invasive – Page 2 – The Garden Professors™

Defining Your Terms

Loyal reader and thoughtful commenter Ray Eckhart posted a while back (something along the lines of) wouldn’t it be nice if we could come to some agreement on all this “what’s invasive” terminology. This has been flitting in and out of my brain but has not found sufficient gray matter to come to rest. Regardless, here goes. I’ve attempted to capture these concepts in as few words as possible. My opinions in no way reflect those of the Garden Professors, blog host Washington State University, or anyone else important, for that matter.

Native. From these here parts.
Or this half/region/corner of the Continent, depending on your definition. And there are LOTS of them. We had this discussion in my Herbaceous Landscape Plants class last week. My students alone came up with 10 different definitions and/or criteria, all probably legit. I teach a plant as native if it is found east of the Mississippi (these are garden plants, it’s not a botany class). I try to describe its “nativeness” more in terms of whatever biome it came from (tall grass prairie, deciduous piedmont forest, etc.) as cue to how to best use it in the landscape. If it’s found even closer to home, I find myself describing it as “really native” which makes no sense, but I can’t seem to stop.

Alien. Not from here (wherever here is).
Is a broader term than “native” because “here” seems to refer to land masses, whether continental or island. Alien has a rather negative and even inter-planetary vibe to it. “Must…thinkofway… to… eliminate… Japanese Privet. Helpme…, Spock.”

Non-native. The kinder, gentler alien.
As in, Hosta are not going to take over the planet. Even though non-native is often used in the same breath as alien, we need it to stand on its own in this case: what do you call a Calochortus (Mariposa Lily) in Virginia? It’s not native to the east coast, but species of the genus are native to the western part of continental North America and further south. So I deem it non-native, but not alien. Maybe “mail-order” should be a category.

Exotic. An attractive but promiscuous alien.

Invasive. Aliens amuck.
Lists are created, but are unfortunately often ignored (see previous post on ligustrum). There are lots of good invasive plant definitions out there. But can native plants ever be invasive? Or are they just “vigorous” or “aggressive”? See below.

Vigorous. This has positive connotations. Holds its own in difficult situations, makes lots more of itself, can share with friends.

Aggressive. See vigorous, but cue theme to Jaws. Makes a WHOLE LOT more of itself. Can share with unsuspecting friends.

Passive-aggressive. Just when you think “I don’t know what all the fuss was about”, BAM you’re pulling out it of every nook and cranny. I can think of many examples – good fodder for a future post.

Based on these definitions, I see a few of our favorite garden natives as aggressive or passive-aggressive. You’re not sure whether to be pleased or perturbed that a delightful, wildlife-friendly native is reseeding all over your garden. Two examples right here in our campus garden: Joe Pye (Eupatorium purpureum) and Cup Plant (Silphium perfoliatum). Completely out of hand.

I am no authority on this stuff, just happy to be part of the conversation. Feel free to agree/disagree. But please be gentle; I’m suffering from a late night of watching the Hokies get splatted by Boise State (invasive Broncos).

GP Hit List: Ligustrum sinense (Chinese Privet)

[And I don’t mean “greatest hits”, I mean “Mafia hit”]

Gardeners, yardeners, and designers are on a perpetual lookout for a good hedge species. Hedges are useful in so many ways: providing backdrop for a border, making good neighbors, containing football players, etc. In reference to the latter, the field at Sanford Stadium at the University of Georgia (cue woofing!) is known as “Between the Hedges”. In the extensive lore of UGA football, everyone makes like it’s some sacred plant, but (gasp!) it’s just plain ol’ privet. Aside to Jeff, my fellow UGA alum: CNN just reported the dubious distinction that Georgia is the #1 party school in the country (again). Maybe this is why I can’t remember the Krebs Cycle.

Variegated Chinese Privet in all its shrubby glory; photo from University of Arkansas Cooperative Extension.

