Caragana arborescens Seeds | Siberian Peashrub
LATIN NAME: Caragana arborescens
COMMON NAME: Siberian Peashrub
Plant Profile
The genus Caragana holds around eighty woody species across Central Asia, and several are grown. Caragana korshinskii and Caragana microphylla are the sand-fixing species of the Chinese deserts, smaller and more sprawling, while Caragana sinica is the southern Chinese shrub of dry hillsides with larger flowers. Caragana arborescens is the tallest and most upright of the group, which is why it is the one used for hedges and small trees.
The shrub carries nitrogen-fixing nodules on its roots, which is the main reason it grows where nothing else will. Like other woody legumes it partners with soil bacteria in the root cortex, and the partnership lets the plant use atmospheric nitrogen on soils too poor to support most crops. Fallen leaves therefore enrich the ground beneath the thicket.
Caragana seed is built to sit in the ground. The testa carries a palisade layer of thick-walled cells impregnated with suberin, which is waterproof, and that layer keeps water out until it is breached by abrasion, heat or the passage of time. This physical dormancy lets a batch of seed emerge in flushes across seasons, an asset in the steppe and an annoyance in a seedbed.
Science & Biotechnology
The microsymbionts of Caragana belong to Mesorhizobium, a genus of rhizobia that specialises in these shrubs. Inside the nodule, the bacterium reduces nitrogen using the enzyme nitrogenase, and a plant protein called leghemoglobin keeps the nodule interior free of oxygen so the enzyme can work. The pink colour inside a freshly cut nodule is leghemoglobin, and it is the quickest proof that the partnership is active.
Uses & Benefits
The seeds were once eaten in the same way as dried peas, and settlers in cold districts used them in soup and ground them for poultry feed. Young pods can be picked and cooked while still flat and tender. Roasted and ground, the seed served as a coffee substitute, and the bark strips into cordage.
Where wind lifts sandy soil, caragana is planted as a low barrier that slows the air at ground level, and its suckering habit knits the surface together. Rows are set across the prevailing wind, close enough that the bushes merge into a continuous hedge. Because the shrub fixes its own nitrogen and tolerates alkalinity, it will establish on raw subsoil where a grass mix fails.
Growing from Seed
No other Asian steppe shrub has been planted as widely outside its homeland. For much of the twentieth century, government shelterbelt programmes across the Canadian prairies handed out caragana seedlings by the million, and the nursery at Indian Head, Saskatchewan, grew them in rows by the mile. The shrub earned that place by surviving what killed everything else: forty below and soil that would blow.
Caragana arborescens comes from the scrub of the Altai, the Sayan ranges and the Mongolian steppe, where it grows on dry slopes and along the edges of river terraces. It often forms thickets where grazing keeps taller trees out. Its tolerance of cold, drought and alkaline ground is inherited from that setting rather than bred into it.
The leaves are pinnate with eight to sixteen small oval leaflets on a short rachis, and on vigorous shoots the stipules harden into slender spines that make the bush awkward to brush against. Yellow pea flowers open in late spring, singly or a few together, and set pods three to five centimetres long. Those pods dry and split explosively, the two valves curling apart and flicking seed a short distance from the parent.
Treat the seed before sowing, or the coats will hold water out. A hot-water soak is the simplest method at home.
For a hedge, set plants 40 to 50 centimetres apart within the row and cut the leading shoots back by half at planting to force branching from the base. A windbreak row stands further apart, a metre or more between plants, so each bush can spread without crowding. Firm the soil and water well; leave the row alone for its first year.
Older stems can split in hard frost where the site is exposed, and rabbits and deer will browse young plants to the ground in a bad winter. Renew a thicket by cutting a third of the oldest stems to the base each year, and prune hedges in late winter while the plants are dormant.
Our Take
Caragana is the plant I recommend to anyone gardening on blowing sand or thin chalk, because it asks for nothing and gives back nitrogen. Sow more than you need and plant the extras as a nursery row for later transplanting. The desert relatives are worth a look for drier ground: Caragana korshinskii for sand fixation, Caragana microphylla for a lower, spreading cover, and Caragana sinica for larger flowers on a drier bank.
Related Reading
More species in the same orbit as Siberian Peashrub: