HELPFUL ALLIES IN THE ORCHARD

NITROGEN FIXATION

The importance of nitrogen in the soil cannot be stressed enough. It is essential for all life forms, as it is needed to make important molecules for life, including RNA, DNA, and proteins. It is among the top three most vital nutrients required by plants, alongside potassium and phosphorus. Nitrogen assists with chlorophyll production, photosynthesis, and the formation of amino acids (proteins that are vital to plant health and growth).

So why all this talk about nitrogen? Well, most nitrogen is unreachable by life forms! Despite the fact that 78% of the atmosphere’s is made up of nitrogen, it isn’t usable by plants. It is an inert form of nitrogen, called atmospheric nitrogen (N2), therefore, it isn’t accessible to perform any reactions to form the important molecules for life. Additionally, the soil is easily depleted of nitrogen since it is used in such high quantities, and it is easily leached away by the sun and water.

This is where the primary nitrogen fixers jump into the picture. What nitrogen fixing plants do is help transform inert atmospheric nitrogen into ammonia (NH3) and ammonium (NH4+), accessible forms of nitrogen, via the nitrogen fixation process. The ammonia can further break down into nitrates and nitrites, which are also usable by life forms. The primary nitrogen fixers are actually rhizobia bacteria (soil bacteria that fix nitrogen) that work in tandem with certain plants. They live in the soil and on plant roots of legume plants (peas, beans, etc.) within nodules growing on the roots. They fix nitrogen for the plant and nearby plants in exchange for the carbohydrates the roots provide for the bacteria. Once this process occurs, it can be passed down to other life forms through the food web.

NITROGEN FIXING PLANTS

The Process

Nitrogen fixing plants are our allies in the garden. They make more nitrogen available for themselves and other plants. But, as we already know, they don’t do it on their own. These plants have a symbiotic relationship with the rhizobia bacteria mentioned above. These bacteria form root nodules on the plant roots, within which bacteria can convert nitrogen into ammonia. This symbiosis mostly occurs with plants in the legume family, but not exclusively. These plants come in all forms, ranging from groundcovers to trees. Some examples are: peas, beans, baptisia, black locust trees, and the list goes on. Many of the common nitrogen fixing plants used in agriculture in this region are non-native, but native options are also available.

There are many ways the plants distribute the converted nitrogen into the soil and to other plants. It can be via the roots, leaf fall, plant die back, or the human intervention called chop and drop (dropping any pruned off leaves onto the garden floor).

Classification of Fixation Effectives

It should be noted that not all nitrogen fixers are equally effective at nitrogen fixation. This USDA plants database gives information about the varying amount of nitrogen fixed by different species. The nitrogen fixers are classified as either high (160+ lbs/acre), medium (85-160lbs/acre), or low (1-85lbs/acre). The location of the plant and light exposure also impact the nitrogen fixation rates. According to Martin Crawford’s Creating a Forest Garden and Nitrogen Fixing Plants for Temperate Climates, in situations where plants are receiving all their nitrogen from nitrogen fixers, plants in full sun need fewer nitrogen fixing plants nearby compared to plants in the shade. Also check out Eric Toensmeier’s article comparing nitrogen-fixing capacity!

Native Versus Non-Native Nitrogen Fixers

The unfortunate (but also not so unfortunate depending on how you look at it) reality is that the most effective nitrogen fixers in this region are non-native. There are really only a few native species that are ranked high. This isn’t an advocacy for using noxious non-natives; there are plenty of less aggressive non-native nitrogen fixers to choose from. However, it is always ideal to go for native plants due to the belief that they have long-standing relationships with the local soil biome. Below is a table that was created by Eric Toensmeier from the article All Nitrogen Fixers Are Not Created Equal, that lists the most effective nitrogen fixers in the region and categorizes them by plant type, whether they are native or non-native, and their nitrogen fixation effectiveness.

NITROGEN FIXING PLANTS IN THE ORCHARD

There is a wide variety of nitrogen fixing plants to choose from in an orchard setting. Criteria for selecting nitrogen fixing plants in a holistic orchard setting include plants that not only improve soil, but may offer some human or ecological benefit, whether that be food, medicinal products, nectar, etc.

Incorporating nitrogen-fixers within an orchard setting can be done a few ways. There are also preferred species to consider.

• Growing an annual climbing bean plant under a tree and letting it climb the trunk.
• Cover-cropping in the fall with a leguminous cover crop, such as crimson clover, vetch, alfalfa, etc.
• Planting a nitrogen fixing shrub for every 2-3 trees.

