Nitrogen Fixation - How it Works and a Look at Some Super Nitrogen Fixing Trees, Shrubs and Herbs

An essential component of any regenerative landscape will be the Nitrogen-fixing perennial plants within the community of fruits, nuts and herbs and other plants.

During this post, we will look at why Nitrogen is important for plants, How Nitrogen can be biologically sourced and we'll profile some of our favorite Nitrogen-fixing trees, shrubs and herbs

Nitrogen Fixing Trees, Shrubs and Herbs for Permaculture and Polyculture

Nitrogen is an essential nutrient for plant growth and development and although around 78% of the earth's atmosphere is nitrogen, plants cannot utilize this. Plants instead depend upon combined or fixed forms of nitrogen, such as ammonia and nitrate. Currently, the majority of this nitrogen is provided to cropping systems in the form of industrially produced nitrogen fertilizers. The use of these fertilizers has led to worldwide ecological problems, such as the formation of coastal dead zones, and requires a high energy input to produce. Biological nitrogen fixation, on the other hand, offers a natural means of providing nitrogen for plants. 

Legume (aka Pulse Crop) in association with Rhizobium bacteria.   

Biological Nitrogen fixation is an important component of regenerative agriculture, organic gardening/farming, forest gardening, and other polyculture practices. Through a partnership with micro-organisms in their roots, some plants can turn atmospheric nitrogen into nitrogen fertilizers useful to themselves but also becoming available to their neighbours over time through root dieback, leaf fall, and chop and drop pruning. These are known as the nitrogen-fixing plants.

This is a mutually beneficial relationship with the plant providing carbohydrates obtained from photosynthesis to the microorganism and in exchange for these carbon sources, the microbes provide fixed nitrogen to the host plant.
While it does not replace the need to bring in other nutrients depleted by harvests such as phosphorus and calcium, nitrogen fixation provides a valuable biological source of an essential fertilizer.

There are two main groups of microbes that plants associate with in order to utilize atmospheric nitrogen to fuel growth. They are  Frankia and Rhizobium.

Frankia

Many plants partner with micro-organisms called Frankia, a group of Actinobacteria. These plants are known as actinorhizal nitrogen fixers.

Frankia can be seen above as the nodules forming around the roots of one of our Elaeagnus umbellata saplings in our nursery.

Actinorhizal plants are found in many ecosystems including alpine, xeric, chaparral, forest, glacial till, riparian, coastal dune, and arctic tundra environments and can be found in the following plant families  

• Betulaceae, the birch family.

• Myricaceae, the bayberry family.

• Casuarinaceae, the Austrian “pines”.

• Elaeagnaceae, the oleasters.

• Rosaceae, the rose family.

• Rhamnaceae, the buckthorn family.

These plants tend to thrive in nitrogen-poor environments and are often the pioneer species in plant communities playing an important role in plant succession.

Rhizobium

By far the most important nitrogen-fixing symbiotic associations are the relationships between legumes (plants in the family Fabaceae) and Rhizobium and Bradyrhizobium bacteria. These plants are commonly used in agricultural systems such as alfalfa, beans, clover, cowpeas, lupines, peanuts, soybean, and vetches. 
The Rhizobium or Bradyrhizobium bacteria colonize the host plant’s root system and cause the roots to form nodules to house the bacteria. The bacteria then begin to fix the nitrogen required by the plant. Access to fixed nitrogen allows the plant to produce leaves fortified with nitrogen that can be recycled throughout the plant. This allows the plant to increase photosynthetic capacity, which in turn yields nitrogen-rich seed.

Rhizobium colonies, clearly seen as nodules on the plant roots of Spartium junceum

So now you know what nitrogen fixation is, let's take a look at some of my favorite Nitrogen Fixing Trees - Shrubs and Herbs that we use throughout our polyculture gardens.

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Nitrogen Fixing Trees

Italian Alder - Alnus cordata 

Alnus cordata - Italian Alder - Nitrogen Fixing Tree

Overview: Alnus cordata  - Italian Alder is a deciduous tree that grows up to 25m at a fast rate. It has a long season in leaf - from April to December - and is in flower in March. Has a heavy leaf canopy and when the leaves fall in the winter, they help to build up the humus content of the soil. The species is monoecious and is pollinated by wind. Thrives on poor and dry soils, even sometimes on chalk, but prefers to be near water.

Uses: Windbreak, pioneer species for re-establishing woodlands, ornamental, biomass production, coppice, ornamental. The timber has a red/orange appearance and is used for turning and carving.

Nitrogen Fixing Potential: Level of fixation is not specified on the USDA website.

Biodiversity: The catkins of Italian Alder provide early pollen for insects, and the over-wintering cones are a good food source for birds.

Propagation: Seed germination rates are generally high. Transplant into permanent positions when growth has reached a suitable level.