Back to privet. Designers of D.O.T. landscapes such as medians and interchanges seem especially enamored. Clouds of variegated privet (L. sinense ‘Variegatum’) dot interstates throughout the South. Yes, it’s tough, useful, variegated, and touted as “wildlife friendly” by some leading gardening resources. The berries are indeed good bird food; therein lies the problem. Seeds pooped out by birds results in the green, non-variegated version – much more vigorous and deposited everywhere. For the record, the variegated form also reverts like crazy.

My family’s farm in northeast Georgia is bordered by the North Fork of the Broad River. Over the years, the river bottom, and then the fence lines at all elevations, have filled with privet. It is, pardon my language, a total bitch to remove and almost impossible to eradicate. The State Botanical Garden of Georgia in Athens has an entire squad dedicated to eliminating it from acres of river bottom; ironically, only a few miles from Sanford Stadium. Thought of mostly as a Zone 7-9 plant; we have lots of it in the woods here in the mountains – an entire zone cooler.

Distribution of L. sinense; USDA PLANTS Database http://plants.usda.gov/

Ligustrum sinense is on at least two official Invasive and Noxious Weeds lists by the Southeast Exotic Pest Plant Council and Florida Exotic Pest Plant Council as well as several international lists. Nursery growers and landscapers across the South know these things. So why, why, why, do they continue to produce and spec it? “Variegated foliage sells. People see it, like it, and then ask for it. It’s a bread-and-butter item.” This comment came from the owner of a fairly large nursery that sells to independent garden centers.

Our industry yelps whenever a state or federal mandate threatens to impose restrictions on a best-seller (Hedera helix, for recent example). There are LOTS of terrific woody plants out there, people. The Dirr book is now up to 1250 pages – thumb through and pick a few alternatives. Cease and desist with the Chinese privet.

What About the Corn?

Year after year farmers in the US plants a lot of corn. A safe estimate is around 80 million acres with another 70 million acres or so going to soybeans. Corn comes from South America, soybeans are from East Asia. When we plant these crops we plant them in such a way that we exclude or, at the very least, limit the ability of native plants to grow. A safe estimate is that 99 percent of our cropland is planted in non-native species. I’d like to get your opinions on whether it’s OK for us to go to such efforts to control invasive species like kudzu and buckthorn when, on the flip side, we’re going to so much effort to encourage other species which don’t come from here.

Beantroversy

Castor Bean – Ricinis communis. Folks who make their living creating fabulous color displays for public gardens, municipalities, and commercial parks love ‘em. Civilian gardeners/plant geeks love ‘em. People who get their knickers in a twist about poisonous plants do not.

Pros: ridiculously rapid growth, huge leaves for that tropical look, tolerant of less-than-ideal conditions, cheap and easy to grow from seed, weird wild flowers and seed pods.

Cons: pretty darn poisonous. A few seeds (have seen figures from four to 20), chewed up to release the toxic protein ricin, will allegedly kill you. A Caster Bean seed looks like a really huge tick (head and all), which should be sufficiently repellent to anyone over ten years old. Pleh.

Beans with flowers. U.S. Botanic Garden, 2005, Washington D.C.

This topic arises because I was standing in our campus garden yesterday with said pot o’ poison (cultivar ‘Carmencita’, a red-leaf form), trying to find a place to plant it. Last summer, my department head noted them growing in front of our garden pavilion and asked that I kindly remove them. “They’re really poisonous, and kids will stick anything in their mouth, and I wouldn’t want my kids around them” (quoted best of my memory). He’s a wonderfully laid-back guy, so it struck me that he must really be concerned to bring it up. I teach Castor Bean in my Herbaceous Landscape Plants class for both the pros and the cons…heaven forbid a Hort graduate is unable to identify it, so I persuaded him into letting me keep it up if I removed the seed pods before they ripen. I’m not going to go into all the medicinal/industrial uses of castor oil (ick…I hope my generation was the last to have this foisted on us) nor the insidious uses of ricin – Google away if you’re interested.

