I have a Black Walnut tree, about 25' high, 12" diameter and, I would like to plant two dwarf Alberta Spruce trees, one on each side of the walnut tree. The Alberta Spruce trees are about 4' tall. Can I do this without killing the spruce trees and, if so, how far away from the walnut tree should I plant the spruce trees?
Walnut tree are allelopathic, meaning that they inhibit their neighbours by putting up some type of chemical defense. You should think of how big the bole of the Black walnut will get and the influence of the fallen leaves, which will smother and inhibit the proposed Spruce planting. Get the Spruce trees well away from the Walnut as much as possible. Spruce trees like almost full sun, with moisture available throughout the season, something that the Walnut would greatly inhibit. Good luck
Picea abies Norway Spruce is said to be somewhat tolerant of Juglone (the "herbicide" produced by walnut trees). The Black Walnut is the most allelopathic of the walnuts, so consider your nearby plantings carefully. Have a look at this list from the U of Wisconsin: http://www.uwex.edu/ces/wihort/landscape/Juglone.htm Landscape: Juglone Tolerant Plants Ralph
Dwarf Alberta spruce is a shade-intolerant alpine rock garden type conifer, which would look quite out of character and probably receive to much shade and litter underneath a walnut tree.
For what it's worth I've seen it done having Alberta Spruce planted along with a Walnut. The too much shade aspect is a real concern. If you can control the Walnuts canopy and place the Spruce about 15-20' from the base of the Walnut on each side (wish I knew your prevalent sun direction, here it is West and East) then you can get by having these trees grow among each other. It is the husks from the Black Walnut that will cause you the most worry, more so than the invasive roots from the Walnut tree. In order to have these trees co-exist will require you to be thorough with your clean up of the dropped nuts, the husks and the leaves. You cannot let any of them stay on the ground in close proximity to your Spruce until your Spruce have become better established, which for them is around 5 years in the ground. Give the Spruce as much sunlight as you can or they will brown out on you. Jim
