Invasive Species, Weeds, Nativars and Other Terms of Confusion 

The native plant gardening world is full of terminology that those gardening with non-natives have seldom had to consider. In this month’s article, I hope to shed some light on what some of these terms actually mean so that you can speak confidently and knowledgeably with garden center staff and fellow gardeners. 

Native vs Naturalized 

I started my journey into native plant gardening with the purchase of a package of “wildflower” seeds. When I recognized California poppies, bachelor’s buttons, and a few others I knew were not from these parts, I became confused. That’s when I discovered that wildflower is NOT the same as native. After a little more research, and a broken heart, I discovered that many of the wild plants I grew up loving in the fields and forests were actually European, Asian or other non-natives that had escaped from gardens over the past couple of hundred years and not the cherished native plants I thought they were. So what IS a native plant? 

In its simplest terms, the general consensus used by many but not all, is that any plants growing in an area before the European settlers arrived are considered native. These are the plants that evolved here and developed ecological relationships with the insects and birds and other animals as well as with the surrounding plants. These relationships developed over thousands of years. Europeans, as well as those from other parts of the world, came to this continent and brought plants they were used to using and or seeing, and all of these plants are considered to be non-native. Yes, plants migrate, but they do so very slowly and local ecosystems typically have time to react to any newcomers. The sheer volume and strangeness of the plants that were introduced by settlers really disrupted these natural changes. And, yes, changing climate is likely to allow plants not previously growing here to survive, but the rate of change will be nothing like the speed of putting a packet of seed in a truck in Arkansas and spreading in your southern Ontario garden a week later. 

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I don’t know when the currently accepted definition was proposed or by whom, and it leaves many unanswered questions, such as: should plants that were brought here by First Nations peoples (e.g. pawpaw – Asimina triloba) also be considered native? By keeping the definition simple, i.e. is it pre-European, the answer is also simple: yes. (There have been many long, philosophical discussions on the topic in many native plant forums. But I like to keep things as simple so this is the definition I use.) 

So what, then, is a naturalized plant? Naturalized simply means that that particular non-native species can exist in the wild without human intervention. Many of my favourite childhood wildflowers, like mullein (Verbascum Thapsus) and common chicory (Cichorium intybus), and even dandelions (Taraxacum officinale) fall into this category. Though many non-natives have adapted well to the wild, many, like zinnias (Zinnia elegans), petunias (Petunia spp.) and many of our vegetable crops, like bell peppers (Capsicum annuum), cannot survive long in our region without human intervention. Some, on the other hand, have adapted so well they are considered invasive. 

Invasive vs Aggressive 

We often hear about invasive species, and sometimes (incorrectly) an overly aggressive native plant like Jerusalem artichoke (Helianthus tuberosus), or Canada goldenrod (Solidago canadensis) are called invasive. However, the term invasive species is reserved for non-native plants that are so prolific, either because they self seed, spread vegetatively, and/or produce toxins to inhibit the growth of any competition, that they spread unchecked and have a large negative impact on the natural ecological balance of an area. Note that not all non-native species are invasive, but those that are can have devastating effects on local flora and fauna.  

They way I wrapped my head around the terminology is with an analogy: If our military forces came into a major city and started roughing up and arresting people, we’d say they are being aggressive, but if a foreign army did this, we’d say they were invading. The domestic army can’t invade because it’s already here. Same for plants – Canada goldenrod can’t be invasive because it belongs here. But a plant from another part of North America that was never here before CAN be considered invasive. For instance, cup plant (Silphium perfoliatum) was never native in New York state but it is a very aggressive spreader and is therefore considered an invasive species there1, 2 (and it is illegal to sell or grow it for sale in that state).   


Weed is another term that has some confusion surrounding it (I’m not talking about the weed your brother-in-law smokes, either). Weeds may be non-native OR native plants. We’re all familiar with dandelions – some consider it a weed while others do not. This Eurasian-native species was brought by European settlers as early as the 1600s as a food and medicine plant3. Plants like Canada thistle (despite its name, it, too, comes from Eurasia) were brought for the same reasons, or because they were familiar flower garden plants “back home”. These, and many more, have become ubiquitous throughout North America.  But the term weed is actually an agricultural term and describes any plant that has a negative economic impact on agricultural (food) production. In our gardens, is simply a plant that is unwanted where it is growing and in traditional gardening, these often happen to be native species, though not always.  

