Who Says Native Plants have Longer Roots?

16
Prairie grass root display at U.S. Botanic Garden
“Secret Life of Roots” exhibit at the U.S. Botanic Garden

Still Ranting about Generalizations

Over my 14+ years of garden-ranting here, my number one target has been sweeping generalizations about plants – they’re stupid and misleading!!

On that subject, I recently signed a letter objecting to misstatements about native plants in my local newspaper by the city’s “environmental coordinator.” The statements we took issue with are here in italics, followed by our rebuttals. The only information we have about the author’s education is that he’s a Certified Master Naturalist.

Deep and Extensive Roots?

Native plants generally have deep and extensive root systems that make them more tolerant to drought and other adverse conditions compared to ornamental and nonnative plant species.

There are plenty of shallow-rooted natives (most of the plants in this list, for example). And since all plants are native to somewhere, it makes no sense to contrast their root depth with “non-natives.” A statement that would be accurate and not overly simplistic is to contrast common turfgrasses with, say, prairie grasses.

Hosta roots

What our letter didn’t say on this point is how ridiculous the claim is, which could only mean – if true – that plants have deeper roots where they’re native, right? So hosta roots grow longer in Asia than they do here or in Europe, where they’re nonnative? Amazing!

Native plants also aid in stormwater management. Adding native plants to a landscape helps to slow down stormwater and increase infiltration to recharge groundwater sources, filtering the water as it percolates through the soil.

Both native and nonnative plants do these things to varying degrees, so writing that native plants do it creates the misinformation that nonnatives do not. 

In other words, none of that is specific to natives. It’s just what plants do.

Lawn on a hillside in Takoma Park, Md.Turfgrass No Better than Concrete

We often think of impervious surfaces as asphalt roads and parking lots or concrete sidewalks, but turf grass, used in lawns and general landscaping, acts as another impervious surface. Turf grass has shallow roots that create thick mats just a few inches below the soil. In heavy rain, stormwater will sheet and flow off the grass like it was concrete…”

While it is true that most turfgrasses have shallow roots, they still hold more stormwater than concrete!

To bolster sheer common sense – that turfgrass can’t possibly be literally impervious, like concrete – I offer an example from my garden of 26 years, the back yard shown above. Trust me when I say that this lawn was on quite an incline – steep enough to make mowing difficult – and most rains sank in, rather than sheeting and flowing downhill. 

It may be, as some sources suggest (sorry I don’t remember which) that studies of turfgrass imperviousness were based on its performance in extremely compacted soil. So blame the condition of the soil, not the plants trying to grow in it. 

Natives are Cheaper

Native plants will cost less money in the long run compared to ornamental and nonnative plants that require more direct input to survive.

We don’t know of any evidence for this generalization.

I suppose it could be true, but in my decades of gardening it’s hard to imagine plants requiring less “direct input to survive” than, say, hostas or spireas. 

In our letter we went on to suggest that “writings on environmental topics by city employees be science-based, and even better, that the sources be identified…For our region, we’re fortunate to have several of the very best Extension universities in the U.S. and we recommend them as your go-to sources: UMD, NC State, Penn State and Cornell.”

Local Information Fares Better

Curious as to where the environmental coordinator was getting his (mis)information, I googled “native plant deep roots” and found examples like these, that make total sense:

From the State of Minnesota, deep-roots are attributed to prairie grasses and Minnesota native grasses, not the overly broad “natives.” Another agency in Minnesota wisely just says “Many native plants have deep roots.” See, it’s so easy to get it right!

And the USDA keeps it regional: “The deep root systems of many native Midwestern plants increase the soil’s capacity to store water.”

The U.S. EPA contrasts natives v. nonnatives in Kansas. So far, so good. But unfortunately its text veers far beyond the State of Kansas: “Native plants have deep roots which can penetrate the native soil to depths of up to 16 feet! During the dry summer months native root systems reach deep into the ground to find water, which is why native plants are more drought resistant than non-natives.” 

I wonder how many researchers and writers have been misled by this one website alone. I mean the EPA’s a reliable source, right?

There are plenty of other reputable sources that overgeneralize or, like the local writer who prompted this rant, list attributes of natives that aren’t specific to natives but apply to plants generally. Misleading, at best.

The Problem of Rain Garden Plant Lists 

Having seen SO many authoritative sites recommend only native plants for rain gardens, I’m betting they’re contributing to the misimpression that native plant roots are deeper – across the board. Penn State is one example: natives only, with no explanation as to why. 

Though now that I’m googling it, I find that Home Depot’s rain-garden plant list is inclusive, as are the plant lists by The Spruce, Almanac, and Fine Gardening

But I think that Michigan State puts it best: “The plants selected for the rain garden may be native or have extensive root systems that help the garden absorb the rain water. Native plants are often selected as they do not need special care, are resistant to most insects and diseases, and attract beneficial insects. Non-native plants can also be used, as long as they are not invasive.”

See, you can still get in a pitch for natives – by saying that they attract beneficial insects and resist most insects and diseases (again, avoiding overgeneralization) – without spreading confusion and misinformation about plant groups. 

Daylily root photo credit.

