What is the difference between “native,” “non-native,” and ”invasive” plants? Not only what is the difference – but why does it matter?

Definitions First

Let’s start with the ones that are easiest to define. We can work our way up to the ones that are more slippery to define.

Invasive Plants

Invasive plants are those that were either intentionally or accidentally translocated to an area where they did not exist naturally, and now that they are there, they are able to spread and cause harm to native plants and animals within the local ecosystem.

According to the definition on the USDA website, “invasive plant species are plants that are not native to a particular ecosystem, and the introduction of them is likely to cause economic, social, environmental, and human health impacts.” For our region think buffelgrass, and salt cedar, and stinknet.  (Those links are three different sites – in case you are interested)  The cover image is  buffelgrass.

In general, invasive species disrupt the growth of native plants, and root and spread more quickly than the plants that are already in the area. These species usually do not have any ecological checks on them, which means no predators, no pathogens, nor any of those sorts of things that can, ecologically speaking, keep a species from spreading unchecked.

Noxious Weeds

A noxious weed is a legal designation. It is any plant designated by federal, state, or local government officials as injurious to public health, agriculture, recreation, wildlife or property. Once a weed is classified as noxious, authorities can implement quarantines and take other actions to contain or destroy the weed and limit its spread. Generally a plant starts out as invasive. Once it becomes troublesome enough, it is legally designated as noxious.

Aggressive Plants

A non-native plant may never become a problem. Being non-native alone is not really the issue. It depends on if the non-native is an aggressive grower or not. The non-native oregano in my yard is not aggressively growing nor is it spreading.

Meanwhile, some native plants can be very aggressive growers. Take the Sonoran native desert milkweed vine for example. I adore it. The native butterflies adore it. The native sweet acacia tree that the vine is currently engulfing is not a fan. The milkweed vine is an aggressive grower and a bit of a bully. It’s about to get trimmed back severely in my garden. In the wild, its aggressive growth would ultimately kill the tree.

Non-Native Plants

Non-native plants are species that have have been introduced to an area by human activity. These plants may or may not become invasive.

One example is the perennial Paiute sunflowers that are not native to Washington state. They have edible roots, and were evidently introduced to the area from northern California by Native hunter-gatherer tribes. These sunflowers are specifically found living along old human migration routes, and very rarely downstream of those routes.

Native Plants

Getting into tough territory now. Some websites define native as “plants that were historically in the area.” The problem with that is how do we define as “historically.” Things do change.

Right here in the Southwest, a scant 4000 years ago, pine trees grew in the area that is now Organ Pipe National Park. Then the glaciers receded. Pine trees don’t grow there now, organ pipe cacti do. Go back an additional 5000 years in history and mammoths were once native to this area. So you can say that they were indeed “historically” native here.

Some people refine this definition to “plants that have existed in a location prior to European colonization in North America.” Those Paiute sunflowers were not native to Washington, and are only found along the migration routes. And were certainly there before European arrival. But they only live in patches where originally planted by human activity. Are they native or not?

Things Change

Another issue. In the 1600’s, as the Southwest region was being explored and colonized by Europeans, this entire region was wetter. Grasses prevailed in many regions across the Southwest that can no longer support grasses. It isn’t about overgrazing, or water table depletion, or even non-native plants. The fact is that historically, since records were kept (I found some going to 1885 in Tucson) the entire region has been drying out. We now understand that this has been going on for 19,000 years when the glaciers first started receding.

Define “Area”

Within North America, a plant can be native in portions of it and non-native elsewhere. Take rhododendrons. Native to the Southeast USA. Not native to the Southwest USA.

Be Specific

Rather than say North America, or even one state, let’s look at ecosystems or ecoregions.

Arizona ash trees are native to Arizona. Not all of Arizona. They are only found growing along riparian (river) areas within Arizona. So yes, Arizona ash trees are native to the area, or the state, or even the region, they are just not native to this entire area, and not the entire ecoregion.

