One of the most frequent questions I get from readers is about nitrogen-fixing plants. More specifically, what exactly are nitrogen-fixing plants, and how do they “fix” nitrogen? I wanted to answer this and help others get familiar with these transformative plants, so I put together this guide.
Some of the best nitrogen-fixing plants are seaberry, acacia, mesquite, lupine, black locust, goumi berry, and buffaloberry. However, you should choose nitrogen-fixing plants based on your climate and garden needs. For example, in dry and rocky conditions consider using seaberry, black locust, and buffaloberry.
While these are just some of the nitrogen-fixing plants, exactly what do they do for the garden, which plants should we use for our garden, and how should we plant them? Let’s take a closer look.
In this article:
- How Do Nitrogen-Fixing Plants Work?
- 68 Nitrogen-Fixing Plants (Trees, Shrubs, & More)
- How to Use Nitrogen-Fixing Plants (5 Steps)
- Plants That Fix Other Nutrients (Phosphorus, Potassium, & More)
How Do Nitrogen-Fixing Plants Work?
Plants require three primary nutrients to grow.
Nitrogen (N) is primarily used for canopy and root growth, phosphorus (P) is used for flowering and fruiting, and potassium (K) is used for immunity and overall health of the plant.
There are many other secondary nutrients such as magnesium, calcium, and copper that assist in other plant functions.
However, nitrogen is by far the biggest nutrient required (with some plants such as citrus and avocado trees needing double the nitrogen to phosphorus and potassium).
Because of this, nitrogen-fixers are an often underutilized resource in the garden and home orchards.
“We can best start the orchard by planting legume (nitrogen-fixing) plants—small species like white clover, lab-lab bean and lucerne, larger species such as acacia, albizia, and black locust, and a scattering of shrubs (tree medic, tagasaste).”Bill Mollison, Introduction to Permaculture
How Plants Fix Nitrogen
Nitrogen-fixing plants have a mutual relationship with bacteria called Rhizobia that live in their roots. These bacteria are able to “fix” nitrogen by taking nitrogen from the air and turning it into a form that plants can use.
The bacteria then store this nitrogen in the soil, and in return, the plants provide the bacteria with the sugars they need to survive.
“The roots of C. equisetifolia (Casuarinas) produce root nodules where the bacteria fix atmospheric nitrogen, which is an essential nutrient for all plant metabolic activities.”National Library of Medicine, PubMed
As a result, nitrogen-fixing plants act as a natural fertilizer for the plants around them, reducing the dependency on chemical fertilizers.
“[Nitrogen-fixers] serve as the fertilize factory in the landscape, reducing or eliminating the need for fertility importing from off-site.”Ben Falk, The Resilient Farm and Homestead
Plus, many of them are great at attracting pollinators and beneficial insects such as ladybugs (which eat common pests such as aphids and spider mites).
Which Kinds of Plants Can Fix Nitrogen?
The majority of nitrogen-fixing plants are leguminous, which means they’re part of the bean family (Fabaceae). The most common examples are beans, peas, and clover.
However, not all leguminous plants are nitrogen-fixing. For example, honey locust and carob are not.
There are also over 150 other plants that aren’t in the Fabaceae family but still fix nitrogen (such as alder, autumn olive, and casuarinas).
Nitrogen-fixing plants are often pioneers in degraded landscapes, which means they are extremely fast-growing and specialize in amending soils and establishing canopies. This significantly assists the establishment of slower-growing species.
Let’s take a look at the best nitrogen-fixing plants, followed by how best to use them.
The Top 68 Nitrogen-Fixing Plants
“[Nitrogen-fixing plants] can be culled as the garden matures. Remember to use nitrogen-fixing trees and shrubs, too, not just perennial and annual herbs.”Toby Hemenway, Gaia’s Garden
Feel free to take a look at the 68 nitrogen-fixing plants below, but don’t get overwhelmed. Learn how to select the plants that work best for you with my 5 steps toward the end of this article.
|USDA Hardiness Zone
|Golden Chain Tree
|Kentucky Coffee Tree
Why provide botanical names? Sometimes multiple plant species have the same common name. To avoid confusion (and to prevent you from buying the wrong plant), consider using the scientific or botanical names.
When using nitrogen-fixing trees, aim to plant them before fruit trees and other more sensitive species. This way they can establish a canopy (microclimate) and amend the soil, paving the way for the fruit trees and other vulnerable species.
