Article
Native trees and shrubs that fix nitrogen, and the legumes that do not
The native shrubs and trees that fix nitrogen, by partner and soil, why honey locust and redbud do not, and how much a fixer gives the fruit trees nearby.
On this page
The native fixers and their partnersThe legumes that fix no nitrogenHow much nitrogen a fixer adds, and who gets itWhat a fixer means for a fruit tree beside itChoosing a fixer for your groundCommon questionsSourcesBlack locust, clammy and bristly locust, the indigobushes and leadplant fix nitrogen with rhizobia, the bacteria that live in the root nodules of legumes. The alders, the bayberries, buffaloberry and New Jersey tea with its kin fix it with Frankia, a different microbe in nodules of their own. Some legumes fix none, among them honey locust and eastern redbud. A fixer keeps most of what it makes, and the plants around it get a share year by year, as its fallen leaves and dead roots decay.
The native fixers and their partners
Blue Whale Trees grows all nineteen fixers in the table: eleven legumes with rhizobia, then eight shrubs with Frankia, each group tallest first.
| Plant | Partner | Height, feet | Zones | Soil moisture |
|---|---|---|---|---|
| Black locust | Rhizobia | 40–70 | 4–8 | Dry to average |
| Clammy locust | Rhizobia | 30–40 | 3–8 | Dry to average |
| Indigobush | Rhizobia | 6–12 | 4–9 | Dry to wet |
| Schwerin’s indigobush | Rhizobia | 3–10 | 7–8 | Dry to average |
| Shining indigobush | Rhizobia | 3–10 | 7–9 | Dry to wet |
| Bristly locust | Rhizobia | 3–8 | 5–8 | Dry to average |
| Mountain indigobush | Rhizobia | 3–6 | 7–8 | Dry to average |
| Ouachita indigobush | Rhizobia | 3–6 | 5–8 | Dry to average |
| Clusterspike indigobush | Rhizobia | 2–4 | 7–10 | Dry to average |
| Georgia indigobush | Rhizobia | 1–3 | 8–9 | Average to wet |
| Leadplant | Rhizobia | 1–3 | 2–8 | Dry |
| Seaside alder | Frankia | 12–30 | 3–9 | Average to wet |
| Odorless bayberry | Frankia | 10–20 | 7–10 | Average to wet |
| Smooth alder | Frankia | 10–20 | 4–9 | Wet |
| Northern bayberry | Frankia | 5–10 | 3–7 | Dry to wet |
| Canada buffaloberry | Frankia | 3–8 | 2–8 | Dry to average |
| New Jersey tea | Frankia | 2–3 | 4–8 | Dry to average |
| Prairie redroot | Frankia | 2–3 | 4–8 | Dry to average |
| Littleleaf buckbrush | Frankia | 1–2 | 8–10 | Dry |
The legumes that fix no nitrogen
Honey locust, which we do not grow, fixes no nitrogen, and four legumes we grow are not known to fix any. Unlike most legumes, honey locust forms no root nodules. Neither its genus, which includes water locust, nor its close relative Kentucky coffeetree is known to nodulate.
Eastern redbud forms no root nodules either and does not appear to fix nitrogen. Yellowwood comes from an early branch of the pea subfamily in which nodules are uncommon, and it forms none.
How much nitrogen a fixer adds, and who gets it
A stand of fixers adds up to several dozen pounds of nitrogen an acre a year. At the Coweeta Hydrologic Laboratory in the North Carolina mountains, black locust stands gained 29 to 67 pounds an acre a year, the most at 17 years old. On the dune sands of the Northeast, northern bayberry adds an estimated 14 to 29 pounds an acre a year.
Little of that nitrogen reaches a neighbor while the fixer lives:
“Almost all of the fixed nitrogen goes directly into the plant.” (New Mexico State University Cooperative Extension Service, Nitrogen Fixation by Legumes)
Some leaks into the soil from living roots. Most reaches other plants only as the fixer’s fallen leaves, dead roots and fruit decay. So the gain stays close by, as with Canada buffaloberry on poor ground:
“Its nitrogen-fixing ability allows it to grow in soils with low amounts of mineral nitrogen, which are common in disturbed areas. It also enhances the growth of associated species by producing “an island of fertility” around its perimeter.” (USDA Forest Service, Fire Effects Information System, Shepherdia canadensis)
The gain also lasts. Where black locust spread into New York pine barrens, the extra nitrogen outlasted the trees after they were removed and in places favored weeds and grasses over the wildflowers.
What a fixer means for a fruit tree beside it
For a fruit or nut tree beside a fixer, the gain is real but gradual. Black walnut has grown better planted with black locust, and interplanted walnuts in Canada carried more nitrogen in their leaves. The locust’s wide crown and root sprouts can also suppress slow young trees planted with it, so give it room.