So I do encourage people to grow plants that are native to their ecoregion. Plants that will survive with the soils, and intense sunlight, and low humidity, and average rainfall, and high and low temperatures that are found in the particular ecosystem that you live in in our very varied Southwest area. Native to Flagstaff or Reno is quite different than native to Phoenix or Las Vegas.

The Struggle is Real

The struggle to plant only native plants is real. If you go to one of the big box stores, you can find many plants that are pretty. You may even find a plant that is touted as “native.” They may be wonderful plants, but they may not be native to the Southwest. Box stores have very little to no incentive to carry native plants. They certainly do not have the ability to be mindful of what plants might be native to the region of where that particular box store might be. They simply carry what the growers will sell them cheaply. This is why I encourage people to go to local nurseries.

Does it Really Matter?  A Great Question

Those of you who read my Substack newsletters might remember that I recently spoke at Desert Green, a fantastic horticultural conference in Las Vegas. (If you don’t subscribe to the newsletter – sign up today! There are some deals coming.)

About Desert Green Conference – several of us speakers in the Design Track, and some in the Plant Materials Track highlighted the many beautiful native species of low-water plants that could be used to help reduce water consumption in that burgeoning city.  In the hallway later, an earnest young landscape designer came up to me and wanted to talk to me about native species in a city environment. Given all the asphalt and busy streets, does using natives really matter at all?

All I could offer was my own life experience. I don’t have any scientific studies – but I bet they are out there. 

When I was a kid, the birds in the neighborhoods around the University of Arizona, as well as on the campus itself, were primarily non-native birds – house sparrows and pigeons, with a few bold hummingbirds flitting in. There were few, if any native birds to be found.  Since the 1960’s, gradually, the non-native trees and shrubs that people planted have been replaced with native trees and native shrubs, even native perennial plants. Now Gila woodpeckers and cactus wrens live throughout the area. Phainopeplas, warblers, pyroluxias, and all manner of native birds are commonly seen. Cooper’s hawks too. Well, and pigeons too. Those didn’t go away.

Do You Like Birds?

Research shows that native plants can help create a healthier environment, a healthier ecosystem, and support a higher diversity of animals. Native plants can host many more types of insects than non-natives.  When it comes to birds, use the native mesquite tree for one example that I do have some data for. Researchers have found that velvet and honey mesquite trees in the cityscape can host over 100 different species of insects, and notably their larvae. Those larval bugs (caterpillars and grubs) are vital food sources for birds, especially the Lucy’s warblers and other songbirds. In contrast, the invasive African sumac (Searsia lancea formerly Rhus lancia – a close relative of poison ivy) only had an average of 7 species of insects on them.

Say Yes to Natives

So let’s just say that native species are the species natural to your area. Natives are specifically adapted to each the unique ecosystems found in every corner of the Southwest. Not only that, but native plants are intrinsically important to the continuation of healthy and diverse ecosystems.

Is it Native?

You can ask me – down below here or via my Substack site.  You can also ask the folks that have been working on this specific topic since the 1960’s. Please don’t simply ask the search engines. Those AI bots are idiots about plants.  I recommend the Ladybird Johnson Wildflower Center. They maintain a very robust native plant database that you can check on with the common name of a plant or – better yet – use the scientific name.  Seinet is very useful if you know the scientific name.  It can’t search common names.

I used to tell you to ask me on Facebook.  I am phasing out that social media platform because they keep showing me gross pornography at the top of my feed when I log in.  Ruins my whole day.  I have tried to tell them but they do not change a thing.  Also, when my Instagram was hijacked they simply shut it down.  Meta is not responsive to actual humans that don’t give them tons of money.  Sigh.

Have some trees for you and the birds in here.

May I recommend my book?  The Southwest Fruit & Vegetable Guide offers not just growing guides but some of the latest varieties – including ones specifically for raised bed and container growing. Price is what you would pay on Amazon – only when you buy from me you get a signed copy!

From the review:

“In this updated 2nd edition of Southwest Fruit & Vegetable Gardening, you’ll find much-needed advice and practical tips on growing an edible garden, no matter which part of the southwestern US you call home.

Profits from the sale of this book go to the Arizona-based Horticulture Therapy non-profit Tierra del Sol Institute.

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