Designers should avoid using poisonous plants in therapeutic landscapes, especially if the spaces will be used by children or people who are developmentally disabled.
Since their bodies are smaller, children are more susceptible than adults to toxins in plants, and they are also more likely (especially when very young) to explore their environment with all of their senses, including taste. Many children’s gardens encourage this type of exploration, and have been designed with plants that can be smelled, touched, tasted, and so forth.
Of course, plants don’t have to be poisonous to be harmful. Those with thorns (for example roses, barberries, hawthornes), sharp edges (some grasses are particularly injurious), or substances that irritate the skin (for example, most Euphorbias) should be avoided when designing for clients who would be apt to come into direct contact with a garden’s plant material. Several of the links and resources listed below address these types of (potentially harmful) plants as well.
Good printed references:
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ASPCA National Animal Poison Control Center. Household Plant Reference. Urbana, IL. You can reach the organization by this number: 217.337.5030.
James, Wilma Roberts (1973). Know Your Poisonous Plants: Poisonous Plants Found in Field and Garden. Happy Camp, CA: Naturegraph Publishers, Inc.
Lampe, Kenneth and Mary McCann. The AMA Handbook of Poisonous and Injurious Plants.
Moore, Robin C. (1993). Plants for Play. Berkeley, CA: MIG Communications.
Moore breaks his lists down into levels of high, moderate, and slight toxicity, which is quite helpful (Moore’s book also provides lists of non-harmful plants that can be used in children’s gardens).
Nelson, Lewis S., Richard D. Shih, and Michael J. Balick (2007). Handbook of Poisonous and Injurious Plants. Springer.
Favorably reviewed in April 2007 issue of Horticulture. Includes a section on plant-induced dermatitis.
Ogren, Tom (2000). Allergy-Free Gardening. Berkeley, CA: Ten Speed Press.
This book is out of print, but can probably be located through sites that sells used books; it may also be available at your local library.
Schmutz, Ervin M. and Lucretia Breazeale Hamilton (1979). Plants That Poison: An Illustrated Guide to Plants Poisonous to Man. Flagstaff, AZ: Northland Publishing.
Stewart, Amy (2009). Wicked Plants: The Weed That Killed Lincoln's Mother and Other Botanical Atrocities. Chapel Hill, NC: Algonquin Books of Chapel Hill.
Visit Amy Stewart's website and blog, where you can order signed copies of her book.
Turner, Nancy J. and Adam F. Szczawinski (1991). Common Poisonous Plants and Mushrooms of North America. Portland, OR: Timber Press.
“Understanding Poisonous Plant Compounds.” Fine Gardening, No. 81, Sept./Oct. pp. 80-82.
Potentially Harmful Plants, listed by The Horticultural Trades Association, U.K.
Potentially Harmful Garden Plants, listed by the Royal Horticultural Society, U.K.
An American bibliographic database of poisonous plants
A phytochemical database with plants ranked by chemical content
LocalVets.com's list of Plants That Are Poisonous to Pets