Erigeron canadensis (Horseweed, or Canada Fleabane) is a prime example of a native plant that is considered a serious weed in agricultural crops, especially since it developed resistance to glyphosate (the major weed-killing ingredient in Roundup) and to acetolactate synthase (ALS) herbicides. Milkweed, especially Asclepias syriaca or common milkweed, was long considered a noxious weed – a special category for the really nasty ones – and was eliminated from farm fields whenever it was encountered. Livestock won’t eat it, its rapidly spreading rhizomes enables it to take over large areas of a field out-competing the farm crops, and it had huge impacts on the farmer’s bottom line. We now know that it is also essential for the survival of the monarch butterfly, and as a result many jurisdictions have removed it from their noxious weeds lists. 

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Essentially, a weed is simply a plant growing where you don’t want it. In my all-native-plants garden, for instance, I consider squirrel-planted tulips and star of Bethlehem plants to be weeds. (I even refer to my hosta bed under a large maple tree in my front yard as my “weed garden”, and one day it will get “weeded” and native plants put in). 

Cultivars and Nativars 

Some other terms in native plant gardening also bring some confusion, especially to those just starting out. For instance, what’s the deal with cultivars and what is a nativar? The term cultivar is simply an abbreviation for the phrase “cultivated variety”. Nativar is a newer term that has come with the surging interest in native plant gardening and is simply a combination of two terms, native and cultivar.  

When many of us start to grow native plants in our gardens, we bring with us a history of growing non-natives. We, therefore, often look to the exotics for unusual colours or forms. I was no different in this regard – I was in in my 50s, with close to 40 years gardening experience, when I discovered native plant gardening.  When I saw a cardinal flower (Lobelia cardinalis) with a deep burgundy colour, a swamp milkweed (Asclepias incarnata) with a pure white flower, or a bloodroot (Sanguinaria canadensis) with a “double” flower, I was amazed and wanted it in my garden. Most often these plants are simply natural mutations that horticulturalists have latched onto and have cloned for resale. Some will breed true to the new form, but some are one-off freaks of nature whose seeds, if they even produce any, will revert to the original. Others are the result of selective breeding. And because the horticulture trade is, for the most part, driven by profit, some growers will select for unusual colours or forms to meet the demand for exotic plants.  

You can usually spot a plant in the garden center that is a cultivar because the tradition is to include the commercial name (either in quotation marks or not) after the scientific name, such as Aclepias incarnata “Ice Ballet” – note that the scientific name is in italics but the cultivar name is not. The use of var. (for variety, or in Latin, varietas) in the name, as in Cercis canadensis var. alba, is supposed to be used for naturally occurring varieties that have been selected for certain characteristics (in this case, a white version of redbud) rather than for plants that have been specifically bred for the trait. Unfortunately, there is not a lot of consistency in how these terms are used in the trade. (See article #4 in the reference list below for some more detailed explanations of these and other related terms.) 

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But as intriguing as some of these unusual variations are, recent research is showing that not all cultivars/nativars are equally valuable to the insects that depend on them5. Nutritional values can vary because the genetics that produce a desirable feature may be connected to those that impact nutritional value (similar to the genetics that make a German shepherd dog look the way it does also makes it susceptible to hip dysplasia). For instance, many of the double flowered nativars either do not produce nectar or pollen – the extra petals in double flowered species is often a mutation where the stamens in a flower are replaced by petals – and are often sterile, while others may be unpalatable to the insects that need it to survive. As the interest in native gardening grows so, apparently, does our interest in nativars and ongoing research is needed to determine which varieties are just as good for the insects as the originals.  


You should now be able to confidently explain that, no, Canada goldenrod is NOT invasive, but garlic mustard is (and why). You should also be able to explain to your neighbour why those tulips in your lawn are actually weeds, why the milkweed in your front garden isn’t, and what the difference is. And finally, when shopping for some new plants, if you see a non-Latin name pinned to the end of the scientific name on the plant tag, you’ll know this is (most likely) not a pure native species and, as such, may not have the same value to the insects and other critters that need it to survive.  

Happy native-plant gardening.