Black Locust - Robinia pseudoacacia 

Robinia pseudoacacia - Black Locust

Overview: Robinia pseudoacacia - Black Locust is a rapidly growing, deciduous tree that is native to North America. This member of the pea and bean family (Leguminosae) has now naturalized in many parts of Europe. It is in flower in June, and the seeds ripen from November to March. The species is hermaphrodite.  It's a good tree for establishing on degraded land but can become invasive due to its prolific seed production, and it also spreads by suckering from the roots. Prefers well-drained soil and sunny positions.

Uses: Shelterbelts and windbreaks, restoration projects due to extensive root system, dynamic accumulator, ornamental. The wood is very durable and rot-resistant, good for posts and beams in construction. Essential oil is obtained from flowers and used in perfumery.

Nitrogen Fixing Potential: The species is classified by USDA as being a MEDIUM Nitrogen fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m²  or 0.014g /m2.

Biodiversity: The pretty flowers open in May or June after the leaves have developed and are attractive to a wide range of pollinators, including bees that produce  "Acacia" honey. Noted for attracting a wide range of insects. 

Propagation: Pre-soak the seed for 48 hours in warm water and sow in late winter in a cold frame. Prick out the seedlings into individual pots when ready and grow them on in the greenhouse for their first winter.  Plant into their permanent positions the following summer. For cuttings, use small new-growth branches at least 8 inches long with a leaf node near the cut. Can take up to 3 months for roots to develop.

Sea Buckthorn - Hippophae rhamnoides

Hippophae rhamnoides -  Sea Buckthorn

Overview: Hippophae rhamnoides -  Sea Buckthorn is a deciduous thorny shrub growing to around 6m in height. It is in flower in April, and the seeds ripen from September to October. Provides an abundance of highly nutritious orange berries in the autumn. The species is dioecious and is pollinated by wind. Requires a sunny position and does very well in sandy soils. Fairly slow growing and plants produce abundant suckers.

Uses: Shelter hedge, Pioneer species to reestablish woodlands, Maritime exposure tolerated - stabilises sand dunes. The wood of Sea Buckthorn is tough, hard, very durable, fine-grained, and used for fine carpentry and turning.

Nitrogen Fixing Potential: The species is classified by USDA as being a MEDIUM Nitrogen fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m²  or 0.014g /m2

Biodiversity: Provides shelter and in the autumn, berries for birds. The ripe fruits also attract insects, and therefore birds that feed on insects such as thrips, earwigs and mites. Deer, mice and other rodents may also feed on sea buckthorn, and bees and hoverflies are attracted to the flower's nectar.

Propagation: Sow seed in spring in a sunny position in a cold frame. Germination is usually quick and successful, although 3 months of cold stratification may improve the germination rate further. Once big enough, transplant into individual pots and grow on in a greenhouse for the first winter. Take cuttings at the end of autumn or very early in the spring before the buds burst. Store them in sand and worm casting mix until April, then cut them into 7 - 9cm lengths and plant them in a sunroom or with plastic bags and bottom heat.

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Nitrogen Fixing Shrubs 

Elaeagnus umbellata - Autumn Olive

Elaeagnus umbellata - Autumn Olive 

Overview: Elaeagnus umbellata - Autumn Olive  - A large deciduous shrub from E.Asia, growing 4.5 m high and 4.5 m wide, tolerates part shade, very drought tolerant. Branches are often thorny, and leaves are bright green and silvery beneath. Yellowish-white, fragrant flowers are produced in May-June, followed by rounded silvery brown (ripening red) fruits in Sep-Oct. Sometimes cultivated for its edible fruit. There are many named cultivars. Flowers are rich in nectar and very aromatic. Plants can fruit in 5 yrs from seed. This specie is considered invasive in the U.S.

Uses:  Hedging plant, ornamental, and tolerates maritime exposure succeeding in the most exposed positions.  The plant is used as a nurse tree, when planted with fruit trees it is reported to increase the overall yield of the orchard by 10%. It can also be grown as a biomass crop on a 3-year rotation.

Nitrogen Fixing Potential: The species is classified by USDA as being a MEDIUM nitrogen fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m² or 0.014g /m2.

Biodiversity - The shrubs will begin to flower in the 4th or 5th year after planting and are attractive to a wide range of pollen and nectar-feeding invertebrates. If you leave some fruits on the tree they provide a good source of winter food for birds. In time as the hedge thickens up with regular pruning, suitable nesting habitat will form inside the lower part of the hedge. Birds such as Wren, Chiffchaff, and Robins are commonly found in dense low hedging. These birds can help to keep common vegetable pest populations low.