Fertilizer and fixers also work against each other, since a legume given plenty of nitrogen slows its own fixing. A fruit tree short of nitrogen this season needs a soil test and the right feed, not a new neighbor.
Choosing a fixer for your ground
Choose the fixer by the ground it will grow in:
- Any yard: the tree finder opens on the fixers we grow, to narrow by zone, size, sun and soil moisture.
- Dry sand: leadplant, northern bayberry and bristly locust, with how to plant them, in native trees and shrubs for dry, sandy soil.
- A wet corner: smooth alder, with five other natives that stand in water for weeks, in the Wet Spot Pack.
Common questions
Does black locust fix nitrogen?
Yes, through rhizobia in its root nodules, and black locust stands in the North Carolina mountains gained 29 to 67 pounds of nitrogen an acre a year.
Is honey locust a nitrogen fixer?
No. Honey locust is a legume, but it forms no root nodules and fixes no nitrogen, and no species of its genus is known to nodulate.
Does redbud fix nitrogen?
No. Eastern redbud is a legume, but it forms no root nodules and does not appear to fix nitrogen.
Are nitrogen-fixing trees good for an orchard?
They help a little each year, as their fallen leaves and dead roots decay, and black walnut has grown better beside black locust. Give a big fixer room, since black locust’s crown and suckers can suppress young trees.
Sources
- Robinia pseudoacacia, black locust: USDA Forest Service, Fire Effects Information System (Katharine R. Stone, 2009): black locust as a nitrogen fixer, net nitrogen gains of 33 to 75 kilograms a hectare a year in stands 4 to 38 years old at Coweeta, its leaves the richest in nitrogen of the trees tested in a North Carolina forest, better growth and more leaf nitrogen in walnuts planted with it, its crown and root sprouts suppressing slow interplanted seedlings, and the nitrogen that outlasted it in New York pine barrens.
- Morella pensylvanica: USDA Forest Service, Fire Effects Information System (A. Scott Hauser, 2006): northern bayberry nodulated by Frankia, and the estimated 16 to 32 kilograms of nitrogen a hectare a year it adds to the nitrogen-poor dune sands of the Northeast.
- Alnus incana: USDA Forest Service, Fire Effects Information System (Janet L. Fryer, 2011): the nitrogen in fallen alder leaves and dead roots released to other plants as they decay.
- Shepherdia canadensis: USDA Forest Service, Fire Effects Information System (Crystal J. Walkup, 1991): buffaloberry as a nitrogen fixer of poor soil, its nodules most abundant in nutrient-poor, sandy soils, and the quotation on its island of fertility.
- Gleditsia triacanthos: USDA Forest Service, Fire Effects Information System (Janet Sullivan, 1994): honey locust forming no root nodules and fixing no nitrogen, unlike most legumes.
- Cercis canadensis: USDA Forest Service, Fire Effects Information System (Janet Sullivan, 1994): eastern redbud forming no root nodules and not appearing to fix nitrogen.
- Nitrogen Fixation by Legumes: New Mexico State University Cooperative Extension Service, Guide A-129 (Robert Flynn and John Idowu): rhizobia in the root nodules of legumes, some nitrogen leaking from living roots, most of it returning as the plant’s roots, leaves and fruit decompose, fixing slowed by plentiful soil nitrogen, and the quotation that almost all fixed nitrogen goes into the plant.
- Frankia Diversity in Host Plant Root Nodules Is Independent of Abundance or Relative Diversity of Frankia Populations in Corresponding Rhizosphere Soils: S. Ben Tekaya, T. Guerra, D. Rodriguez, J.O. Dawson and D. Hahn, Applied and Environmental Microbiology 84 (2018): Frankia as the soil actinobacterium in the root nodules of actinorhizal plants, and the Frankia groups that nodulate the alders, the bayberries, ceanothus and buffaloberries.
- Advances in Legume Systematics 14. Classification of Caesalpinioideae. Part 2: Higher-level classification: A. Bruneau, L.P. de Queiroz, J.J. Ringelberg and others, PhytoKeys 240 (2024): the tribe Gleditsieae of Gleditsia and Gymnocladus, the honey locusts and the coffeetrees, long known as close relatives, and none of its genera known to nodulate.
- Accumulation of ENOD2-Like Transcripts in Non-Nodulating Woody Papilionoid Legumes: C.M. Foster, H.T. Horner and W.R. Graves, Plant Physiology 124 (2000): American yellowwood as a woody legume of the pea subfamily that does not nodulate, from an early branch in which nodulation is not common.