Have you ever thought about the fact that almost all books on native plants define their ranges as “Native in the Northeastern US and Southern Canada”, or “Native in the Midwest States and Southern Prairies”? Very few plants are native throughout those rather large areas of geography. Soil type, microclimates, even underlying geology can have a great influence on which plants would be found naturally in any given location. Wouldn’t a map be much more accurate and useful? After all, native plant gardeners, more so than any other, want to know if a plant is actually native to where they live. In this month’s article I will try to explain why range maps are not very common, why they are a challenge to create, and why they can be a challenge to interpret.  

There are at least 3 reasons why range maps are rarely found in the guides. The first of these is that, with the exception of a few plants that have been studied extensively (e.g. Goldenrods in the Astereae Lab at the University of Waterloo), a lot of the native plants we want to grow in our gardens have historically garnered very little interest from a geographical perspective. We may have studied the ecology of the plant (how it interacts with its environment), its physiology, and in some cases we may even have brought it into the greenhouse to produce cultivars. But, for the most part, we’ve been more concerned with the plant itself than with knowing where to find it (beyond what kind of soil/light regime it can be found in – prairie, forest, etc.). This means that we just don’t have readily available data to create reliable range maps. 

The second problem is that the collection of locational data to determine native ranges is, by necessity, a historical one. In the past 150-200 years we have moved plants all over the place, well beyond their native ranges. This means that it is a lot trickier now to determine if that plant has always been here, or if it has just escaped from someone’s garden. We rely on early botanists’ recordings of what they saw, and where they saw it.  

However, even though the historical records are extensive, they aren’t always 100% accurate. Modern botanists often debate whether so-and-so’s sighting of this plant or that accurately represent where the plant was found before Europeans arrived. Echinacea purpurea, for instance, is usually considered native in parts of Michigan, but some argue that the historical records indicate it was only found in areas not far from railway corridors. Does this mean those plants were simply the result of seeds brought in inadvertently by rail? Or maybe that their mapped location is a case of the botanists recording those sites, but not venturing further afield to see that the plant existed elsewhere. Keep in mind that at the time many of these botanists were working, there were few roads and lots of wilderness. 

The third, and perhaps biggest challenge, is determining what scale to use to map the ranges. Some maps are created at a small scale (i.e. they cover a large area). By necessity, details get obscured at these scales. If we map, say, the range of Agastache scrophulariifolia (Purple Giant Hyssop) at the North American scale, as shown in the 1945 map from The American Midland Naturalist journal (Vol 33, No. 1), we can see that the mapping is very generalized and doesn’t take into account variations in local landscapes or microclimates. 

At the other end of the spectrum (large-scale mapping), we are zoomed in close (so everything looks larger – hence its name), but these maps have their own issues. Perhaps the biggest of these is that many people looking at such a map believe that the boundary shown is a hard boundary – that the plant is definitely found on one side of the line and not found on the other side of it. Therefore the map-maker runs the risk of using vague data to create the illusion of clear and precise knowledge. As a former map-maker, I know only too well the challenges.  

It is extremely difficult to generate a map showing where a plant was native over a large area and yet still capture small isolated pockets. In the map I created (below) for Green Milkweed (Asclepias viridiflora), you can see that it covers a large contiguous area in a band from southern Lake Michigan, crossing southern Ontario, into Ohio, Pennsylvania and beyond (shown with striped shading). Yet if you look closely, you’ll see a couple of small pockets of stripes – one further north along the Lake Michigan shoreline, and another just north of Manitoulin Island in Ontario. The second of these locations, in particular, is extremely exaggerated in area. In fact, the plants were only found in a small, isolated area that would not even have shown up at the scale of this map. The dilemma is, then, whether to exaggerate the area, or to leave the impression that the plant was never found that far north.  

When you combine all of these reasons, you get a better feel for the amount of work involved to 1. verify the native ranges, and 2. determine what scale to map them at, and what information to include or leave out. All such maps, by necessity, are generalizations because you can’t easily account for small, localized variations. As a result, few people have tried to do it.  

Then, of course, there is the whole business of interpreting the range maps once they ARE made. The authority I use to determine if a plant is native to Ontario is the Database of Vascular Plants of Canada (VASCAN). However, all the maps on their website show nothing more than IF the plant is native SOMEWHERE in Ontario. For instance, Pulse Milk-Vetch (Astragalis tenellus, aka Astragalis multiflorus) is shown as native to Ontario. However, that is based on one, small, localized occurrence in northern Ontario, as shown in this map from the Atlas of Rare Vascular Plants of Ontario. 

South of the border, plant species have been mapped to the county level, in most states. However, it is often unclear if these maps represent current locations or historical (i.e. native) ranges. In the map of the same plant (identified as Astralagus multiflorus) from the Biota of North America Program (BONAP) website, the bright green indicates the counties where the plant is known (in the US) but in Canada that level of mapping has not been done, so the entire province of Ontario is shown in dark green.  This could lead some people to think that it is found throughout the province.  

Before you can decide that a plant is native to where you live, you need to check multiple sources and put together a picture. For those in southwestern Ontario, at least, an excellent resource is Michael Oldham’s 2017 “List of the Vascular Plants of Ontario’s Carolinian Zone (Ecoregion 7E)” that you can download here. It gives a county-by-county breakdown of where the plants are considered to be native. 

