My forestry research work took me to the USA and Italy looking for the best ways to grow walnut trees for timber. Foresters in both countries pioneered the planting of nitrogen-fixing trees or shrubs with hardwood trees, where their ability to improve soil fertility for ‘hungry’ species, such as walnut, had proven very beneficial.
The most promising companion plant for walnut seemed to be a little known shrub in forestry, Elaeagnus umbellata, sometimes known as spreading oleaster or Autumn olive. This five metre tall and wide shrub fixes atmospheric nitrogen and makes it available to other neighbouring plants. Its wide-spreading crown quickly smothers competing weeds and it protects young trees from wind. When planted in a hardwood tree plantation its limited height does not compete with the main crop. Finally, as a drought-tolerant plant too, it can be considered future-proof in the light of projected climate change.
In the USA Elaeagnus umbellata is listed as an invasive species but here in the UK we have been growing it in our parks and gardens for 200 years without any such problem. In the forestry trials I established across four sites in England in 2000 the shrub has been shown to increase walnut survival and growth, and there has been no sign of invasiveness.
You can find more about this work and similar research on my publications page:
JR Clark and GE Hemery, “The use of autumn olive (Elaeagnus umbellata Thunb.) in British forestry,” Quarterly Journal of Forestry 100, no. 4 (2006): 285-288.