Nitrogen-Fixing Trees for North Carolina (USDA Zone 7)

A piece of me feels guilty when I look at trees now. I used to love them for the simple beauty: the bark, the leaves, the spread, maybe some colourful flowers in spring, colourful leaves in autumn. Now, I think about function.

Truth be told, all trees have significant functions and worthiness. They clean the air. They provide leaf litter. They prevent erosion, trap heat, block wind, create wildlife habitat, and play a vital role in the water cycle. In short, even within permaculture design, when striving to have multiple functions for components, trees have an answer in-the-ready.

But, I look for more now, almost as if I expect all trees to be super-arboreal. What products can I derive? Food: fruit, nuts, leaves, flowers, roots? Firewood? Can it be coppiced? Does it grow quickly? Is the burn high quality? Is lumber an option? What are the leaves like? How is it for pollinators? The interview only continues.

In terms of characteristics, if a tree isn’t food-producing, particularly a fruit or nut tree, the next trait I look for is: Does it fix nitrogen?

Tree Appreciation

In the wild, I have learned to look at trees with a more appreciative eye. I revel over our native mountain laurel, rhododendron, and flame azalea (all in the same family), which have no edible qualities nor do they fix nitrogen. Nonetheless, they are famed for early spring flowers, as are the dogwoods, which also lack those characteristics (food & nitrogen).

I now reflexively check on hemlocks, currently engaged in a battle with the Hemlock Woolly Adelgid, an invasive and pervasive life-sucking insect. I ceaselessly admire the sycamores growing in and around our property simply because they are grand and engaging. I have some strange attraction to the tree of heaven, a naturalised invasive also known as stink tree, because we have big, old, gorgeous specimens along the roadside.

In other words, I can forgive myself a little for this focus on function because that is a designer’s concern not a nature-lovers. I wouldn’t plant a sycamore in a food forest, but I’ll gladly admire one on a walk. I would never choose the tree of heaven as a pioneering species, but a majestic one is hard to discount as all bad. In fact, because of my interest in function, I now know the story of these trees, and I actually appreciate them more, even the unpopular ones.

Invasiveness & Invention

Within permaculture design, particularly when it comes to nitrogen-fixers, we often walk a tightrope with forestry nativists. Lots of nitrogen-fixing trees are considered invasive, which should be no surprise because that is typically their function in natural systems. Where there has been disturbance, deforestation, or soil degradation, nitrogen-fixers are the first to respond, and they do so in abundance. What this often means is that fields and roadsides are usually the victims of the invasion because these are areas that are naturally attempting to reforest themselves.

The same issue arises with notorious vines like kudzu and bittersweet, which are only invasive along forest edges, where trees don’t shade them out. Kudzu is actually nitrogen-fixing and edible, was cultivated as erosion control through government sponsorship during the Dust Bowl, and soon got out of control. By the 1970s, it was classified as a weed. It’s also high-quality forage for animals; however, the massive deposits available are rarely if ever used for that.

While I’m not in any way suggesting one should plant kudzu, this is all to say that naturalised plants, around for decades, in essence have become recognised citizens. They will not be eradicated, and I have to wonder how much of a threat they are to established forests. They are invasive where humans have created the opportunity to be: along roadsides and in cleared agricultural fields. Even so, they are often demonised for performing as they have biologically evolved to do, but have humans not set the stage for them to invade? Are we not responsible, not the “weeds”?

In that vein, I know that at times my willingness to include recognised invasive trees as viable choices for nitrogen-fixers can ruffle feathers. However, I do so with the concept that food forests are managed ecosystems, particularly in the beginning, in which nitrogen-fixers are used for specific functions—fertility, forage, food, and mulch—and not left to run rampant as kudzu was. I also would be less likely to choose an invasive tree that hasn’t already fully naturalised in the area. Like weeds, I can’t help but wonder if invasiveness has been more human invention than botanical devilishness.

Nitrogen-Fixing Species for North Carolina

With all that in mind, I’ve put together a list of nitrogen-fixing species for Western North Carolina, where my wife Emma and I live and are developing a homestead with a food forest. Our food forest is only about a thousand square meters (1/4 acre), so larger over-story trees don’t necessarily feature in it except for coppicing. However, this mix of choices covers the over-story and understory, includes notable edibility, as well as firewood, timber, and other useful aspects.

Native:

• Black Locust (Robinia pseudoacacia)

The black locust is native to the eastern United States but has naturalised throughout the lower 48. It’s now present in Europe, Asia, Africa, Australia, and South America. It is considered invasive in some states because it spreads rapidly via suckering, but it supplies premium firewood and long-lasting fencing timber. It tends reach about 30 meters (100 feet) and is not shade-tolerant, so established forests, such as the oak-pine forests here, aren’t threatened by it but grasslands are. The flowers are edible and can be sold as niche crops in season, and they are also great for honey production.

