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Would the spot behind this walnut tree be suitable for planting a goldcrest conifer?
If I cleared up a large patch of the overgrown grass you can see behind the tree, does it look like it'd be suitable for planting conifers? (There is actually a lot more overgrown grass space than you can see in the picture).
Also, would the conifer be likely to have a detrimental effect on the growth of the walnut tree or vice versa?

Also, would the conifer be likely to have a detrimental effect on the growth of the walnut tree or vice versa?

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“Allelopathy
Walnuts secrete chemicals into the soil which inhibit other plants. This is known as allelopathy. However, the effect is more discernable from Juglans nigra (black walnut) than J. regia (common or English walnut). Tomatoes and apples are said to be especially affected”
https://www.rhs.org.uk/nuts/walnuts
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
I find it rather worrying that someone can just 'plant a tree' without any regulation or information. I assume this isn't your own garden though.
I live in west central Scotland - not where that photo is...
When you don't even know who's in the team
S.Yorkshire/Derbyshire border
At a minimum, I would recommend you move it much further away from your fence, as it's canopy will spread to 4-5 metres within about 5-10 years. We have experienced no problems with ours killing other plants (we have a White walnut), and we have successfully planted immediately around the trunk with no problems whatsoever.
Your problem is going to be the fast growth rate, and eventual size of the tree. Could you have a friendly chat with whoever bought you the tree, so see if you could replace it with something something more suitable, and perhaps find a better home for it? If not, I think you may find it causes you problems in the future. They don't like to be pruned, they can bleed very heavily, and the natural shape is ruined when you start pruning them. We know this from bitter experience. I definitely wouldn't plant another large shrub near to it, they will compete for water and nutrients.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.