Recommendations for trees to plant near established walnit trees

Asked October 4, 2018, 3:58 PM EDT

Hello, I planted 2 butterfly magnolia trees (through Friends of Trees) on my planting strip in spring of 2018. Both did really well in spring and in to early September Then the most northern planted tree's leaves turned yellow/brown, and overall the tree looks unwell. There are lots of new buds on it though, so it's kind of confusing. Anyway, Friends of Trees believes it is because my neighbor to the north has a walnut tree on the planting strip, and it is essentially poisoning the magnolia. They recommend I replant a tree that plays well with walnut trees. I was on a watering program all summer with FOT, so I do not believe it is related to under watering. Do you have recommendations for trees that work with walnut trees? The tree will face east, with about 6 hrs of sun per day. I am located in North Portland. Thank you for your help.

Multnomah County Oregon walnut trees

1 Response

Thank you for your question. The issue that Friends of the Trees is referring to is a type of poison called juglone that walnut trees have in all parts of their structure, and especially in the roots. And, as you observe, the problems vulnerable trees show are similar to that associated with drought and under-watering. The negative impact of juglone can extend out 50 to 80 feet, so it is a very rational conclusion that the walnut trees are the issue. Here is one (of many) Extension article that explains the issue, and has a chart of plants which are resistant to juglone: https://hort.uwex.edu/articles/black-walnut-toxicity/

This second Extension article has an even longer list: http://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/430/430-021/430-021_pdf.pdf

You might want to cross-reference the trees in either list with those in the following Metro guide, which has plants native to our area, that are adapted to our climate and water supply, with photos and size information of the trees: https://www.oregonmetro.gov/native-plants-willamette-valley-yards-booklet

I hope this is helpful. If you need further information, don't hesitate to respond! Good luck!