Gardeners in the know are familiar with Nitrogen Fixing plants, but the majority of gardeners out there have never even heard of them. Imagine, a plant that will add valuable nitrogen into the soil so that your fruit trees or vegetables can grow even better and you don’t have to do any of the work. That’s what Nitrogen Fixers do!
Some of the best Nitrogen fixing trees to plant in your tropical garden would be Madre de Cacao, Ice Cream Bean, Koa, & Faya. The best nitrogen fixing shrubs for the tropics include Pigeon Pea, Sesbania & Rattlepod. The best Nitrogen Fixing groundcovers are Perennial Peanut, Desmodium & Nanea.
Gardens in Hawai’i can greatly benefit from the addition of Nitrogen fixing plants into the landscape. With nutrients in tight supply and organic matter always a struggle to obtain, having a way to produce your own in your garden will go a long way towards providing you a thriving food forest.
Related: Nitrogen Fixing trees need constant chopping if to be utilized correctly as a nitrogen fixer. You need a good tool to accomplish the job, with my favorite being this Chainsaw from Amazon.
Benefits of Nitrogen Fixing Plants
There are a few benefits to having Nitrogen fixing plants mixed into your garden. Besides the sequestration of nitrogen from the air and then fixed into the soil, Nitrogen fixing plants have a few other benefits for your garden.
Nitrogen fixing plants are typically very fast growers. They can provide much needed shade to nurse your fruit trees along as they are getting established. Most tropical fruit trees grow up in an environment that has partial shade. By planting a nitrogen fixing tree at the same time you plant a fruit tree, you are giving that fruit tree a much better chance at getting established.
Tropical soils are notorious for having very little in the way of nutrients for plants to feed from. Most of the nutrients in the tropics are located in organic material such as leaves, branches, manures, etc. Adding more of these to the soil where the plants can reach them require lots and lots of mulching.
Nitrogen fixing plants respond very well to coppicing (heavy pruning). Once pruned they sprout right back in no time providing even more organic material than before. The leaves and branches of these plants are rich in nitrogen and other minerals that plants need to thrive.
Not only do you have an easy way to feed your fruit trees from resources already on site, The mulch also helps suppress weeds that are a constant battle in any garden.
There are many species of nitrogen fixing plants that also provide food, not only for ourselves but for animals as well. Plants such as the Ice Cream Bean Tree or Pigeon Pea are great nitrogen fixers that also have edible seed pods for humans. My kids love eating the cotton candy layer of an ice cream bean.
The branchy, leafy material is also very good fodder for livestock. Cows and goats do well with a mixed diet of not just a variety of pasture grasses, but also off of grazing leaves from nitrogen fixing trees that are high in nutrients. Interplanting nitrogen fixers in a pasture is a great strategy for obtaining healthier pasture and healthier livestock.
Nitrogen fixing plants have a few more benefits for the off grid homestead. They can be planted as a living fence. The roots of nitrogen fixing plants typically run very deep. They do their best to break up hard dirt and create healthier soils.
The wood of nitrogen fixing trees can be used as a great source of firewood and even as a building material. I have used many branches off my prunings as a natural element into many of my building projects around my homestead.
How Nitrogen Fixing Plants Work
Working with a group of bacteria called rhizobia, nitrogen fixing plants can pull nitrogen out of the air and accumulate it within its leaves, branches and roots. The bacteria innoculate the roots of the nitrogen fixing plants, housing themselves in small ballike root structures called nodules.
To ensure that your soils have the proper rhizobia, it is common practice to add rhizobium into the hole at the time of planting. You can tell if the plant is fixing nitrogen by inspecting the roots for the white nodules and cut them in half. If they are pink inside, they are fixing nitrogen.
Use With Caution
Nitrogen fixing plants are typically known as the weeds or invasive species in many areas. Not all of the time but most of the time. They are the pioneer species that comes in to repair degraded lands.
They tend to only be found in areas where the land has been disturbed, but can spread over time. Nitrogen fixing plants are known to self propagate very easily. I try to stick with nitrogen fixing plants that do not disperse their seeds by wind to minimize their invasiveness.
It is also important to note the notion that planting as many nitrogen fixers as you can is the best thing to do. It’s not. Too many nitrogen fixing plants can over nitrify the soils and pollute ground and surface waters. I like to go with planting 3-4 nitrogen fixers per tree, or add them into a rotation in my annual garden after every 3rd planting.
Types of Nitrogen Fixing Plants for Hawai’i
Albizia (Falcataria moluccana) The Albizia is notorious in Hawai’i as one of the most invasive trees around. The tree colonizes disturbed landscape through wind dispersed seeds and outcompetes natives. It is NOT a Nitrogen Fixer that I would consider planting on purpose.
