There is not a lot of information on what can grow or not grow near a walnut tree. A few needle evergreens are known to tolerate a walnut - it is a hemlock. Hemlock need shade hen young but full sun as they age (15+ years). Most arborvitaes are also tolerant but need full sun. Norway spruce, mugo pine, red pine, white pine and yews do NOT tolerate being within 50 feet of walnut