important sources of nectar and pollen. Found in areas with sandy soils (savannahs, oak barrens, dunes), Wild Lupine ecosystems require controlled burning or mowing to prevent the build-up of organic matter which threatens their populations. 

Caution: Most lupins sold in garden centers are non-native hybrids of the west coast Lupinus polyphyllus. On the east coast, these are being designated an invasive species because of their ability to spread quickly, and to significantly decrease native plant diversity. For more information see: Mixed blessings: the pros and cons of lovely lupins (CBC News)

Baptisia spp. (False Indigo)

Another beautiful NF genus in the Fabaceae family is Baptisia. There are several species native to Eastern North America that have been introduced to Ontario with white, yellow or  blue pea-like flowers, and glaucous bluish-green foliage. The most common species here are Baptisia australis (Blue False Indigo), 1-1.5 m tall,  B. minor (Dwarf False Indigo), similar to B. australis but much smaller at 60 cm tall, and B. alba (White False Indigo).  Baptisia are drought tolerant, virtually indestructible specimen or anchor plants for meadow and prairie gardens. They are long-lived (20 +years), and are important sources of nectar and pollen, especially for larger bees like the bumble bee below whose size and strength are perfectly adapted to push deep into the flower to access its floral rewards. As its common name ‘indigo’ suggests, Baptisia is an important North American dye plant.

To learn why Baptisia pollination is especially fascinating: Bumble Bees and Baptisia: A Pollination Story (Xerces Society)

Baptisia are very promiscuous, and there are many bi-specific hybrids available in garden centers. Information on some of these hybrids can be found in the Mount Cuba Baptisia Trial. Criteria used in their evaluation include growth habit, floral display, disease resistance, hardiness, and foliage quality. Some hybrids are sterile, so if you want the pretty seed heads, consult the plant patent for more information before purchase.

Actinorhizal nitrogen fixers

Historically, larger, woody, nitrogen-fixing plants have played a significant role in creating shelterbelts and windbreaks, and as sources of food, lumber and fuel. Their contributions, in some cases, however, have led to ecological challenges when non-natives are planted. Such is the case with invasive Elaeagnus umbellata (Autumn Olive) and Robinia pseudoacacia (Black Locust). The first two NF actinorhizal examples pictured below are excellent food plants, but the first suckers (as noted) and requires more space than a small urban yard provides. As they are both non-native, thrive in most soils, and fruit prolifically, care is needed to be sure they do not spread beyond your property. The third shrub (Bayberry) is native but rare in Ontario and is becoming more popular in the urban landscape.

Shepherdia argentea (Silver Buffalo Berry), a native of Central and Western North America, is a fairly long-lived (25 years), slow-growing shrub  which produces nutritious red berries high in anti-oxidants. It requires space, as it forms thickets up to 6 m tall, is a medium nitrogen-fixer, and a good plant for infertile, difficult sites. It is very cold and drought tolerant. This plant is dioecious, so both male and female plants are needed for fruiting.

Elaeagnus multiflora (Goumi Berry) is native to Korea, China, and Japan. It too is a medium nitrogen-fixer, and grows in difficult nutrient-poor sites in sun to part shade. It is also salt and wind tolerant. A shorter, more compact shrub  (3m), Goumi is more popular in urban settings. It lives up to 20 years, and has fragrant, late spring, flowers followed by small red fruits. Several species from the Main Botanic Garden in Kiev, Ukraine are available from Whiffletree Farm and Nursery

Morella pensylvanica (Bayberry) is a fairly long-lived (30 yrs) shrub that is salt-tolerant and can be used along roadways. It has glossy, aromatic leaves and beautiful waxy grey berries loved by songbirds when not harvested by humans making bayberry candles or soaps. The plant is dioecious, requiring both a male and female plant for pollination and fruit set. Northern Bayberry is native but rare in Ontario, growing along the northern shore of Lake Erie.

Last, a few years ago I was fortunate to find the NF Caragana microphylla ‘Mongolian Silver Spires’ (Mongolian Silver Spires Littleleaf Peashrub) for a hot, dry, sunny spot in extremely poor soil at the front of my house. This small tree or shrub grows to about 3.5m tall, 2m wide, lives for about 30 years, and is extremely cold-tolerant like its Caragana relatives. It has greyish-silver foliage, and beautiful, yellow pea-like flowers along the branches in spring, followed by reddish, narrow, seed pods. It gets no supplemental water during dry periods, is easily pruned to fit its current location, and as yet, has not self-sown.