Maple Tree near Black Walnut
Q: I would like to plant a "Pacific Sunset Shantung Hybrid Maple" somewhat close to an established Black Walnut tree. It's hard to get information on compatibility, especially on this hybrid maple.
I believe the sugar maple, red maple, and Japanese maple are compatible, while the silver maple is not. I'm worried about juglone from the walnut tree affecting the maple.
Can you give me some advice?
A: Norway maple is not on any list I have as being tolerant of juglone. Red, sugar, black and Japanese maples seem to tolerate it and, as you mentioned, silver maples are intolerant (they can't grow within 50' of the dripline of a walnut).
Since the 'Pacific Sunset' is a Norway maple hybrid, I suspect it may be effected if its roots get near enough to the walnut's. My suggestion would be to contact the nursery and ask them -- if anyone would know, it would be they.
Juglone only effects other plants if the roots are intermingled, or at least growing closely, so much depends on how close you intend to plant the maple to the walnut.
The spread of the root system of your walnut reaches, and probably goes beyond, its dripline, so planting within the dripline definitely places the new tree within the root zone, so I wouldn't suggest planting anywhere beneath the tree. The further away, the better.
You might think to sever the roots on the side you intend to plant the new maple. Consider renting a "ditch witch" trencher and trench on a tangent to the dripline on that side, going as deep as possible (probably 2-3').
If you're at, or beyond, the walnut's dripline, this trenching will not hurt it one bit, and will give your new tree a chance to survive. You could also place a root barrier into the trench before filling it back in to keep roots from ever passing beyond this point.
Actually, you could trench as close as 5 times the walnut's trunk diameter (measured at 4 1/2' above ground level) without seriously harming it. So, if your walnut is 24" in diameter at this height, you could trench (on a tangent to the trunk) as close as 10' and not bother it very much.
Yes, there will still be roots in the ground that will continue to give off their juglone even after being severed by trenching, but this should dissipate over time.
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