Shrubs are often ideal in an orchard setting because they fix more nitrogen than herbaceous plants and don’t need to be replanted every year. It is best practice to place one nitrogen fixing shrub per 2-3 trees, especially trees that have high nutrient needs and/or are more susceptible to pest and disease pressures.

Nitrogen Fixing Orchard Shrubs

POP focuses on fruiting plants, therefore the majority of the shrubs growing at POP orchard sites are edible or medicinal in some way. Below is a list of nitrogen-fixing shrubs used at POP orchards, including the POP Learning Orchard at The Woodlands:

• Goumi (Elaeagnus multiflora)

Native to Russia, Korea, Japan and China, the goumi is a large shrub than can get 10 to 12 feet tall and wide. It is in the oleaster family, related to silverberry, and buffaloberry. Due to their large size, they are best planted with ample space around them. Goumis not only improve the soil; they offer an abundant yield of delicious, tart, fruit in spring. The fruit are red, oval, and adorned with silver dotting. It can be eaten raw or processed into jellies and baked goods. In addition to its fruit, they are known for their silver-hued leaves dotted with tiny scales, offering ornamental value. They are partially self-fertile, so best planted with another goumi by its side. This plant is very hardy and can tolerate most growing conditions, including partial shade. NOTE: Although related to some other plants considered invasive in this region, goumi is specifically noted in multiple sources as non-dispersive. The USDA rates this plant as a medium nitrogen fixer.

• Siberian Peashrub (Caragana arborescens)

Siberian peashrub is a deciduous shrub endemic to Central Asia that can get around 10 feet tall and 10 feet wide. It fits well between trees or on the margins in an orchard setting. It has 3-5 inch compound leaves with small, rounded leaflets and a lacking terminal leaflet. Yellow flowers bloom in May and June then produce 1-2 inch long green seed pods that turn brown. Over the fall, the pods curl, open, and release seed. It prefers well-drained soil and partial/full sun. This plant can spread via seed, so should be considered when planted. It is self-fertile. The flowers, pods, and seeds are edible. The USDA rates this plant as a medium nitrogen fixer.

• Seaberry (Hippophae rhamnoides)

Seaberry or Sea Buckthorn, distantly related to goumis and autumn olive, is a deciduous shrub or small tree that might be the most widely grown, northern-hardy, fruiting plant in the world. It can get 10-20 feet tall and wide, with silvery-hued leaves, thorns, and orange-yellow berries. These fruits have a citrus flavor and been used historically as medicine and food in Eastern Europe, Northern Europe, and Central Asia, where it is native. It is wind-pollinated, therefore requires one male plant and ideally 6-8 female plants. Therefore, it requires space so that it can be planted in a small grove. USDA doesn’t have a nitrogen fixing rating for this plant.

• Northern Bayberry (Myrica pensylvanica)

Bayberry is a semi-evergreen, native shrub, related to wax myrtle. It can get 5-6 feet tall and if living in ideal conditions, it can create a colony via suckering.  Because this plant gets rather unwieldy and wide, it is best to plant it on the edge of an orchard, rather than the middle of it or between trees. It is a suitable hedge, privacy screen, and plant for erosion control. It is native to the coastal regions of the mid-Atlantic, often living in dune-like habitats. It is no surprise then that it tolerates high winds and salt. It likes full sun or partial shade and prefers dry, acidic soil. Because it is a nitrogen fixer, it thrives in poor soil. The waxy berries, although not edible, were historically used to make candles. When planting, if the berries are wanted to candle-making, then at least two need to be planted for pollination. The USDA rates this plant as a medium nitrogen fixer.

• New Jersey Tea (Ceanothus americanus)

New Jersey Tea is a short, native deciduous shrub that often grows to only 3 ft. tall. It grows in a compact, round form. Because of its small, compact shape, this shrub can be planted between trees. It has hairy leaves that give the plant a grayish hue. Small white flowers occur as clusters, covering the entire shrub. The flowers offer a nectar source for pollinators. The base is woody, while the upper portion of the plant is made up of herbaceous, spreading branches. It prefers well-drained sandy loam or rocky soils in full sun to partial shade. It is self-fertile. It fits well in both a traditional formal orchard or more naturalistic orchard. The leaves can be used to make tea. The USDA rates this plant as a low nitrogen fixer.

