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Saturday - August 02, 2014

From: New Haven, MO
Region: Midwest
Topic: Problem Plants, Shade Tolerant, Herbs/Forbs, Trees
Title: Magnolia species are allelopathic
Answered by: Nan Hampton

QUESTION:

Have a healthy Southern Magnolia tree around 8 years old. It seems like everything I plant next to it dies.: Variegated Spirea, Stokes Aster, Hydrangeas. Is there something it secretes like the walnut trees do that kills certain plants? Thanks.

ANSWER:

You are absolutely right that magnolias produce chemicals that leech into the soils around it from fallen leaves, twigs and flowers that affect the growth of many other plants. The chemicals are sesquiterpene lactones. Here is a link to a previous question sent to Mr. Smarty Plants explaining allelopathy and magnolias. As well as the allelopathic effects of the magnolia, it is often difficult to find plants that will grown in the dense shade provided by the magnolia canopy. The SF Guide's article, What Flowers Grow under Magnolia Trees, suggests some flowers that will grow there, but they are not all native plants and if native, are not native to Missouri. You can find native plants that are recommended for Missouri on our Recommended page. Using the NARROW YOUR SEARCH option in the sidebar, choose "Herb" from the GENERAL APPEARANCE slot and "Shade" from LIGHT REQUIREMENT to limit the list small plants that grow well in the shade. Here are a few that might be able to resist the allelopathic effects of the magnolia.

Asarum canadense (Canadian wild ginger)

Aquilegia canadensis (Eastern red columbine)

Athyrium filix-femina (Common ladyfern)

Lobelia cardinalis (Cardinal flower)

Sanguinaria canadensis (Bloodroot)

If you do the search above, you can find more plants.

The best way to fight those allelopathic effects are to keep the leaves, twigs, etc., from the magnolia picked up from under the tree.