Nitrogen Fixing Species for Agroforestry Systems

Paul Alfrey
Tuesday, 12th August 2014

Paul Alfrey shares a variety of nitrogen-fixing plant species beneficial to an agroforestry ecosystem.

I am currently working on a regenerative landscape design for a site in Todorovo, Bulgaria. The plan is to establish an agroforestry system known as alley cropping wherein rows of mixed species edible trees and shrubs are planted at intervals with spaces for herbs, forage and/or grain crops to be grown in between. It's a dynamic system which is inherently diverse, providing multiple yields and excellent habitat for wildlife while at the same time being relatively resilient to a changing climate.

Topography Map of Suhi Dol showing Contour lines and Existing Vegetation

An essential component of the design will be the nitrogen fixing perennial plants within the community of fruit and nut trees. These plants will be pruned at regular intervals to provide biomass for surface mulch and to release a biological source of nitrogen to the surrounding productive plants and soil life by means of root shed associated with top pruning.

When selecting plants for the nitrogen fixing component of this design, I was looking for species that could; withstand record lows of -28oC (Zone 5); tolerate some shade; were fast growing; tolerant of trimming and coppicing; able to grow in clay soils; known to provide significant quantities of nitrogen; easy to propagate from seed; and provide some food for humans and other animals. The following plants fit the criteria.

Elaeagnus angustfolia - Oleaster, Russian Olive
Elaeagnus commutata - Silverberry, Wolfberry
Elaeagnus umbellata - Autumn Olive. Autumn Elaeagnus
Caragana arborescens - Siberian Pea Tree

We are planning to grow the nitrogen fixing plants for this site from seed and to involve the local community in doing so. Many local people, particularly the older generation are skilled horticulturalists with many seasons of experience behind them. We hope to include a number of them in the process of propagation, each one functioning as a individual unit. This will keep the propagation process small scale, making it far easier to use biological methods. The propagation will begin in the autumn as Elaeagnus spp. all require cold stratification unless they are sown immediately after they are picked. Caragana aborescens will be sown in the spring 2015.

The Benefits of Propagating from Seed

When I first started growing shrubs from seed I was pleasantly surprised at how fast the plants establish. In my experience from growing these and other nitrogen fixing shrubs, seeds germinating in the spring can establish well and be ready to plant out in the autumn of the same year (subject to species hardiness and, of course, the weather conditions in a given year). The following spring after autumn planting, I practice formative pruning to encourage the shrubs to become denser and by the third summer after sowing I have recorded growth of up 80cm high and 60cm wide specifically for Elaeagnus angustifolia. The growth I have witnessed from plants in my own stock have, in some instances, outperformed established six year old plants I have growing in the garden, purchased from a commercial nursery.

When propagating from seed you have the advantage of selecting the strongest seedlings. Another significant reward is that you are promoting genetic diversity within your populations, something you are not likely to find in the majority of cloned nursery stock.

Plants Profiles for the Nitrogen Fixing Component of this Design

Elaeagnus angustfolia - Oleaster, Russian Olive

 

Overview: A deciduous large shrub or small tree from Europe and W.Asia, growing approx 7m high and 7m wide. Hardy to zone 2 (-40oC), 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 climate.
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: Edible fruit - raw or cooked as a seasoning in soups. The taste is dry, sweet and mealy. The oval fruits are about 10mm long and contain 17 amino acids with total sugars making 54% of the composition. In China they are made into a beverage. Expected fruit yields are 7-9kg per plant. The seed is edible raw or cooked. The seed oil, flowers and leaves are used medicinally. Plants can be grown as a hedge in exposed positions, tolerating maritime exposure. An essential oil obtained from the flowers is used in perfumery. A gum from the plant is used in the textile industry in calico printing. Leaves are used as goat and sheep fodder. The wood is hard, fine-grained and used for posts, beams, carving, domestic items and makes good fuel. The plant is attractive to bees and is known to be grown as a biomass crop on a three year rotation. In Pakistan it is valued as a pollard fuel and fodder crop.

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

Propagation: Establishment and reproduction of Elaeagnus angustifolia is primarily by seed, although some spread by vegetative propagation also occurs. Cold stratification required for 30-60 days.

Elaeagnus commutata - Silverberry, Wolfberry 

Overview: A medium deciduous shrub from N.America, typically growing 3m high and 1.5m wide but sometimes double that. Hardy to zone 2 (-40oC). Branches are thornless and reddish-brown, leaves are silvery on both sides. A profusion of fragrant silvery flowers appear in May-June, followed by round silvery fruits ripening in September. It typically grows on dry to moist sandy and gravel soils in steppes, meadows or woodland edges. It tolerates very alkaline soils. Plants prefer a continental climate. It can regenerate from old wood making it a good coppice plant. It resents root disturbance. Plants produce suckers quite freely often sending them up at some distance from the plant. Plants start to fruit often after two years.

