I am currently working on a regenerative landscape design for a site in Todorovo, Bulgaria. The plan is to establish an agroforestry system known as alley cropping wherein rows of mixed species edible trees and shrubs are planted at intervals with spaces for herbs, forage and/or grain crops to be grown in between. It's a dynamic system which is inherently diverse, providing multiple yields and excellent habitat for wildlife while at the same time being relatively resilient to a changing climate.
An essential component of the design will be the nitrogen fixing perennial plants within the community of fruit and nut trees. These plants will be pruned at regular intervals to provide biomass for surface mulch and to release a biological source of nitrogen to the surrounding productive plants and soil life by means of root shed associated with top pruning.
When selecting plants for the nitrogen fixing component of this design, I was looking for species that could; withstand record lows of -28oC (Zone 5); tolerate some shade; were fast growing; tolerant of trimming and coppicing; able to grow in clay soils; known to provide significant quantities of nitrogen; easy to propagate from seed; and provide some food for humans and other animals. The following plants fit the criteria.
Elaeagnus angustfolia - Oleaster, Russian Olive
Elaeagnus commutata - Silverberry, Wolfberry
Elaeagnus umbellata - Autumn Olive. Autumn Elaeagnus
Caragana arborescens - Siberian Pea Tree
We are planning to grow the nitrogen fixing plants for this site from seed and to involve the local community in doing so. Many local people, particularly the older generation are skilled horticulturalists with many seasons of experience behind them. We hope to include a number of them in the process of propagation, each one functioning as a individual unit. This will keep the propagation process small scale, making it far easier to use biological methods. The propagation will begin in the autumn as Elaeagnusspp. all require cold stratification unless they are sown immediately after they are picked. Caragana aborescens will be sown in the spring 2015.
Read more: http://www.permaculture.co.uk/articles/nitrogen-fixing-species-agroforestry-systems
