Desirable Plants, Thugs and Weeds: The Fine Line Between Good and Bad

The first in the critically acclaimed ‘Over-Thinking Series’

What is a weed? What differentiates it from any other plant? What about plants like Golden Creeping Jenny? I know those who call it a weed. Is it a matter of generally accepted horticultural opinion? One is on the list and the other isn’t? Is it more than simply a plant we did not plant? We all have those fortuitous seedling volunteers that appear bringing with them a little garden magic. Some of these prove themselves to be quite remarkable and, if found by the right person, if they retain their desirable characteristics when propagated and are stable over time, may become the next rage among the garden fashionistas. They aren’t weeds are they? While not planned for we may choose to welcome certain such plants. To just define weeds as plants out of place, is to render the word almost useless as an aid to understanding our problem.

There is no conspiracy going on in the garden. No malignant intelligence plots and schemes to upset our plans. Not every seed germinates. Not every bit of rhizome or tuber will take over the garden/world without our intervention. Weeds are plants and within the limits of their own genetics they either grow or die. If a seed of Calypso bulbosa finds itself on the ground in the desert. It will not germinate. Eventually it will lose its viability. No one will notice its absence because it never belonged there. All plants possess a degree of vigor. All plants have within themselves a range of conditions they can grow and prosper in. Weeds, are plants that possess the vigor and adaptability to be successful under the range of conditions that we as humans typically occupy. Because we occupy these places, disturb them, because we move about and conduct trade with those elsewhere, we have ‘weeds’. These are merely the plants that do well in the places that we habitually have created. What we should be talking about instead is the quality of ‘weediness’, that blend of adaptability and vigor, that these plants possess.

(Readers should be aware that there are no pretty pictures to either break up the following text or to graphically ‘hammer the lesson home’.

Every plant, even those that require very specific species associations, have some ability to exhibit “weedy” tendencies, albeit for some these are very limited. ‘Weediness’ goes to a plant’s ability to survive, to grow to maturity and produce viable seed whilst withstanding all of the slings and arrows a particular climate and place can throw at it. Without this characteristic, this vigor, those valued exotic perennials and shrubs would soon disappear from our gardens, but with too much…they’ll overwhelm. Or, you change the location, change the conditions and another plant may become problematic. Kahili Ginger, Hedychium gardenereanum, has become an invasive pest in Hawai’i, but here seedling volunteers will rarely, if ever appear and then only in a highly pampered bed. Turn off your summer irrigation and it will struggle and soon die. Vigor is key, but it is not the only critical characteristic of weeds. Adaptability also comes into play.

A plant that is able to thrive under a wide range of growing conditions is much more likely to become a ‘weed’ problem. The more different conditions it accepts the more places it can grow and the more accessible your garden will be to infection by its seed. Site conditions vary across a continuum. Light to shade. Acid to Alkaline soils. Sand to clay. Deep soil to crevices in rock. Bog to arid desert and tundra. Every plant has a range of conditions within which they prosper and increase. Go beyond them and they will struggle and die. This is as true of your choice coddled perennials as it is the weeds you battle. Weeds are very ‘successful’ plants in the sense that in a world defined by physical disturbance they are perfectly equipped to move in and survive. When we grade a site and put up houses we have simplified it. The complex relationships between soil, fungi, bacteria pre-existing flora and fauna have been erased. While your potting soil may be uniform, sterile and inert, the soils in our undisturbed native landscapes are not. Plants whose niches are narrow and specific, have lost a compatible supportive site when they are disturbed. Weeds have been indirectly ‘selected’ over time in this way for just these kinds of places. This ‘disturbed’ world/ landscape is much simplified. Specialized plants with specific narrow site requirements have fewer places where they can thrive. We don’t struggle to get the conditions right in order to have Dandelion and Vetch do well. Meconopsis is a different story. Imagine a world where Meconopsis sheldonii, one of the Blue Himalayan Poppies, proliferated like Dandelions, many of us would be struggling with ‘controlling’ them (Yes, you would and you’d probably be trying to figure out how to better grow Dandelions). So, vigor and adaptability define a weed.

The performance of any plant, weed or not, can be predicted when one understands where the plant originated, the range of conditions it may grow in and the vigor with which it typically grows. It is part of the calculation that a gardener should make when they chose any plant. Often homeowners don’t want to worry about plants that might die if they don’t provide the proper care so they buy plants that are very adaptable and have vigor. These often become problematic over time as they overwhelm their space, crowd structures and battle their plant neighbors…. Low-care, no-care, vigorous plants, are dangerously close to being weeds and in fact now number themselves amongst some of the worst we now face.

At the weediest end of the continuum are the truly invasive weeds and by that I mean that they are able to invade undisturbed plant communities that have been growing much as they have for centuries and more. Even though these sites are composed of integrated plant communities that have existed in balance, an invasive plant can find a place to grow, mature and later dominate the landscape, burying and overwhelming the pre-existing plant community. That’s vigor!

The next step down would be plants that readily invade disturbed and bare soil sites under natural conditions, i.e., no soil amendments, no supplementary water. These areas would include ‘waste’ areas and vacant properties, not managed landscapes. These would include many common weeds from Queen Anne’s Lace to Canada Thistle.

After that are the plants capable of invading a managed irrigated landscape or garden. If water is removed, these weeds would weaken and likely disappear over time. These would include the Stellaria spp., Smooth and Mouse-Eared Chickweed, which tend to be a problem only on sites with moist rich soils.