We temperate-zone gardeners love anything tall and big-leaved to give vertical “oompf” to summer beds and container plantings. The tropical African native is considered an invasive plant throughout the subtropics; I’ve seen it in pastures and fence rows in far south Florida, the Cayman Islands and Dominican Republic. Apparently the cows and goats have learned to avoid it (natural selection?). I noticed it used en masse in the gorgeous beds lining the street in downtown St. Louis last summer, as well at several other public gardens I’ve visited over the past few years. So I’ll continue to use it in our garden’s palette of plants, but will remove the pods to keep everybody happy.

p.s. At the top of my summer reading list is Amy Stewart’s book “Wicked Plants”.

Permaculture – the discussion continues

We’ve started a robust discussion on the topic of permaculture, especially as applied to home gardens. Let’s continue looking at some of the advice provided in Gaia’s Garden targeted towards home gardeners.

The book contains several lists of plants suggested for specific functions. For brevity’s sake, I’ll just mention two:

“Host plants for Beneficial Insects” (pp. 157-159)
This list is prefaced in the text with “many of these florae are very attractive and can (and should!) be included even in the most formal garden bed.” With this strong endorsement, the author then presents an unsourced list of plants, several of which are identified as noxious weeds in many states in the country. They include Washington noxious weeds false indigo (Amorpha fruticosa), fennel (Foeniculum vulgare), Queen Anne’s lace (Daucus carota), toadflax (Linaria vulgaris), cinquefoil (Potentilla recta), sulfur groundsel (Senecio vulgare), and tansy (Tanacetum vulgare).

“Dynamic Nutrient Accumulators” (pp. 131-134)
We are told “certain species draw specific nutrients from deep in the soil and concentrate them in their leaves” and given an extensive table of these plants and exactly which nutrients they accumulate. The references for this table are not scientific, and in at least two cases are mystical in nature (Cocannouer’s Weeds: Guardians of the Soil and Pfeiffer’s Weeds and What They Tell). As in the previous table, many of these plants are designated noxious weeds in Washington or other states and include nodding thistle (Carduus nutans), Canada thistle (Cirsium arvense), fennel (Foeniculum vulgare), toadflax (Linaria vulgaris), creeping thistle (Sonchus arvense), and tansy (Tanacetum vulgare).

As readers of this blog know by now, we GPs are not “plant purists.” But it is highly irresponsible to encourage people to plant listed noxious weeds in their gardens. Even the author seems to understand this, and states (on page 15) that “it is foolish to deliberately introduce a species known to be locally opportunistic.” It’s mystifying, then, that he does exactly that in these two tables.

The inclusion of the table of “dynamic nutrient accumulators” demonstrates that this book tends to wander far afield of the philosophical roots of permaculture. It is an excellent example of pseudoscience, as it creates a scientific-sounding phrase (“dynamic nutrient accumulator”) and misleads non-experts into believing a scientific claim (nutrient accumulation of specific minerals) without providing actual supporting data.

Permaculture – beginning a discussion

Among other things, part of my job involves reviewing educational materials for use in WSU’s Extension programs related to urban horticulture. One of the books is “Gaia’s Garden: a guide to home-scale permaculture” (T. Hemenway). It occurred to me that my review might also be of interest to our GP readers.

I’ve created a fairly extensive review and I will break it into separate posts over the next few weeks. So let’s start the discussion off with a topic we already know is inflammatory: invasive species. To be clear, we are not talking about the many introduced species, plants and animals alike, who appear to be well-behaved in our country. Here’s my take on “The Natives versus Exotics Debate” (pp. 12-17):

The author, with no formal training in biology past his bachelor’s degree, states that “calling a species ‘invasive’ is not good science.” This will come as news to researchers in the field of invasion biology. He blithely disregards the real environmental and economic damage caused by invasive species and erroneously believes that invasive species selectively appear only as a result of human-caused environmental disturbance. Apparently natural disturbances (from fire, volcanic eruption, flooding, etc.) don’t open themselves up for invasion (again, a notion that is incorrect and refuted by a number of obvious examples, such as the 1988 zebra mussel invasion of Lake St. Clair and the subsequent colonization of many freshwater habitats). The author seems not to understand that there may be unfilled niches in certain ecosystems that can be exploited by invasives, endangering native species whose niches may overlap; there are obvious lessons from Hawaii, Australia, and other parts of the world. In any case, the author’s naive tolerance of invasive species is a poor example to follow and certainly not based on current, mainstream science.