However, if you already have Albizia growing on your property, you probably have pretty good soil. Albizias drop organic matter like it’s going out of style and their deep tapping roots break up the lava rock underneath. Unfortunately, full sized albizias are difficult to remove and cost a lot of money to do so.
If you’re like me though when I was first establishing my property, Albizias were popping up all over the place. I would let them grow to 20’ tall and then cut them to the ground when they just started to flower. Then they’d resprout and I’d do it all again.
This was much easier for me than starting some nitrogen fixing trees by seed, then digging a hole and transplanting them. All I had to do was coppice them once a year. Besides, I was too busy building a house in the early years. If we’re smart we can let nature do the work.
All of the albizias have since been removed from my property. They served their purpose for the time they were there. Now I have other nitrogen fixers that I don’t have to worry about going all “invasive” on me (as much).
Madre de Cacao (Gliricidia Sepium) – The Mother of Chocolate, it is most often used in tropical forms of agriculture to act as a shade tree for crops such as chocolate and coffee.
Most often going by the name Gliricidia in Hawai’i, this tree is also used as animal fodder, mulch material, nitrogen fixation, fence posts and rat poison. With as many rats as we have around here and the growing Rat Lungworm problem, almost every landscape of size should include at least one of these trees.
Like all Nitrogen fixers, this tree responds well to regular HARD pruning and does have a potential of being invasive although not nearly as much as an albizia. This tree prefers the more dry forest type climates on all the islands.
Ice Cream Bean (Inga Edulis) – The Ice Cream Bean is one of my children’s favorite trees. They love the edible pulp that surrounds the seed, they claim it tastes like cotton candy. Besides kids, animals love it too. I’ve seen goats plow through the leaves and seeds of this tree.
The tree produces large seeds that drop to the ground, minimizing their chances for getting invasive, although your yard will probably get a bunch of seedlings under the tree if you let it go to seed.
I prune the tree once a month to provide leaf mulch for my vegetables and as carbon material for my compost piles. They grow back very quickly.
The tree is does well planted as a living fence and provides decent lumber. I have used raw branches on numerous projects and have found them to be solid and longstanding. These trees also make great climbing trees for the kids or foundations for treehouses.
Koa (Acacia Koa & Acacia Koaia) – There are two types of native koa trees growing in Hawai’i. An upland Koa and a lowland Koa. The upland koa is a stately tree that can get up to over 100’ in height. It is the tree most highly prized for its wood in making surfboards and canoes.
The lowland koa is more scrubby than its upland counterpart. It does well on the more dry sides of the islands.
These native plants would be a great addition as a nitrogen fixer into the landscape. Utilizing native nitrogen fixers should be included in any effective landscape design that aims to regenerate local landscapes.
Besides their great nitrogen fixing capabilites, these trees are home and food source to countless native insects and birds.
Firetree (Myrica Faya) – Faya is a fast growing shrub or small tree that is invasive to the Hawaii Islands. Due to the fact at how quickly it can invade an area, I would not recommend using this plant as a nitrogen fixing solution for your garden. I mention it here because it can be recommended to use by some people.
While the frietree fills a great niche in providing a small nitrogen fixing tree or shrub, unless you are managing it properly, it can quickly get out of hand. May as well plant something else that does not have the same problems as Faya has
Tamarind (Tamarindus indica) – Once commonly planted in Hawai’i of old, the Tamarind is typically only found in Hawai’i as very mature specimens. Queen Liliokalani loved Tamarinds and had one planted in her garden on Oahu at Washington Place.
The one I am most familiar with is the tree at the Punalu’u Bake Shop in Na’alehu on Hawai’i Island. It is a large, stately tree that produces tasty tarty/sweet seedpods and flowers that bees just seem to flock to.
The try prefers to grow in the south or leeward sides of the islands and is very tolerant to drought.
The tree is used for its edible seedpods, it’s wood for furniture making, as a medicine, metal polish, mulch material and animal fodder. Its amazing that these trees have fallen out of fashion because they could provide a homestead with numerous yields.
Sesbania (Sesbania Grandifolia) – Sesbania Grandifolia is a small tree or shrub that fixes nitrogen into the soil and produces edible bean pods, leaves and flowers. It is a great plant to fill in the mid sized understory of your food forest.
Best grown at the beginning of the rainy season, Sesbania Grandifolia starts producing edible bean pods within nine months and will continue growing beyond that. It responds well from heavy pruning which should be employed to ensure the tree stays within a manageable size.
There is another Sesbania that is native to Hawai’i, Sesbania Tomentosa. It grows in the lowland areas as a ground cover or very small shrub. It also fixes nitrogen but does not have any edible parts. Both Sesbania plants prefer to grow in the drier parts of the islands.
