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Saturday - June 29, 2013

From: San Diego, CA
Region: California
Topic: Poisonous Plants, Problem Plants
Title: Can oleander poison the ground below it?
Answered by: Nan Hampton

QUESTION:

Can oleander poison the ground below it? Would it kill/damage grass or other plants below it? Thanks.

ANSWER:

It was shown that Nerium oleander (Oleander), native to the Mediterranean region, southeast Asia and northern Africa, did have strong allelopathic effects on seed germination and the growth of radishes when extracts made from oleander's leaves, stems with bark or roots. [Tran Dan Khanh et al. 2005. Paddy weed control by medicinal and leguminous plants from Southeast Asia. Crop Protection. 24 (May 2005) pp. 421-431.]

Another study [Rajyalakshmi, M. et al. 2011. Inhibitory effects of Nerium oleander L. and Its Compounds, Rutin and Quercetin, on Parthenium hysterophorus L. Journal of Agricultural Science. 3(2) June 2011.] showed that extracts of oleander leaves significantly inhibited germination and early growth of the invasive weed, Parthenium hysterophorus.

Certainly, oleander has allelopathic effects on vertebrates that ingest any part of it. See:

Canadian Poisonous Plants Information System

Poisonous Plants of North Carolina

Plants Poisonous to Livestock: Cornell University

However, I couldn't find clear evidence that living oleander plants release any sort of chemical into the soil to kill or inhibit the growth of other plants around it. The only statement that I could find saying oleander is allelopathic was from The Lazy Gardener column that appears in several Marin County, California newspapers. The author states that oleander is allelopathic and inhibits the growth other plants underneath it because it contains the compound called catechin. Catechin is thought to be the compound that is the reponsible for the allelopathic effect of the invasive European Centaurea stoebe [synonym = Centaurea maculosa](spotted knapweed) against many North American native plants.

However, another paper [Gopinath, S. M. et al. 2011. Chemical Prophylaxis and AntiBacterial Activity of methanolic and Aqueous Extracts of Some Medicinal Plants Against Bovine Mastitis. International Journal of Advanced Biological Research. 1(1) 2011:93-95.] did phytochemical analysis of extracts from several different plant species for a number of bioactive compounds (catechin being one of them). Their results show that oleander leaves contain no catechin.

So, I suppose the bottom line is:

Probably the roots of the oleander do not exude an allelopathic compound; however, research has shown that extracts from the roots, stems and leaves do exhibit allelopathic effects when applied to some plants. So, if the fallen leaves and stems are crushed and it rains or irrigation water falls on them, there may be enough allelopathic compounds leached from the crushed leaves or stems to affect plants in their immediate vicinity.