Hi,

Walnuts and their juglone chemicals that are dispersed into surrounding soil come with a little bit of a debate, Walnut Toxicity. Walnuts do however have good wildlife value and building raised beds near them for vegetable growing is highly unlikely to exhibit problems with juglone impacting growth.

You can explore soil composition for raised garden beds on our webpage. If the soil is at least 4 inches deep, that should be sufficient to aid in the decomposition of the cardboard you are putting underneath. Just be sure to use unwaxed cardboard so it does break down. If any beds are within 10 feet of the dripline of the walnut tree, we suggest beds greater than 8 inches deep in case juglone is an issue. You could also try planting a few tomato plants this spring in the native soil on the edge of the garden closest to the tree to see if they pick up any wilt symptoms, if you're curious or want to experiment with juglone possibly effecting the growth.

Let us know if we can be of more help with this or other gardening questions. 

-Emily