16 COMMENTS

  1. Excellent post Susan. Many people are scared to speak out on this subject lest they be blasted — particularly from the newly proselytized who repeat terms that sound correct and therefore should be correct. Plants should be evaluated on their merits and their faults, and how they adapt to, function in, and sometimes remediate specific conditions of soil, exposure and climate, whilst providing for wildlife populations. Though I rip out invasives on a near-constant basis, and utilize many native plants; I equally utilize many non-natives. I could make a museum to Mid-Atlantic native plants in my garden, and it would be a lovely thing and perhaps worth making (Mt. Cuba is FABULOUS!); but I would be delusional to believe that the moment I kick the bucket Nature won’t re-green the space with whatever the hell she feels like based on the Darwinian concept of survival of the fittest. She won’t ask to check passports. The fittest is not necessarily the prettiest, or the virtuously pedigreed — and it’s definitely not the most fragile. Any native species that needs constant curation will simply not make it when I or others can no longer coddle them. For nativists, this is a tough concept to come to terms with, so it is either faced with anger, or ignored completely.

    Bottom line, I’m not into the rigidity and accompanying righteousness of floral xenophobia. On a constantly changing four-and-a-half-billion-year-old planet, it only makes sense when viewed through the lens of human chronologies and border-making. We should never accept such concepts within human populations, why is it okay when our subject is plants? – MW

  2. I recently attended a virtual presentation by Peter Del Tredici, retired Director of Harvard’s Arnold Arboretum and tree expert extraordinaire. His subject was tree roots, about which there is a great deal of misunderstanding. Claims are often made about roots of certain tree species being unique in some way that made that species unsuitable. That’s not true because the roots of trees do not vary much between tree species.

    For example, nativists frequently claim that eucalyptus has a deep tap root that enables the tree to drain more water from the water table than other tree species. That’s not true because all tree species have a tap root at the seedling stage that disappears over time as the tree establishes its wide perimeter of shallow roots as it matures. Del Tredici called it “the myth of the tap root.”

    Nativists also make the contradictory claim that the roots of eucalyptus are shallower than other species, making it more unstable than other tree species. That’s not true because all trees develop a wide and shallow root system as they mature UNLESS the trees are physically constrained by rock, concrete, or some such obstacle. Without such physical constraints root systems are typically like those of the excavated apple tree that was shown at the Chelsea Garden Show in 2013. The roots surround the tree, making a circle larger than the tree canopy.

    When you hear criticism of non-native plants and trees, don’t take them for granted. As Susan so rightly points out, most of the baseless accusations about non-native plants are either untrue or equally true of both natives and non-natives. They are all plants, after all, not alien visitors from another planet!

    Thank you Susan for writing to your local publication about that ridiculous article, apparently written by someone who knows nothing about plants, native or non-native.

  3. I suspect that the source of confusion is the demonstrable fact that plants which are established from seed in situ have dramatically deeper root systems than those propagated in containers from seed, cells, cuttings or divisions, and later set in place. Naturally occurring native plants have extensive root systems, but so do self-sown or broadcast non-natives in ideal situations. Transplants of either, however, will likely not in comparison.

  4. “Native plants have deeper roots” has become a mantra we use at work to crack each other up. Generalizations make arguing one’s points quick and simple. Unfortunately, most subjects worth arguing about are not simple nor best resolved quickly.

  5. Garden Professors blog and Facebook page wrestles with this frequently. Of course, there are a lot of issues with tree and shrub planting from a nursery, another factor in survival.

  6. Marianne, thank you for going to bat on this issue. The overwhelming amount of misinformation on gardening that is bandied about so loosely and enthusiastically makes me grind my teeth when I’m not just wincing. But mostly I remain passive for my own peace of mind. So glad there are gladiators willing to speak up.

    P.S. To All The “Ranters”:
    I’m fairly new on this site…should have checked it out years ago. I read many/most? of these lovely “rants” with a grin on my face and often a nod of recognition. Thanks to all for the writing worth reading.

  7. Here’s my generalization:
    Nativists are lazy. They don’t bother to research or ask questions. They meme this BS over and over with zero effort to determine if it’s true.
    And they also may be xenophobic…

  8. Here, Here, Susan! In fact, native prairie grasses MAY reach 15 feet down in Kansas soils but, after living here 30 years, I would guess that is by far the exception than the rule. In the Flint Hills, the rock starts at 12 inches in many, many places and the ground water table is far below 16 feet. And yet the grass survives!

  9. The last quote by MSU also supports your statement about over generalization. If natives were better at disease and pest resistance, we wouldn’t need to concern ourselves with emerald ash borers, Japanese beetles, etc.

  10. I bet the writers of blanket “natives have deeper roots” haven’t dealt with much Canada thistle.

    And yes, there are way too many generalizations and assumptions made and passed around. I’d bet many native plants known for their root depths based on undisturbed soils, especially deep prairie soils, would not have nearly as long of roots when grown on modern depleted, eroded soils. Similar when it comes to woodland plants grown in natural undisturbed forests, vs grown in many woodlands today where any downed trees and branches are removed or burned on the spot.

  11. Another one that gets me going, “Native plants require less watering”. Oh really? So a native fern (VA) requires less watering than a non-native cactus?

LEAVE A REPLY

Please enter your comment!
Please enter your name here