• Silverberry (Elaeagnus commutata)

Silverberry, related to goumis and buffaloberries, is a deciduous native shrub that gets 10 feet tall. It has beautiful silver-hued foliage and fragrant yellow flowers that turn into a white, oval, berry-like fruit covered with silver dots. The fruit and seeds are edible, but very astringent unless fully ripe. It is suckering so should be given ample space, unless the suckers are removed. It has a shallow root system so can be easily transplanted. It grows wild in colder climates in North America, making it a valuable food source for mammals in the winter. It can tolerate gravely and sandy soils. It is self-fertile. The USDA rates this plant as a medium nitrogen fixer.

• Buffaloberry (Shepherdia argentea)

Buffaloberry is a medium sized, thorny, deciduous shrub related to goumis and silverberries and native to northern and western America. \ The shrub can get up to 13 feet tall, so needs ample space in an orchard setting. It is drought tolerant and can be found on rocky, sandy, saline soils. It prefers partial shade or partial sun to full sun. The bright red berries are tart but become sweeter after a frost. They can be eaten raw or cooked, they can also be processed into jams and syrups. Make sure to not overeat them due to small amounts of the toxin saponin, which is not readily absorbed the body. Some indigenous groups used the berries as a primary ingredient in a version of ice-cream. It is a good late winter food source for birds. The USDA rates this plant as a medium nitrogen fixer.

Nitrogen Fixing Trees

There a wide variety of nitrogen-fixing trees, both native and non-native, to be aware of when planting a garden. Some can work in a orchard and/or food forest setting, while others are less fit for that.

• Black locust (Robinia pseudoacaci)
  • Native tree
  • Up to 80 feet tall
  • Suckering
  • Thorny
  • High nitrogen fixer
• Honey Locust (Gleditsia triacanthos)
  • Native tree
  • Up to 100 feet tall
  • Suckering
  • Thorny
  • No information on nitrogen fixing rating
• Catalpa (Catalpa speciosa)
  • Native tree
  • Up to 90 feet tall
  • Showy blooms
  • Low nitrogen fixer
• Kentucky Coffee tree (Gymnocladus dioicus)
  • Native tree
  • Up to 80 feet tall
  • Suckering
  • Seeds historically used by Native Americans and early settlers as a coffee-like beverage
  • Low nitrogen fixer
• Whitethorn Acacia (Acacia constricta)
  • Tree native to southwest USA and Mexico
  • Inch long white thorns
  • Drought tolerant
  • Yellow, round blooms in winter and summer
  • Low nitrogen fixer
• Black alder (Alnus glutinosa)
  • Non-native tree from Europe, northern Africa, and Asia
  • Up to 90 feet tall
  • Short-lived, pioneer species
  • No information on nitrogen fixing rating
• Grey alder (Alnus incana)
  • Non-native tree
  • Up to 25 feet tall
  • Prefers colder climate and wetter soils
  • Medium nitrogen fixer
• Mimosa tree (Albizia julibrissin)
  • Non-native tree from southwestern and eastern Asia
  • Showy, whimsical blooms
  • Suckering
  • No information on nitrogen fixing rating

Nitrogen Fixing Herbaceous Plants

There are also a variety of nitrogen fixing herbaceous plants to consider planting in an orchard or food forest. In addition to some perennial flowers, the ones used most often are leguminous cover crops because they fix nitrogen and improve soil fertility and soil texture. In the wrong context, these plants may sometimes be considered “weeds”, but we are generally happy to see them pop up and sometimes sow them intentionally when a cover crop is needed.

Check out our blog post about POP’s favorite herbaceous nitrogen fixers, including baptisia, thermopsis, dalea, and cover crops like clover and vetch.

SOURCES

POP blog: Goumi Shrub

POP blog: Bayberry Shrub

Nitrogen Fixation

Siberian Peashrub

Nitrogen Fixing Plants

Weeds aren’t actually that bad

Cover Crops

Comprehensive List of Nitrogen Fixers

DISCLAIMER

The information presented on this website is for informational, reference, and educational purposes only and should not be interpreted as a substitute for diagnosis and treatment by a health care professional. Always consult a healthcare professional or medical doctor when suffering from any health ailment, disease, illness, or injury, or before attempting any traditional or folk remedies. Keep all plants away from children. As with any natural product, they can be toxic if misused. 

This POP Blog was written by Orchard Assistant Simone Shemshedini with help from Co-Executive Director Phil Forsyth.

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