Nitrogen fixing plants are a significant asset in a permaculture system. Trees offer the most benefit because of their deep roots and substantial leaf drop. They can be allowed to grow tall and create an overstory canopy, or chopped and dropped.
One of the main systems used in creating a food forest or agroforestry system is to plant about 10 times the number of nitrogen fixing trees you need for a canopy, and chop and drop most of them, allowing them to grow at different rates in response to what your fruit trees need. Young avocados, for instance, prefer some shade until they get larger. Allowing a nitrogen fixing tree to shade your avocado when young can speed its growth - when it gets older, the tree can be chopped to allow more light in.
Which plants should be used? You have a wide leeway here, but there are a few things to consider.
Are there already native nitrogen fixing plants growing on your property or in your area? Those are the most desirable if suitable in other ways. Natives have created relationships in your system that go beyond their nitrogen fixing capabilities. If there is any way to use them as priority, do.
Are the plants invasive? Ideally, you wouldn’t use them, but there are exceptions.
We have used a number of invasive nitrogen fixers especially if already growing on the site, or if we plan to chop and drop them before they seed. One that we use on our property has never seeded. Another has seeded, but not produced seedlings (it isn’t invasive in our particular ecosystem - too cold, too dry). Another is already endemic in the neighborhood; we chop and drop it so keep the ones on our property from spreading.
Four of the five nitrogen fixing trees we use freeze out in the winter and die back to the ground, inhibiting their ability to reproduce but giving us plenty of biomass, which is why we have them.
Are they suited to your ecosystem? Will they grow with few or no inputs and little or no care? Examples would be native species that aren’t growing in your immediate area but could, or other species that are compatible with your ecosystem.
Are they fast growing? If used as a chop and drop or support system, this is an important point.
Do they produce edible fruit? This is not a necessity but is a plus. Inga edulis, or Ice Cream Bean, is a well known example in the tropics. The fruit is delicious. Pigeon pea is another well known example of a fast growing small tree. We usually let some go to seed, and some are used as chop and drop. They use the nitrogen they produce when fruiting, though Inga is much longer lived and ends up leaving much more nitrogen in the system than pigeon pea.
List Of Common Nitrogen Fixing Small And Large Trees In Different Climates
Tropics
- Acacia
- Cajanus cajan (Pigeon Pea)
- Gliricidia
- Inga
- Leucaena
- Sesbania
- Tagasaste
- Tipuana Tipu
Florida
Any of the tropical trees in S Florida, though some are considered invasive. Most tropical trees will grow in central and North Florida though will die back in the winter
Native:
- Acacia (Sweet Acacia, great tree)
- Myrica (Wax Myrtle)
Non-Native:
- Albizia (julisbrissan is invasive)
- Enterolobium (cyclocarpum is preferred)
- Ebenopsis ebano (Texas ebony)
- Leucaena (invasive)
- Sesbania
Temperate climates
- Acacia
- Alnus (Alder - note, Red Alder has very high fixation)
- Caragana
- Cercocarpus montanus (Mountain Mahogany)
- Elaeagnus
- Gymnocladus dioica (Kentucky coffee tree)
- Robinia (Locust)
- Shepherdi (Buffalo Berry)
Drylands
- Acacia (drylands varieties)
- Caragana
- Cercocarpus
- Elaeagnus (Russian olive can be invasive, but tough)
- Ceratonia siliqua (Carob)
- Maackia amurensis
- Prosopis glandulosa (Mesquite)
Identifying Nitrogen Fixing Trees
Young enterolobium tree. This leaf pattern - compound, fern-like leaf, is very common among nitrogen fixing trees and bushes. There are many hundreds of species that look like this in the acacia family alone.
Not all legume trees fix nitrogen in equal amounts, and not all trees that look like this are legumes, but there is a very good chance that if you see this pattern, it is a valuable nitrogen fixing variety. You can find this leaf pattern on nitrogen fixing bushes, vines and ground cover too.
Many legumes have a three leaf pattern, like this pigeon pea.
Flowers are this distinct shape, or puffballs. Seeds are “pods.” (Pigeon Pea).
Sensitive plant, or mimosa strigillosa, with a “puffball” flower (native ground cover in Florida).
A cycad, or “dinosaur plant.” These plants are some of the oldest on the earth and were around during the dinosaur era. They also fix nitrogen. Note the compound, opposite, fern-like leaves.
A common native cycad in Florida is the coontie plant. It loves growing under trees.
How to use nitrogen fixers in your system
Use them as hedge plants, or intersperse them amongst your crops.
Some common ways to incorporate nitrogen fixing trees in your plantings.
Alley cropping is a form of agroforestry where nitrogen fixing trees are planted between corn, coffee, cassava, etc. They are chopped regularly when they start shading the corn – they grow back stronger than ever and the “chop” is used as fertilizing mulch.
In the case of coffee, the trees can be allowed to grow into an overstory, as coffee prefers shade. This is a coffee farm with nitrogen fixing and timber trees as an overstory.
We plant nitrogen fixing trees (Enterolobium cyclocarpum, left) in rows by our orchard trees. We chop and drop them 2-3 times per year into our rows of orchards. They are due for their last trimming of the year here. Mulberries are on the right, with a small swale in between.
We plant pigeon pea near our fruit trees. We usually see the benefits of this with extra growth from our fruit trees.
Surinam cherry (left front) and pigeon pea (right rear) are both thriving. Pigeon pea was planted in the spring, about seven months ago.
This tree (Enterolobium cyclocarpum) is young but will grow as large as a mature live oak so we give it plenty of space and put it on the north side of the orchard. It will help feed and nurture our mulberry trees as a full grown nitrogen fixer.
Nitrogen fixing trees are routinely used as living fences in Central America. We saw many fences like this one, usually strung with barbed wire. In the older fences, the tree trunks had grown around the barbed wire. The trees are cut back every year and the trimmings are usually fed to grazing animals as a high protein, high value feed.
This fence is alive but has been trimmed.
No need for barbed wire with this fence, and no need to worry about the post rotting after a few years. Living fences from nitrogen fixing trees have the potential to stack many functions.
Here’s a beautiful version, with willow. This could be done with long, thin branches from nitrogen fixing trees as well.