This technique is often called ecological succession, or planting “support” species before “productive” species. As the productive species become established, you can begin pruning and cutting down the support species (more on ecological succession later).
For example, we have an acacia (a warm climate nitrogen-fixing tree) in our backyard and it provides lots of flowers, dropped leaves, and mulching material for our fruit trees.
Here’s what Bill Mollison, the founder of permaculture, had to say about Acacia trees.
“Acacias fulfill many functions: they provide seeds for poultry forage, foliage for larger stock, and fix nitrogen in the soil, while blossoms provide pollen for bees. They are also pioneer plants which prepare and protect the soil for slower-growing, more sensitive plants.”Bill Mollison, Introduction to Permaculture
With our acacia, we’ve noticed an increase in pollinators as well as the quality of soil directly around the tree. While it’s difficult to measure exactly how much the acacia tree has helped our backyard plants, it’s clear upon a quick observation.
Remember to select nitrogen-fixing trees based on their potential height and preferred climate.
|USDA Hardiness Zone
|Carolina bush pea
|Siberian Pea Shrub
There’s some overlap between nitrogen-fixing trees and shrubs as some shrubs are seen as small trees (such as pigeon pea).
Some of the most popular nitrogen-fixing shrubs are sea buckthorn (seaberry), buffaloberry, and goumiberry. Not only are these great companion plants for your fruit trees, but they also provide edible berries of their own!
Others such as Siberian pea shrub (as well as the tree tagasaste) are often used as a windbreak, poultry food (seeds), and fodder (leaves) for larger livestock.
As with the trees, choose nitrogen-fixing shrubs based on your climate as well as spacing (again, I’ll provide steps for this later).
|USDA Hardiness Zone
It was a little difficult to find nitrogen-fixing “herbs” as many of them could be also classified as shrubs, flowers, or ground covers.
The definition of an herb is a green, soft stalk (as opposed to a hard, woody kind). They’re great at filling the niche of herbaceous plants in a landscape.
|USDA Hardiness Zone
|Blue False Indigo
These nitrogen-fixers are fantastic at not only amending soil but attracting pollinators and beneficial insects.
For example, we recently planted lupine near our citrus trees. We also have sweet peas throughout the garden and notice bees, bumblebees, and hummingbirds pay frequent visits.
Ground Covers (Cover Crops)
|Austrian Winter Pea
|Sweet White/Yellow Clover
|White Dutch Clover
|White Ladino Clover
|White New Zealand Clover
|Nitro Persian Clover
Many of these nitrogen-fixing ground covers can be found in Ben Falk’s book, The Resilient Farm and Homestead.
Nitrogen-fixing ground covers, or cover crops, are best noted for their ability to retain soil moisture, regulate soil temperature, and provide mulch, all while fixing nitrogen in the soil.
Cover crops are best used with crop rotation. In other words, after harvesting crops such as corn, cabbage, etc, instead of leaving the soil bare (leading to dry and eroding soil), use cover crops while you wait to plant your next crop.
This way the soil is protected and amended with nitrogen as you wait.
Cover crops are also amazing to use in-between rows of fruit trees. Many vineyards have recently recognized the benefits and begun practicing cover cropping with hopeful results.
While growers often recommend tilling the ground covers back into the soil to maximize nitrogen absorption, it’s not necessary as the roots naturally amend the soil.
For example, Toby Hemenway in Gaia’s Garden mentions “Live nitrogen fixers are at least as growth boosting as dead ones.”
How to Use Nitrogen-Fixing Plants (5 Steps)
1. Find Your Zone
If you haven’t already, find your USDA Hardiness Zone. Simply type in your zip code in the map tool and you’ll find your zone.
USDA hardiness zones tell you the average minimum temperature your area receives.
Here’s how it works.
2. Match Plants to Your Zone
Once you find your zone, match the plants that do well in your zone.
For example, if you live in zone 8b (down to 20ºF), it’s too cold to grow citrus (requiring zones 9-11), but it’s not too cold to grow apples or pears (3-8).
Do the same, except use the examples of nitrogen-fixing plants listed above.
Note the plants that overlap with your zone.