Rattlepod (Crotalaria longirostrata and Crotalaria tetragona) – Crotalaria has many species of plants, but only a few are edible. These plants fix nitrogen and are amazing at building soil.
These plants are high in protein. The leaves, flowers and young shoots are typically eaten in stir-fries.
I have this plant growing in my garden as volunteers. I never planted them but there they are. They seem to thrive in well-draining soils on the windward sides of all the islands.
Pigeon Pea (Cajanus cajan) – The Pigeon Pea is a very popular plant in the tropics and is sought after by many home gardeners. It is a great companion to any plant, whether it be in your vegetable garden or planted as a support species for trees.
The Pigeon Pea grows into a small shrub and produces a large number of edible pea pods per plant. They produce well for 2-5 years before their production drops off considerably.
Asy to grow from seed, Pigeon Pea does well in poor soil in arid climates but also performs well in the windward sides of the islands. Pigeon Peas are a fantastic nitrogen-fixing plant to add to any tropical garden rotation.
Perennial Peanut (Arachis glabrata) – Perennial Peanut is a great ground cover for the tropical orchard. I would keep it away from any annual crop production area though.
Once established by cutting, the perennial peanut roots deep into the soil making it difficult to eliminate once established. However, once established, they outcompete weeds to make a blanket of perennial, nitrogen fixing groundcover.
Perennial peanut grows very fast, is great animal fodder and the flowers attract hundreds of bees when they bloom. Don’t confuse this plant for the peanut that you’re used to at the stores, this plant does not produce edible peanuts.
Desmodium – There are many species of Desmodium that are typically used as a green manure in agricultural systems. They are typically tilled into the soil to fix nitrogen. Desmodium is a popular forage crop for turkey, goats and cattle. If you are looking for a cover crop for your tropical garden, Desmodium would be a good choice.
Nanea (Vigna marina) – A native to Hawai’i, related to yard long bean, black eyed peas and mung beans, Nanea grows well from the sandy shoreline up to 1500’. They are a short lived perennial with a vining growth habit.
Nanea are a great alternative to perennial peanut in elevations below 500’ on the dry side of the islands. They do not produce edible food, but have been used medicinally by the polynesians to cure wounds, boils and ulcers.
Nanea is an easy crop to grow. It readily propagates by seed or cutting and should be considered essential to any lowland garden on the leeward side of all the islands.
How To Use Nitrogen Fixers to Feed Your Garden
Now that we know what Nitrogen Fixing plants do well in our Hawaiian gardens we have to learn how to use them properly.
In the tropics, most available nutrients are not in the soil but in the organic matter, ie leaves, branches, flowers and fruit of surrounding plants. Through proper management, we can use nitrogen fixers to increase the available nutrients for our main edible species.
When using nitrogen fixing trees or shrubs, you can consider a few methods of garden design to implement a successful strategy when using nitrogen fixers. Clump plantings, on contour Hedgerows, Single plantings are all methods for growing nitrogen fixers (See illustration below).
I have a small, flat property where I implemented the clump plantings technique. I found this strategy to work very well, as long as I was able to keep up with the management. Remember, don’t overdo it with the planting of Nitrogen Fixers. Too much is not necessarily a good thing.
Coppicing (Chop & Drop)
As the plants get established, they respond well to regular coppicing or pruning. The idea is to chop a large percentage of the tree or shrub species while they are flowering and leave the prunings on the ground, preferably as a mulch around a neighboring tree. Waiting till flowering increases Nitrogen fixation. This is the chop and drop method popularized in Permaculture.
The leaves and small branches breakdown into the soil, creating mulch and eventually rich compost that is full of valuable nutrients. As this happens, larger branches get left behind that I throw out of the garden from time to time to keep it all tidy. A few end up remaining to decompose more fully.
Sometimes I will use the leaves from coppiced trees in my compost pile. I wait for them to dry and fall off the pruned branches, then proceed to rake them up and add them to the compost as my carbon layer. The leaves and valuable nutrients to any compost pile.
As your fruit trees age and fill in more space in the landscape, some of your nitrogen fixing trees and shrubs can be removed permanently. In the early stages of orchard development, you should have a ratio of 3-5 nitrogen fixing trees to every 1 fruit tree, but as things get more established you’re looking at a more 1:1 ratio.
The Nitrogen Fixing groundcovers tend to be used slightly differently from their tree and shrub counterparts. In perennial orchards, nitrogen fixing ground covers are used for their nitrogen fixing capabilities as well as their ability to suppress weeds. They tend to be a permanent fixture in these orchards.
When using nitrogen fixing ground covers in annual food production, these plants tend to get used as a green manure, getting tilled into the soil when flowering. What follows are heavy feeding plants such as squash or corn.
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