Propagation: Seed - best sown as soon as it is ripe in a cold frame. It should germinate in late winter or early spring, though it may take 18 months. Stored seeds can be very slow to germinate, often taking more than 18 months. A warm stratification for 4 weeks followed by 12 weeks of cold stratification can help. The seed usually (eventually) germinates quite well. Prick out the seedlings into individual pots as soon as they are large enough to handle and plant out when they are at least 15 cm tall. 

Caragana arborescens - Siberian Pea Tree

Caragana arborescens - Siberian Pea Tree

Overview: Caragana arborescens - Siberian Pea Tree - A deciduous shrub originating from Central Asia belonging to the Fabaceae (legume) family growing to 5-6m high and 4m wide with an upright habit. It grows vigorously. Flowers are borne from buds on the previous year's wood and are typical of flowers from this family. Flowering occurs in May. Pollination is via bees, usually wild bumble bees. Pods develop from flowers - looking like small pea pods, they are 4-5 cm long. The pods ripen to amber or brown from June -July onwards and seeds fall by August. The plant is extremely hardy tolerating winter temperatures of -40. Prefers a continental climate with hot dry summers and cold winters.

Uses:  Windbreaks and shelter belts, wildlife-erosion control plantings, Extensive root system that stabilizes the soil. Plants make good wildlife fodder and can be used to as poultry food. A fiber is obtained from the bark and used for rope making.

Nitrogen Fixing Potential: The species is classified by USDA as being a MEDIUM Nitrogen fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m²  or 0.014g /m2

Biodiversity - The shrubs will begin to flower in the 4th or 5th year after planting and are attractive to a wide range of pollen and nectar-feeding invertebrates from April - May.
In time as the hedge thickens up with regular pruning, suitable nesting habitat will form inside the lower part of the hedge. Birds such as Wren - Troglodytes troglodytes, Chiffchaff - Phylloscopus collybita and Robin - Erithacus rubecula are commonly found in dense low hedging. These birds can help to keep common vegetable pest populations low.

Propagation: Seed propagation is the norm. Seeds germinate better after a short period of stratification and/or soaking in warm water prior to planting.

Cytisus scoparius - Broom 

Cytisus scoparius - Broom 

Overview: Cytisus scoparius - Broom ​ A hardy Nitrogen fixing shrub native to Europe growing to 2.4 m by 1 m at a fast rate. Its bright yellow flowers appear in spring, from May to June, and attract a range of invertebrates. A versatile plant is well suited to many soil types that can grow in nutritionally poor soil. Prefers a sunny position but tolerates some shade and will succeed in exposed conditions including maritime exposure. A deep root system means they are very drought tolerant once established and grow well on dry banks. Very tolerant of cutting, it regenerates quickly from the base.

Uses:  Ornamental, Maritime exposure tolerated - stabilizes sand dunes. Urban plant - tolerates atmospheric pollution.  Essential oil from the flowers is used in perfumery. Great plants for fiber, basketry, and good brooms (hence the common name!).  You can either place stems on the ground and go over them with a rotary lawn mower to break the biomass into smaller pieces or leave them as rough mulch.

Nitrogen Fixing Potential: The species is classified by USDA as being a HIGH Nitrogen fixer with estimated yields of +160lbs/acre or +72kg/4050m² or 0.018g /m2.

Biodiversity - A good bee plant and also a good food plant for many caterpillars - it reportedly provides the food for the larvae of the green hairstreak butterfly. Ants are attracted to the seeds, feeding on the juicy attachment that holds them to the pods and thus distributing the seed. 

Propagation: The plant is very easy to grow from seed and large quantities of plants can be grown very quickly. Seed harvested in the summer can be sown straight after picking and overwintered indoors (or protected and planted out the following autumn). Seeds germinate better after soaking in warm water for 8-12 hrs prior to planting.

Elaeagnus angustifolia - Oleaster, Russian Olive

Elaeagnus angustifolia - Russian Olive

Overview: Elaeagnus angustifolia - Russian Olive - A deciduous large shrub or small tree from Europe and W.Asia, growing approx 7m high and 7m wide. Tolerates part shade, salt, and air pollution. It has silvery branches often thorny, with silvery scales when young, silvery willow-like leaves, silvery flowers in June, and yellowish-silvery fruits ripening in October. Plants prefer continental climates.  This species is often cultivated in Europe and Asia for its edible fruits (there are many named varieties some of which are thornless). The plants begin to flower and fruit from three years old. It is very tolerant of pruning even right back into old wood. The flowers are sweetly scented. Fruits hang on the plant for much of the winter providing a valuable source of winter food for birds. The fruit is readily eaten and disseminated by many species of birds. This species is considered invasive in the United States.