While it’s possible to grow plants outside of their recommended hardiness zone, it’s often an uphill battle and usually not worth the extra effort. If you find plants that are on the edge of your zone, consider establishing a microclimate (such as a greenhouse, shaded canopy, or sunny wall).
3. How to Plant
Once you have your list of nitrogen-fixing plants for your zone, we’re going to plant based on two goals.
- Plant support species before productive species (ecological succession)
- Aim for greater density
Ecological succession is when pioneer species (such as dandelions and pine trees) arrive in a barren or damaged ecosystem (flooding, wildfire, or manmade). These fast-growing species often specialize in establishing the early stages of ecosystems, allowing more sensitive and productive species such as slower-growing hardwoods and fruit trees to grow.
The best part is that we can mimic this process. By selecting nitrogen-fixing trees, shrubs, and herbs and planting them where you’re planning to have an orchard, vegetable plot, or other production of land, you can give the productive species a head-start on their growth.
Part 2 is where you stack these pioneers and plant them in layers.
“The immense leafy area of a multistoried garden that mixes trees, shrubs, and low plants will capture sunlight and turn it into life far more effectively than a one-layer landscape. Multiple layers will also slow moisture loss from evaporation and perhaps even harvest fog to boost total precipitation.”Toby Hemenway, Gaia’s Garden
For example, in our backyard, we have acacia in the tree layer (or niche), sweet pea as the shrub, and lupine in the herb or flower niche.
As far as how many nitrogen-fixing plants to use, Ben Falk, in his book The Resilient Farm and Homestead, recommends planting at least one nitrogen-fixing plant for every “feeder” plant such as fruit trees or berry plants.
4. Chop & Drop (Pruning & Mulching)
As the productive species grow, you can simultaneously prune the pioneer plants. You can then use these cuttings as nitrogen-rich mulch (commonly called green manure) for other plants.
This process is called “chop and drop”. Essentially, you’re taking the excess growth from other plants (ideally nitrogen-fixers), and using it to mulch other plants (ideally, productive species).
This is an incredibly effective method to speed up the quality of soil while not importing or relying on chemical fertilizers. The mulch also dramatically reduces evaporation, regulates soil temperature, and prevents erosion.
The preferred time to prune is just after the hot season (as other plants benefit from their partial shade). Pruning the fast-growing nitrogen-fixers before winter means allowing more sunlight to reach the lower-level productive plants.
Repeat this process as needed, pruning the nitrogen-fixing plants at least once per year.
Over time, we can replace some of the nitrogen-fixing pioneers with productive species such as citrus trees, blackberries, and lavender.
The goal here is to fill each niche (such as trees, shrubs, and herbs) with first nitrogen-fixing plants, and then slowly replace them with productive plants.
Keep in mind that you still want some nitrogen-fixers, even after the productive species become the majority. This helps the soil stay fertilized and provides sufficient nitrogen to grow.
Plants That Fix Other Nutrients (Phosphorus, Potassium & More)
Nitrogen isn’t the only nutrient that plants can amend.
Nutrient-accumulating plants are great at using their roots to “mine” deeper nutrients from the soil and store them in their leaves and the surrounding soil. Then, as their leaves drop or are pruned, they can be used as a fertilizer for other plants.
For more context, here are some of the best nutrient accumulators and the specific nutrients they mine from the soil, as found in Gaia’s Garden.
|P, K, Ca
|N, K, Ca, Mg, Fe, Si
|P, K, Ca, Mg, Fe, Cu, Si
|Mn, Fe, Cu, Co, Zn
|N, P, K, Ca, Mn
|K, Ca, Mg, Fe
|N, K, Ca, S, Fe, Cu
|Ca, Mn, Cu, Zn
|P, K, Ca
|N, P, K, Cu
To help with this, I’ll add an image of the period table of elements here if you need it.
If you’d like to learn more about support and productive species, and how to balance them in your garden, check out this video by Geoff Lawton.
- Gaia’s Garden by Toby Hemenway
- The Resilient Farm and Homestead by Ben Falk
- Introduction to Permaculture by Bill Mollison
- Karthikeyan A, Chandrasekaran K, Geetha M, Kalaiselvi R. Growth response of Casuarina equisetifolia Forst. rooted stem cuttings to Frankia in nursery and field conditions. J Biosci. 2013 Nov;38(4):741-7. doi: 10.1007/s12038-013-9362-3. PMID: 24287654.