Another group are the ornamentals that are new enough on the scene that their ability to escape the garden is unknown, but have exhibited the ability to spread aggressively in the managed landscape. The potential may be there for them to exhibit the same tendency on disturbed and undisturbed natural sites if their basic requirements are met. This is a frightening group for land managers, ecologists and restoration workers. They can only watch as these plants gain acceptance amongst gardeners forming a growing local population and wait to see if they begin to escape, and if so, how aggressively. With resources spread thin and shrinking, agencies are unable to monitor the region the way they should, while at the same time, the average gardener may be unaware of the potential danger to the regional landscape. These plants may prove benign over time or may join the ranks of those classified with strong weedy qualities, as invasive.

The next group consists of proven ornamentals and agronomic crops that increase over time in a manageable way in the garden setting. The likelihood of their escape to wilder or vacant sites is minimal as demonstrated over time.

The last group are the plants considered to be marginal or tender in our climate, or whose site conditions are so narrow and specific that we have to work to make them survive in our gardens. These have no ability to escape under almost any conditions. Many plants from Bananas to Zinnias typically freeze out here. Others may only grow in very specific association with other plants and mycorrhizae. They cannot establish a resident population without them. Some may survive the mild winters only to rot out in our wet cold soils. Some in this group appear to thrive, but are unable to produce seed because the length of our growing season is too short. Tropicals like Bananas, genus Musa, Ensete or Musella, require more than a year’s growth to produce mature vegetative growth, which can flower and then ripen fruit. They are not going to become a problem. This last group probably includes Arundo donax, the Giant Reed, which is a particularly vicious invader in southern California where the climate is more to its liking (I’ve heard that his plant has been found more recently up into southern Oregon so it deserves watching!).

All plants can be placed somewhere on this continuum. Its use could be a very educational tool if gardening groups, growers and garden centers provided this information to the public. Just as the public is often ignorant of the risks that the biggest offenders present to all landscapes, they are also of the plants that can become renegades in their own gardens. Providing a ‘weediness’ rating on plant labels at garden centers could be both a great aid to many gardeners in helping them avoid potential problems in their own yards, while at the same time helping them to understand the larger issue of invasive plants facing the region. These ‘weedier’ plants are often the same ones that new gardeners/homeowners are looking…to produce quick easy cover.

Pushing a limited ‘enemies’ list on the public is not as likely to be successful. Many of the plants not on that list are still problematic for the gardener and may also be for particular native landscapes within the region. The State’s list of banned or noxious plants is a limited legal document. It won’t identify emerging problems or economic plants that the industry is still profiting from. An enemies list has limited educational value. Whereas a ‘weediness’ labeling system would help gardeners look at plants in a different, more holisitic way and wake them up to their role and responsibilities regarding the care of the larger landscape. We need to remember that all plants are separated only by degrees of ‘weediness’.

There is no unique gene that will allow you to place a plant solidly in one group or another (that I’m aware of anyway). It is a question of behavior in the environment. We are left with the careful observation of plants displaying this tendency in similar regions of the world and even more careful observation when they arrive here. Species that have proven invasive elsewhere under similar conditions should be watched carefully before their general introduction here, if allowed in at all. A genus that contains aggressively spreading members is worth close watching as well. Lysimachia is a perfect example as it contains such garden thugs as L. nummalaria, L. ‘Outback Sunset’ and L. clethroides. Various Euphorbia species including E. characias ‘Wulfenii’, E. myrsinites and E. cyparissias can seed strongly in the garden as well as several others such as E. esula, Leafy Spurge, which is an invasive weed in the Great Plains states. (I just had a conversation with Greg Shephard from Xera Plants about Euphorbia’s weedy tendencies. He doesn’t see them becoming a problem here in our region, though yes they can spread around the garden.)

Every plant, then, possesses some degree of weediness, which determines its ability to escape its intended site or to spread beyond where we may like it to be. Weeds, given the conditions on our site, are simply better at this than a chosen Penstemon or Podophyllym we have planted. A certain amount of the ‘weediness’ is necessary in our gardens as all gardens are contrived sites, even those we intend to be populated by natives. Without this characteristic ‘weediness’ in our plants, our gardens would fail. Plants would only grow in relationship with the others they evolved with on undisturbed sites in established communities.   It is also a necessary characteristic of plants in native communities to re-establish after intense fires and other cataclysmic events like landslides, volcanic eruptions or the upcoming predicted climate change. Otherwise these, and the sites we have disturbed in our procession around the globe, would be sterile wastelands. We garden, by definition, on disturbed or disrupted sites. Weeds are the plants best suited for survival on these.

That is a third essential feature of weeds…we are essential to them. The way we live. The way we garden. Our aesthetic. Our tendency toward structure. Our effective denial of our role in the world’s landscapes. We brought these plants here intentionally or not. We have radically altered the natural landscape to meet our own desires, our aesthetic and need for structure… and we continue to do so. We have created the conditions that favor the weeds that now plague our landscapes. Some argue that weeds are nature’s attempt to heal the damage we have caused, covering the wounds in an attempt at stopping the ‘hemorrhaging’. Our vastly simplified landscapes are out of balance. They are ‘bleeding’ nutrients away because the needed complex community is no longer there to keep them from leaching away. These soils have lost their ability to rebuild or maintain the complex soil relationships required of healthy soil. This ability needs to be grown back into our soils. Weeds may be the first stage in healing…and we keep tearing the scab off. We have been making little progress in building the needed complexity back into our soils. Our practices are effectively making the wound larger, because we don’t recognize our role in the problem. In a sense, I think this is right, but I don’t feel an impending doom. Rather, I see an opportunity. It is within our power to do something about this. To stop the bleeding. To become careful observers and begin the healing. One of the ways to begin is to take off our black and white glasses and see weeds as what they are and to ask ourselves how did we get here.

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