The tree anchors the corner of the front bed and draws the eye to the front door.

Poisonous Plants in the Home Landscape

I just had a conversation about poisonous plants with a client and this is certainly something worth considering when designing your landscape. 
I went down the rabbit-hole a bit on this one so if you want to skip ahead to the conclusion I won’t be offended.  A couple definitions to get us started:

Poison:   A substance that through its chemical action usually kills, injures, or impairs an organism.

Toxin:   A poison, especially a protein or conjugated protein produced by certain animals, higher plants, and pathogenic bacteria.

Poisoning:  Occurs when any substance (in this case a plant toxin) interferes with normal body functions after it is swallowed, inhaled, injected, or absorbed. 

So first off, why do some plants produce toxins?  The primary reason is as a defense mechanism to deter things from eating them or their seeds.  The organism that consumes them has a bad reaction and in the future avoids that plant.  Win for the plant, it goes on living.  In general these plants also taste awful which usually results in small quantities sampled being rejected before any significant adverse effects are realized. (Incidentally, in case my family is reading, kale is not poisonous)  Toxins may be high in parts like the seeds.  In the case of an apple one may eat the fruit with no ill effects but the seeds contain toxins, notably cyanide. A quick google search tells me I would need to consume approximately 200 apple seeds (20 cores) to receive a fatal dose.   If you’ve ever tried an apple seed you’ll likely agree they taste bad so you likely discard them.  Win for the apple, you just helped to spread its seed to a new location.  I’ll point out I’m not an expert on apple seeds or cyanide – I’m generalizing and DO NOT recommend anyone sample poisonous plants.  (and, once again, kale is not poisonous)

My little joke about kale brings up a good point with regard to poisoning – anything in large quantities can be dangerous.  Eat enough kale and you can achieve a toxic thallium concentration in your system.  If you want to calculate how much I’m sure there’s someone on Reddit that can help you debate that.  In many cases of poisonous plants it is the improbably large volume of exposure you must have for it to have a negative effect that makes it safe to have in the landscape.

Another point I’ll make with regard to poisonous plants is the society we live in.  If a garden plant were to be even occasionally causing significant harm to people or animals there is a pretty good chance that plant would be banned from propagation, and therefore would no longer be allowed to be sold.  It may still exist in natural populations but we are dealing with the man-made landscape over which much control is exercised.

So in the context of a home landscape what are we trying to do?  Certainly we don’t want to harm anything especially children and animals.  We can also make a case that just because a plant produces toxins doesn’t make it inappropriate for use.  Apple, as outlined above, is a good example.  Narcissus (daffodils) is another good example.  The bulb is poisonous and the mostDaffodils (Narcissus) on boulevardDaffodils (Narcissus) on boulevard common way poisoning occurs is when the bulb is mistaken for an onion and eaten.  So will a toddler eat it from the garden in quantities to be harmful?  Highly unlikely.  Will a gardener unwittingly serve it for dinner?  Again, unlikely.  So is it safe to use?  I’m going to suggest yes.  I’m also going to back up my logic by suggesting that the Canadian Cancer Society would be unlikely to use it as their symbol of hope if it were a common cause of poisoning.  Yet, there it is on various lists of poisonous plants.

One of my examples for consideration is “the chewy dog”.  We all know one or have owned one, the usually high-strung dog that eats the seat off the kid’s bike or tears up an entire shrub and chews the branches to mulch.  Clearly this dog is a candidate for poisoning because of its enthusiasm for chewing things with strong ‘not food’ attributes.  Aconitum, or monkshood, is a common garden perennial but all parts, especially the seeds and roots are toxic.  One case in Britain attributes cause of death of a dog to this plant, but it is difficult to find anything beyond anecdotal accounts of same.  My kids, and over the years two of my dogs, have happily coexisted in a garden containing monkshood with no ill effects.  Both kids and dogs would chew things on occasion. My dog chewing on a poplar stickMy dog chewing on a poplar stick BUT this is one plant that I will eliminate from a landscape plan if they have a chewy dog or very young children.  Better safe than sorry.

Last year a story was shared with me by a friend who has cats concerning the toxicity of lilies to cats.  Judging by the number of sources identifying this risk it is real and must be taken seriously.  Of note, oddly, lilies aren’t mentioned on many of the poisonous plant lists.  Again, taking the safe road, if a client has concerns about poisonous plants and has a cat, lilies are one I will avoid using.

For a listing of poisonous plants, plant parts, and symptoms this is a fairly comprehensive resource from Alberta Agriculture and Forestry:

Trees and Shrubs:$department/deptdocs.nsf/all/agdex13352

A nod to this site, which points out choking hazard and other common sense approaches to interacting with plants.

In the context of the average home landscape, plants with poisonous attributes can be used safely with consideration to some situations where the elimination of a few species is prudent.


About the Author

Clayton Ditzler

Clayton Ditzler

Clayton Ditzler is a Certified Landscape Designer (CLD) and has worked for The Landscape Artist Inc since 1989. He welcomes you to connect with questions or feedback any time.

Contact  phone: 403.256.2252 • email:

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