What they are
A short list of shrubs that fix their own nitrogen. Sea buckthorn (Hippophae rhamnoides). Goumi (Elaeagnus multiflora). Autumn olive (Elaeagnus umbellata). Russian olive (Elaeagnus angustifolia). Siberian pea shrub (Caragana arborescens).
Four of those five are not legumes. They host Frankia, a filamentous soil actinobacterium, in root nodules. Legumes partner with Rhizobium. These shrubs run a separate, older symbiosis called actinorhizal. Different biology, same outcome: atmospheric nitrogen pulled into living tissue and dropped on the soil as leaf litter.
Siberian pea is the legume in the group. The other four are actinorhizal. They all do the same job on the landscape.
Why it works
Frankia lives free in most temperate soils at low densities. When a host root crosses one of its hyphae, the bacterium infects through a root hair, builds a nodule, and starts splitting N2 into ammonia using its own nitrogenase enzyme. The shrub feeds it sugar. The bacterium feeds the shrub nitrogen.
Fixation rates run 70 to 180 kg N per hectare per year. Sea buckthorn on a young dune sits near the top. Autumn olive on disturbed eastern soil hits 100 to 130. Siberian pea on cold steppe pushes 60 to 100. A heavy compost application, for reference, delivers maybe 50.
The nitrogen does not stay in the nodule. Leaves drop. Roots turn over. Pruned material rots in place. A tree growing within three meters of a mature sea buckthorn pulls measurably more nitrogen than the same species ten meters away.
See Nitrogen-Fixing Bacteria for the wider microbial picture, and Nitrogen-Fixing Trees and Plants for tree-scale partners.
Pick the right one
Each shrub fits a niche. Get this wrong and you create a problem.
Sea buckthorn. Native belt from western Europe across to China. Loves sun, sand, salt, and cold down to minus 40 Celsius. Thorny. Suckers hard from the roots. The orange berries carry more vitamin C by weight than blackcurrant. Plant it on degraded sandy ground, eroded slopes, or a windbreak edge. Dioecious. You need one male per six to eight females for fruit.
Goumi. East Asian native. Smaller and tamer than its Elaeagnus cousins. Self-fertile. Red speckled berries by midsummer. Fits cleanly into a fruit tree guild as the shrub layer. Not aggressive. Hardy to minus 30. The one in this family worth planting almost anywhere.
Siberian pea shrub. Steppe species from Mongolia and Siberia. Hardy to minus 50. Yellow pea flowers in spring, edible seeds, browse for livestock. Tolerates drought and salt. Slower than sea buckthorn but cleaner: clumping habit, no aggressive suckering. The classic prairie windbreak nitrogen-fixer.
Autumn olive. Stop. In the eastern half of North America this shrub is invasive. Planted by the millions for wildlife and mine reclamation between 1950 and 1980, it now chokes old fields from Ohio to Virginia. Birds spread the seed for kilometers. Do not plant it east of the Mississippi. In its East Asian native range it is fine.
Russian olive. Same warning, different region. Naturalized across riparian zones in the western US and Canadian prairies. Crowds out cottonwood and willow along streams. Legal in some states, banned in others. Check before you buy.
The fix when you want the function without the invasion: goumi or Siberian pea in North America. Sea buckthorn on already-disturbed ground where it can be contained.
Place them
Treat them as part of the structure, not the canopy.
Three meters from any fruit tree you want to feed. Closer and the shrub competes for light. Further and the nitrogen pulse fades. A ring of three goumi around a young apple is a working dose for the first ten years of that tree's life.
In a windbreak, run sea buckthorn or Siberian pea as the second or third row. They take the wind and feed the inner rows. See windbreaks for layout principles.
On restoration sites, cluster plantings with one nitrogen-fixer per three to five productive species. Cut and drop the shrub every two or three years to push nitrogen into the soil. The shrub regrows. The neighbors get a feed.
Tend them
Chop and drop in late winter or early spring before bud break. Cut a third of the stems to the ground each year. New growth runs nitrogen-rich. Drop the cuttings around the base of nearby trees as mulch. See Chop and Drop for the broader practice.
Do not fertilize with nitrogen. Free soil nitrogen shuts down nodulation. The shrub gets lazy. The whole point evaporates.
Yellowing on a young plant usually means the Frankia inoculum is missing. Common on heavily disturbed ground and ex-conventional fields. Fix it with a handful of soil from under an established shrub of the same genus, dropped into the planting hole.
Harvest sea buckthorn berries by cutting whole fruit-bearing branches and freezing them. The berries pop off cleanly when frozen. Picking them fresh from the thorny stems is a slow misery.
When it goes wrong
The shrub keeps suckering into the vegetable beds. That is sea buckthorn doing what it does. Trench a 40 cm root barrier or move it to ground where suckering is welcome.
Berries never set. Dioecious species need a male within bee-flight range. Check what you bought. Many nurseries ship females only and forget to mention it.
It escapes into the woods. Then you planted autumn olive or Russian olive in their invasive range. Cut at the base, paint the stump with concentrated vinegar or a targeted herbicide, and replace with goumi or a native nitrogen-fixing tree. Birds carry the seed faster than you can pull seedlings.
Neighboring fruit trees look no better after five years. Either the shrub is too far away, the soil was already nitrogen-rich, or you have been feeding the trees synthetically. Stop the feed. Move the shrub closer. Mulch with its prunings.
See also
- Nitrogen-Fixing Trees and Plants
- Nitrogen-Fixing Food Trees
- Fruit Tree Guilds
- Chop and Drop
- Native Plant Selection