So, fans of permaculture, what do you think? If permaculture is a legitimate science-based practice, how do we reconcile the very real issue of invasive species? If you disagree with me, keep in mind one of the hallmarks of pseudoscience: attacking the motives or character of anyone who questions the claims. The arguments should contain content, not insults.

Are natives the answer?

Last week Jeff kicked off a lively discussion about invasive plants. Let me state up front that no one on this blog is promoting invasive plants. But the issues surrounding invasive plants are extremely complex and have profound implications for many groups with whom we work in landscape horticulture and urban and community forestry. It is essential in these discussions that we separate fact from hyperbole. In some quarters, lines have been blurred and people fail to make key distinctions and lump exotic, alien, or non-native species together with invasives. According to the Federal Executive Order on Invasive species “Invasive species” means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health. All invasives are alien but only a small fraction of alien species are invasive (all humans are mammals but not all mammals are humans). Nevertheless, there is a temptation to ‘hedge all bets’ and promote only native species for horticultural planting since native plants, by definition, cannot be invasive. In addition, there is a ‘feel good’ aura that surrounds native plants – if they’re native they must be good – that clouds some of the logic in the argument.

Some examples:

Natives are more stress tolerant and better adapted than exotics.
Really. If native plants are always better adapted, why do we have invasives? Shouldn’t the “better adapted” natives out-compete them? Stress tolerance and adaption are a function of natural selection pressures of the environment in which a species or population evolves. The world is full of stressful environments and, therefore, lots of stress tolerant species. There is no a priori reason, for example, to believe that a native species needs less water than an exotic. The ability to withstand drought depends on the particular species in question. I’ve done a lot of research on stress physiology of Scots pine – few, if any, native species here in Michigan can match it for drought and cold hardiness. Moreover, as Jeff pointed out, most of our urban and suburban environments no longer reflect native conditions. Urban heat islands can result in temperatures 10-20 deg. F warmer than the native countryside. In our research on heat island effects in downtown Lincoln, NE we logged temperatures in tree canopies in excess of 125 deg. F. These temperatures were coupled that with the usual urban conditions of impervious surfaces and compacted soils – what tree species is native to that ecosystem?

Native restoration? This nurse-log ecosystem is typical of forests in western Oregon & Washington. Trying to keep it alive in downtown Portland requires constant mist irrigation..

Native plants are more pest resistant than exotics. This would be true if native pests were all we had to contend with. But the exotic pest train has already left the station. Emerald ash borer, Dutch elm disease, white pine blister rust, chestnut blight, Asian long horned beetle, and sirex wood wasp are here and here to stay. And their friends are coming. The continued expansion of global trade will almost undoubtedly mean that exotic pests, for which native trees have not evolved resistance, will become more, not less, of a problem in the future. Relying exclusively on native trees means more, not fewer, catastrophic tree failures. Heavy planting of green and white ash, which are both native in Michigan, has resulted in the loss of 30% or more of the urban tree canopy to EAB in some Michigan communities.

Natives increase diversity This presupposes that exotic species do not or cannot fill niches occupied by natives. Exotics can certainly add structural diversity and age class diversity to an urban and community forest. I would also argue that they add to species biodiversity as well. If we consider an urban community such as Lansing or Detroit, there are maybe six or seven native tree species that we could expect to have reasonable longevity as street trees. If we expand our choices to include non-natives we can expand the list to twenty or so. Not a huge number to be sure, but still a better hedge against catastrophic urban tree loss that the ‘native only’ policy.

