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Wednesday - July 22, 2009

From: Harrisville, MI
Region: Midwest
Topic: Trees
Title: Planting under Walnut Trees in Harrisville, MI.
Answered by: Anne Bossart

QUESTION:

I have 2 50+ yr old Black Walnut trees in Northern Michigan (zone 4). I am planting a new bed (raised of course) and was considering adding a hydrangea. I am curious if this will thrive due to the juglone caused by Black Walnut or if I should be ok. Some sites say YES and some say NO.

ANSWER:

You didn't mention if you are building a raised bed in the root zone of the walnut nor how deep it is. Most of a tree's feeder roots (which need water, nutrients and air) are in the top 36 inches of the soil, no matter how big the tree is. So any time you change the existing grade and cover those roots with soil, you are putting the tree in jeapordy.

You also didn't mention what type of hydrangea you are planning to plant ...and hydrangeas don't appear to be juglone tolerant. Some native shrubs suitable to your area that are juglone tolerant include:

Amelanchier arborea (common serviceberry)

Hamamelis virginiana (American witchhazel)

Hypericum prolificum (shrubby St. Johnswort)

Physocarpus opulifolius (common ninebark)

Rhus aromatica (fragrant sumac)

However, from personal experience (I have a property at approximately the same latitide as you, on the eastern shore of Lake Huron, in Ontario) I have found that many things grow under my two very huge old (one is over 100 years old) walnut trees that should not. I wonder if the soil conditions this far north have a diminishing effect on juglone. So if you are planning to plant a hydrangea that is native to Michigan and suited to the garden conditions and plant community existing on your property, it is probably worth a try.

Amelanchier arborea

Hamamelis virginiana

Hypericum prolificum

Physocarpus opulifolius