Shaun Booth (from In Our Nature) and I have been working on a reference book for native plant gardeners and we have taken on the task of mapping the native ranges for over 200 species of plants in the southern Great Lakes region. Over the past 4 years or so, we have compiled historical records as well as on-line data, and then worked with expert ecologists in the province and adjoining states to verify our maps. We are currently working on finding a publisher so that you, too, can have access to this information. Wish us luck. 

There’s a lot of activity on the various native plant gardening Facebook groups these days about winter sowing.  I’ve never tried it, but it makes total sense to do what Mother Nature does – set your trays of soil with seeds outside and let the natural refrigeration of winter do the cold moist stratification for you. 

In my research for native plants sources, I have come across a handful of companies that specialize in native plant seeds. In fact, some ONLY sell seeds and not plants. 

Have you ever asked yourself where they get the seeds? Does it even matter? A milkweed is a milkweed, regardless of where it is growing, is it not? Well, that depends. 

Rare Native Plants 

In southwestern Ontario, where I live, we are in what is often referred to as the Carolinian Life Zone. Here our climate and soils are similar to places far south of us – as far as the Carolinas in the US. That means that many plants are native here that aren’t found naturally anywhere else in Canada.  It also means we’re at the northern limit for many of these plants, and occasionally the populations of these plants are found only in isolated pockets here and there.  

Take, for example, Stylophorum diphyllum, or Wood Poppy, known to be found naturally only in a few isolated locations around London, Ontario before we started spreading it through our gardens.  

Or what about Cercis canadensis, the Eastern Redbud, a beautiful small tree originally known only from Pelee Island in Lake Erie (Canada’s most southerly point – at the same latitude as northern California). However, because of it’s size and spectacular spring colour it now adorns many yards throughout southwestern Ontario and even points north and east of here. 

Two Sides to the Argument 

There are two sides to the argument about whether this is a good idea or not, and I still haven’t entirely made up my mind which side I’m on. Both sides have much merit. 

The first argument is that these isolated pockets may hold unique genetic traits that could disappear though cross-breeding with plants that evolved elsewhere. And until we have done genetic testing to show otherwise, we should not introduce these other plants. To loosely paraphrase Joni Mitchell, “We won’t know what we’ve lost if it’s gone”. 

At the other end of the spectrum, there is the argument that rare, isolated pockets of such plants are in danger of being lost due to habitat destruction, climate change, pollution, etc. and that in order to keep seeing them, we need to plant more. After all, humans are the main reason many have disappeared over the last 200 years, or are on the brink of disappearing now. And since there are so few left in the wild here, we need to source the seeds elsewhere.  

The other argument for bringing seeds or plants in from other areas is that very small, isolated pockets of plants may reach a genetic bottleneck where inbreeding can lead to weakened plants that are more susceptible to the stressors that are placed on them. Only by bringing in outside plants can we bolster the genetic diversity and, hopefully, stave off extinction.  

Native Plant Sources – Choices 

I recently placed an order for some seeds from a company in Ontario because they were the only company offering the plant I have been looking for for some time. I was told the seeds for my plant were ethically sourced in Ontario.  

However, for another plant I was curious about, I was informed the seeds were from the US. They ignored my question about WHERE in the US, but my guess is that it was likely Prairie Moon Nursery in Minnesota as that is one of the larger sources of native plants seeds and many nurseries here in Ontario use seeds from there.  

The latitude of Minnesota is roughly the same as southern Ontario so those plants should be hardy here. However, the original stock from which those seeds were sourced may be genetically quite different than the ones that actually evolved here. It has been reported that Asclepias tuberosa – butterfly milkweed – plants tend to be a lighter yellow in the west and a darker orange in the east. By the same token, Ratibida columnifera (not actually native in Ontario) – sometimes called Mexican Hat – is mostly pure yellow in Canada but mostly a burnt red in the southern end of its range. By bringing in different genetics, we may lose the plants that have evolved here, and any uniqueness they offer. 

Another native Ontario plant from my wish list that this company offered for sale was out of stock. I was shocked, though, when they said they were just waiting on their shipment from England. What? A native Ontario plant and it is being sourced from across the ocean?  

A few years ago, someone told me that seeds for a plant I was looking for was available on Amazon. After doing a bit of digging, I discovered that these seeds were “shipped from China”. I didn’t buy them.  

All of this is to say, whether you’re winter sowing or spring sowing, ASK where your seeds came from. If there just AREN’T any seeds or plants available for that one plant you really MUST have, only you can decide what limits you’ll set on the acceptable distance from where you live to where they are sourced from. As the demand for native plants continues to grow, local sources will continue to pop up. If they know that the provenance of the seeds is important to native plant gardeners, they will look closer to home to source their seeds. And eventually (I hope) more local farm operations will start to grow native plants to supply the market with seeds. 

In the photo below, the Irvine Ranch Conservancy in California is one of many commercial native-plant producers in the US (photo courtesy of Irvine Ranch Conservancy). I was unable to get a photo from a local commercial producer in time for posting. This level of production is possible here, too, and will happen if we demand local seeds.

Happy native plant gardening.