Where to go from here? We cannot ignore that fact the invasive plants are a huge economic and environmental issue. Presently we do not have models that will accurately predict which exotics will become invasive and which ones won’t. Trees that are demonstrated to be invasive in a given environment need to be dropped from planting programs. Except for the desert Southwest and parts of the Plains, every region of the country has great native trees that can. and should, be an integral part of their urban and community forests. While it’s tempting to play it safe and promote natives only, this policy has significant shortcomings. Urban and community forests provide enormous economic, environmental, and societal benefits. In order for our urban forests to provide these functions over the long term we need as broad an array of trees species as possible, including appropriate exotics.

A thought about Invasive Plants

Recently there was an article published in the journal Science (widely considered one of the most prestigious science journals in the world) by two professors who I knew while I attended college in Pennsylvania (Franklin and Marshall College — Anyone ever heard of it?). I found this article particularly interesting because it explained how the beautiful Pennsylvania scenery that we assume is natural was actually created over the course of three hundred years. Saw mills and dams changed water flow patterns — those pretty streams that flow through the Southeastern PA (and nearby areas) that I grew up in aren’t natural at all.

Of course this is just another thing that we’ve done to make this country different from what it was when people first came here. We’ve also farmed the heck out of the land, built large industrial areas and, on top of that, there’s the issue of global warming (which, for the sake of this post, we’ll assume is caused by humans), increased carbon dioxide in the air, decreased top soil and forest land (mostly because of the farming), and a general increase in soil, water and air pollutants.

So, with all that said, It seems to me that we’ve done a lot of things to change the environment. With all that we’ve done why are we so upset when some plants, which we call “invasive” thrive in these settings? It’s not their fault that they do well in the conditions we’ve created. Sometimes I feel like we’ve built this great big smorgasbord of lutefisk (fish treated with lye — it’s pretty nasty) and then get angry when only people who grew up eating this type of fish come to the party.

I’m not trying to promote invasive plants — And I’ll be the first to admit that this post is oversimplifying the whole question of invasives — Still, it irks me sometimes that we aren’t more concerned about the environment where we plant our greenery rather than the plants themselves who, after all, are just feeding on the smorgasbord that we’ve created.

A Brief Discussion on the Wisdom of Barberry as Median Plant

We never know who to blame (or, rarely, thank) for roadside or median plantings. State D.O.T.? Local municipality? Subcontractor to either of the previous?

A few years ago, this appeared in the median of the Highway 460 bypass – the main road leading to Virginia Tech:

I am somehow reminded of Sideshow Bob from The Simpsons…

Two hedge rows. One of green Japanese barberry (Berberis thunbergii, cultivar unknown); the other of the purple form (Berberis thunbergii var. atropurpurea) Side by side, they make their way up the median, alternating from the right-hand side of the culvert to the left, like two caterpillars in love. Ooooh, lovely! Or at least SOMEBODY thought so.

Issues: 1) Barberry is a prolific fruit-setter and has made several state’s “Invasives” list. It’s outright prohibited in Massachussettes (can’t buy, sell, or import it). In our Blue Ridge Mountains of southwestern Virginia, it is popping up with startling regularity in state forests, road sides, open fields, etc. Interestingly, most of what I’ve seen is the purple form. 2) It’s very thorny. Which means most of the time it is adorned with plastic bags and other perforatable garbage. 3) They are actually mulching, pruning, and weeding this mess, which stretches for at least 2 miles. Best use of state budget/manpower??

I realize purple Japanese barberry is one of the bread-and-butter staples of many nurseries and garden designers. Folks just love the deep reddish-purple foliage. Virginia Tech’s school colors are maroon and orange, so this is the go-to shrub for campus landscapers and ardent alumni desiring that particular color scheme (see below for our “living VT”). Very fun! Growing next to a trillium on our section of the Appalachian Trail? Not so much.

Approximately one gazillion people get their photo made next to the VT logo each year – pretty good public relations for a noxious weed.