• Tag Alder (Alnus serrulata)

Tag alder trees are small (less than five meters) and more shrub than tree, but several are growing in and around our dam. They are particularly good for growing near streams, rivers, and ponds, and as one might guess, they require a lot of water. They produce subpar firewood and have some edibility, though in a last resort survival kind of way. They can be used as a stabiliser in wetland environments and there are some medicinal teas that can be brewed from the bark.

• Eastern Redbud (Cercis canadensis)

Eastern redbuds produce stunning early spring blooms (more pink than red) that are edible, delicious, and nutrient-rich. They grow to about 9 meters (30 feet) tall, have lovely heart-shaped leaves, and are commonly used as ornamentals. Unlike many legume trees, they do not have bacterial nodules on their roots, but some studies have shown that they do fix nitrogen into the soil somehow. They are otherwise beautiful and useful, so it makes sense to include them in the mix.

Naturalized:

• Honey Locust (Gleditisia triacanthos)

Technically, the honey locust trees’ natural range is just outside of North Carolina, but does include Appalachia and the Blue Ridge Mountains. These are medium-sized legume trees (20 meters/60 feet) that grow quickly and get classified as invasive, especially in Australia. They are very tolerant of bad soil conditions, including road salt, compaction, and alkalinity. The pulp in the pea pods is edible and makes good livestock feed; however, despite its name, the tree isn’t especially good for honey production. Like the black locust, honey locust produces fantastic firewood and rot-resistant timber.

• Mimosa (Albizia julibrissin)

Mimosa trees, despite having stunning and pleasantly aromatic flowers, are terribly unpopular with some because they are considered invasive. However, mimosa trees have been in the US for hundreds of years now and are rampant around North Carolina. They are short-lived, good for chop-and-drop mulching, and attract pollinators (hummingbirds, especially). The flowers are highly edible, and the leaves can be used as a soup/stew vegetable. Mimosa trees are drought and wind tolerant as well.

• Russian Olive (Elaeagnus angustifolia) 

Native to Eurasia, Russian olive has only been in the US for about 100 years, and it is not popular. Brought in to be a windbreak, soil stabilizer, and habitat species, Russian olive trees have proven to be even better at escaping cultivation. They grow to about 7 meters, have aromatic flowers, and produce sweet (edible) drupes. They shine in poor soil, reproduce abundantly, and mature quickly. In North Carolina, this is a coastal species, where it can be a chop-and-drop NFT that will continue to provide biomass year after year.

• Autumn Olive (Elaeagnus umbellata)

Related to the Russian olive, the autumn olive also has edible drupes, puts out sweet-smelling flowers, and possesses nitrogen-fixing abilities, three qualities that make it hard to resist for a permaculture food forest. It, too, is considered a highly invasive species. Though it isn’t shade-tolerant, posing little threat to existing forests, it can disrupt the natural succession in developing forests. Again, this could be a viable chop-and-drop species and could be managed to avoid spreading as it is naturally propagated by bird droppings (cut it before the fruit develops). It’s great for erosion and as a windbreak.

• Princess Tree (Paulownia fortunei/Paulownia tomentosa)

Amongst the fastest growing trees in the world (up to 3 meters/20 feet a year), the Princess tree surpasses 20 meters (up to 75 feet) at maturity. It puts on amazing flower shows, and it is grown as timber in its native China and Taiwan. It’s very good for contaminated environments with potential heavy metals in the soil and air pollution. An adapted pioneering species, it specializes in disturbed soils and requires full sun. It is another chop-and-drop, biomass producer that could be a food forest pioneer, and grazing goats or sheep are known to be the solution to Paulownia regrowth. The soft wood is also supposed to excellent for carving.

Specialised:

• Goumi Berry (Elaeagnus multiflora)

Relative of the Russian olive and autumn olive, goumi berry trees are known to be a little less invasive with tastier drupes to enjoy. That said, they are not native nor naturalised here in North Carolina, so they have to be ordered from out-of-state. They grow to about 8 meters (25 feet). They are tough, nice-looking trees/shrubs that tolerate a wide variety of soil conditions and temperatures well below freezing.

There are several other options to look into for nitrogen-fixing in North Carolina or other humid Zone 7 locations. Sea buckthorn (Genus: Hippophae), New Jersey Tea (Genus: Ceanothus), and pea shrubs (Genus: Caragana) are all shrubs worthy of consideration, consist of several viable species, and have edibility on their side to boot. I looked several times for a good list of nitrogen-fixers for the area and failed to find one, so here’s a good start for those in a similar situation.