Uses: Edible fruit, raw or cooked, good with soups and for making jelly. Edible seed, raw or cooked. Plants can be grown as a hedge in an exposed position, tolerating maritime climate. The fibrous bark is used in weaving and rope making. Dried fruits are used as beads. Flowers provide nectar for bees. Cultivated as an ornamental plant for its silvery foliage.

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

Propagation: Seed is 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 seed can be very slow to germinate, often taking more than 18 months. A warm stratification for 4 weeks followed by 12 weeks cold stratification can help. The seed usually (eventually) germinates quite well.

Elaeagnus umbellata - Autumn Olive. Autumn Elaeagnus

Overview: A large deciduous shrub from E.Asia, growing 4.5m high and 4.5m wide, hardy to zone 3 (-35oC), tolerates part shade, very drought tolerant. Branches are often thorny, leaves are bright green, 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 nectar and very aromatic.Plants can fruit in 6 yrs from seed. This species is considered weedy in the U.S

Uses: Edible fruit raw or cooked which is very tasty and can be made into jams, preserves etc. The fruit contains about 8.3% sugars. 4.5% protein. 12mg per 100mg Vitamin C. Mature bushes in the wild yield about 650kg of fruit over 2-3 pickings. The harvested fruit stores for appox. 15 days at room temperature. It can be used as a hedge plant and tolerates maritime exposure succeeding in the most exposed positions. The wood is a good fuel. The nectar from the flowers is attractive to bees comprising 28% sugars. 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 three year rotation.

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

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 seed can be very slow to germinate, often taking more than 18 months. A warm stratification for four weeks followed by 12 weeks cold stratification can help. The seed usually (eventually) germinates quite well. Prick out the seedlings into individual pot as soon as they are large enough to handle and plant out when they are at least 15cm tall.

Caragana arborescens - Siberian Pea Tree


Overview: 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 is vigorous. Flowers are born from buds on the previous years 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 4-5cm 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 -40oC (hardiness zone 2). Prefers a continental climate with hot dry summers and cold winters.

Uses: The young pods are eaten as a vegetable, lightly cooked. The pods become tough later in the season. The seeds are rich in fats and proteins (12% and 36% respectively) about the size of lentils and can be cooked and used in any way that beans are used (the cooked flavour is somewhat bland, so best used in spicy dishes). The young raw seeds have a pea-like flavour although it is not clear whether they should be eaten raw in much quantity. Widely used in windbreaks and shelter belts and used in wildlife-erosion control plantings stabilizing soil with an extensive root system. 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

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

These species profiles include extracts by Martin Crawford, Director of Agroforestry Research Trust from the excellent quarterly publication Agroforestry News Vol.4 No.3. I highly recommend subscription to this journal as essential reading for all who are interested in temperate tree crops and agroforestry.

If you would like to grow your own we are offering great prices for excellent seeds. Our seeds are certified, tested for viability and germination rates, and are priced very competitively. If you purchase 500 seeds or more of each species we offer a 10% discount from the total. To find out more visit www.balkep.org or contact balkanecologyproject@gmail.com with your order.

This article originates from http://balkanecologyproject.blogspot.co.uk/2014/07/nitrogen-fixing-species-for.html

Further resources

Nitrogen fixing plants and microbes

Watch: Tips on perennial polycultures

Soil restoration - using plants to fix nitrogen in the ground

Planting naturalistic polycultures in the vegetable garden

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Smitty Smith |
Fri, 22/08/2014 - 14:36
Thoroughly enjoyed reading this.. I saved it to my evernote to read again!!
Simon Benjamin |
Wed, 10/12/2014 - 09:24
I am going to grow Elaeagnus umbellata and Caragana arborescens in my new 1.5 acre forest garden this year. I intend to plant some near fruiting trees and also near my walnuts and hazelnut trees. Plus some in the hedgerows. I think these trees/shrubs are great, they fix nitrogen and make land more fertile, whilst producing some crop as well. As they are grown from seeds its also possible to plant too many and fix plenty of nitrogen, then thin them out as your other trees start to grow. I believe Geoff Lawson is into this sort of system which he calls over-stacking. Would also be worthwhile to select the best fruiting shrubs out of a batch of seeds to keep and remove the rest, collecting seeds from the best ones. In order to create better shrubs in the long run.

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