Uses: Hedging plant (NB does not form a dense screen), biomass crop, tolerates maritime exposure. Edible fruit -raw or cooked as a seasoning in soups, and the expected fruit yields are 7-9kg per plant. The taste is dry sweet and mealy. The seed oil, flowers, and leaves are used medicinally.  An essential oil obtained from the flowers is used in perfumery. Leaves are used as goat and sheep fodder. The wood is hard, fine-grained, and used for posts, beams, carving, and domestic items and makes good fuel.

Nitrogen Fixing Potential: This specie is classified by USDA as being a HIGH nitrogen fixer with estimated yields of 160+ lbs/acre or 72>kg/4050m²

Biodiversity:  When in flower the plants are attractive to a range of pollinators and I've often observed our plants teeming with flying insects during the flowering period. When trimmed the plants will ramify well and can form a dense hedge-like appearance. The interior of the plants in this condition is perfect for nesting birds and for small mammals and lizards to retreat into when under threat.

Propagation: Establishment and reproduction of Elaeagnus angustifolia are primarily by seed, although some spread by vegetative propagation also occurs. Cold stratification of the seed is required for 30-60 days. Sowing when fruits are ripe will probably provide the best germination results.

Nitrogen Fixing Herbs 

Trifolium repens -White Clover 

Trifolium repens - White Clover 

Overview: Trifolium repens - White Clover - White clover is a dwarf, prostrate, mat-forming perennial that can spread via stems that freely root along the ground at the nodes. Easily grown in average, medium, well-drained soils in full sun to part shade. Prefers moist soils in light shade, but tolerates full sun and moderately dry soils.

Uses:  White clover has been described as the most important forage legume of the temperate zones. Besides making an excellent forage crop for livestock,  clovers are a valuable survival food: they are high in proteins and although not easy for humans to digest raw, this is easily fixed by boiling the harvested plants for 5–10 minutes. Dried flower heads and seedpods can also be ground up into nutritious flour and mixed with other foods or can be steeped into an herbal tea. The plant's ability to spreads aggressively by creeping stems makes it a good ground cover plant.  The plant is also used as a companion plant when undersown with cereals or tomatoes.

Nitrogen Fixing Potential:  The species is classified by USDA as being a HIGH Nitrogen fixer with estimated yields of +160lbs/acre or +72kg/4050m² or 0.018g /m2.

Other sources state up to 545 kg of N per hectare per year is possible.

Biodiversity: The plants provide a source of nectar and pollen for a number of native bees as well as the honey bee.

Propagation: Best propagated by seed.  Pre-soak the seed for 12 hours in warm water and then sow in spring in situ. Division is also possible in the spring and autumn. 

Planting Material: For covering an area quickly seed is the best option.

Onobrychis viciifolia - Sainfoin

Overview: Onobrychis viciifolia - Sainfoin is a perennial herbaceous legume. It has deep tap-roots which are helpful in harvesting minerals from the subsoil. It's in flower from June to August and is hermaphrodite. Sainfoin flowers are pink, and attractive and start blooming with the lower flowers, then moving up the stem. Produces a good bulk of foliage, and makes an excellent green cover/manure. Prefers a well-drained neutral to alkaline sandy loam in full sun and loves full sun.

Uses: Green manure, soil stabilizer (due to deep tap root), companion plant. Grown for pasture, hay or silage since it is very palatable to livestock.

Nitrogen Fixing Potential:  The species is classified by USDA as being a MEDIUM Nitrogen fixer with estimated yields of 85-160lbs/acre or 39-72kg/4050m²  or 0.014g /m2

Biodiversity: Sainfoin produces Nectar and Pollen for Honeybees and Bumblebees. This nectar is thought to be one of the highest-yielding honey plants. The flowers of Sainfoin attract huge numbers of insects and some reports claim Sainfoin may attract up to ten times more bees than white clover. . Deer enjoy sainfoin, and some game birds such as turkey and pheasants shelter among the plant.

Propagation: For the best results pre-soak seed for 12 hours in warm water and sow in situ in the spring

All of the plants mentioned above and more are available from our bio nursery as plants or seeds and some species are available at special discounts for bulk orders. You can find out more info about our plants by clicking on the plant names below.

Alnus cordata  - Italian Alder 

Albizia julibrissin - Silk tree 

Amorpha fruticosa - False Indigo  

Caragana arborescens - Siberian Pea Tree

Cercis siliquostrum - Judas Tree  

Colutea x media - Bladder Senna

Cytisus scoparius 'Luna' - Broom ​

Elaeagnus angustifolia - Russian Olive

Elaeagnus x ebbingei - Elaegnus 

Elaeagnus umbellata - Autumn Olive 

Elaeagnus commutata -Wolf Willow 

Hippophae rhamnoides -  Sea Buckthorn

Onobrychis viciifolia - Sainfoin

Robinia pseudoacacia - Black Locust
Spartium junceum - Broom 

Trifolium repens - White Clover

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