Weed? Alien? Invasive? Malignant?

Words make a difference. They help shape our thinking.

If you're trying to restore health to an ecosystem, such "alien species" as Japanese thicket parsley, oxeye daisy, and Queen Anne's lace are probably not worth worrying about. 

In contrast, "aliens" like teasel and reed canary grass in prairies and "natives" like maple in oak woods can both destroy the entire ecosystem.

What is a Weed?

Weeds in nature are valuable plants that deserve respect. Of course, the ecological definition of “weed” is different from the traditional gardener’s definition.

A weed patch in the ecosystem often functions like a scab on a mild wound that you or I might suffer. Weeds are an ecosystem’s response to hurt, degradation, or disturbance. In healthy ecosystems, weeds make up the scab that helps the wound heal. They prevent erosion and start a succession process that historically ended up with the "high-quality" or "conservative" plant species that were there before the wound.

Weeds are often annual or biennial. Classic prairie weeds include black-eyed Susan, ragweed, and common evening primrose. These days, the “healing scab” species may also include aliens like wild carrot and bull thistle. These “alien” species are long naturalized to North America and function like the native weeds. They should not be seen as problem species. A true weed goes away with time (and doesn’t even leave a scar if the disturbance is mild).

Many people worry needlessly (and sometimes counterproductively) about weeds in restoration areas. Instead, welcome them. Don’t believe the people who say, “Oh, yes, carrot is a problem. I’ve seen it be very invasive.” No they haven’t. They’ve seen it become very common. That’s just a phase, and a step forward for the ecosystem. Relax. You have more important things to do. Weed species are easily outcompeted by higher-quality species within a few years – if high-quality plants are nearby (or have been inter-seeded).

Spraying herbicide on wild carrot or bull thistle may kill it. But that same herbicide will also likely kill the desirable young plants that would otherwise have out-competed the weeds. In response to herbicide, the same weeds – or possibly more damaging invasives – may well fill the herbicide-created void, instead of the young native plant that otherwise would have been thriving in a year or three. (Of course, in a demonstration area or wild garden, a person may choose to pull unsightly weeds without waiting years for succession.)

What is Invasive?

In the early days of ecosystem restoration, we often used the word “alien” as the standard word to describe species that degraded ecosystems. But the word alien became problematic for two reasons. The first was that some people increasingly used it as an opportunity to argue politics. Attempts at scientific discussion were regularly hampered by unrelated Republican-Democrat arguments, especially as America divided over the “undocumented.”

More importantly, it became clear the “native” vs. “alien” wasn’t really the issue. Most non-native species were not a problem. And, in the modern context, many native species were very much so. Unburned prairies died in the shade of native gray dogwood or green ash as surely as they would have from the shade of alien buckthorn.

Many of us started using the words “invader” and “invasive” for what we once called “weeds” or “aliens.” This approach worked better, and, yet, while an improvement, “invasive” was not really quite the right word.

In savannas, although gray dogwood/box elder/sumac could indeed degrade the system in the absence of fire, they did not “invade” from outside. They were natural components that could become lethal to the basic ecosystem under some conditions. There’s a parallel with animals. Zoologists wondered why some species of turtles, snakes, and ground-nesting birds were disappearing from preserves. It turned out that overabundant meso-predators (like raccoons and opossums) were seriously depleting the natural diversity of the ecosystems they’d long been part of – in the absence of large predators (like wolves, that had kept raccoon numbers in balance). Raccoons weren't "invading" - but they were a problem.

Benign and Malignant

Some of us have begun to compare the health of the ecosystem to the health of the human body. It’s a fairly easy model for most people to understand.

In this metaphor, the problem species is like disease bacteria or cancer cell. Just as a perfectly respectable cell from my own body can start multiplying uncontrollably and kill me, just so, a formerly “in balance” member of an ecosystem can reproduce out-of-control and cause the loss of most species. That’s ill health, from a natural community perspective.

In the absence of fire, maples can invade a bur oak woods and wipe out most of the natural diversity of animals and plants. Okay – it seems fair to call that maple an invader or “invasive” in that scenario. But box elders, ash, and cherry can do it too. When “in balance,” they can be constituents of oak woodland. Natural fire keeps them in check. But in the absence of fire, those species can create a malignancy of shade. How should we refer to them in that case? They didn’t invade. Perhaps “out of balance” and “malignant” are the best words our language has.

Native white-tailed deer can erase many species of wildflowers, shrubs and saplings from a savanna or woodland; they can also seriously deplete many bird species in this process. They’re ‘non-invasive’ beautiful animals and should not be despised or ill-treated, but they also may have a malignant impact in the absence of equally admirable wild predators. It makes sense for deer numbers today to be controlled by people.

Conservation requires public support. Thus, we need clear language to describe problems and solutions. Most people care about nature but have only so much time (or inclination) to study intensively. Those few of us who enjoy the detail and the mission will want to use the words that will best communicate to the many who rightly take their cues from us.

Malignant Species we most often combat in the Somme Preserves

Asian honeysuckles

Common buckthorn

Common reed

Crown vetch

Gray dogwood (problematic in prairies but not so much in savannas or woods)

Lesser celandine

Purple loosestrife

Reed canary grass

Red oak (where too dense in bur oak areas)

Sugar maple

Tall goldenrod

Teasel

White sweet clover

Tall goldenrod (and the closely related Canada goldenrod) under some conditions (often apparently the lack of high-quality competitors) seems to be able to eliminate most competition over large areas. Unfortunately one of the species it does not much inhibit is common buckthorn. Both tall goldenrod and buckthorn inhibit fire, so such infestations can lead to the loss of grassland restorations. Tall goldenrod can be somewhat controlled by repeat mowing or scything. 

Woodland sunflowers (Helianthus strumosus and hirsutus) under some conditions can replace tall goldenrod and most all else. Fortunately, it is controlled well by repeat mowing or scything. The trick is to select timing that will most deplete it and least damage the diverse species that would ultimately comprise sustainable biodiversity. 

These goldenrods and sunflowers are said to combat other species by producing chemicals that poison the soil to other species but not themselves. It would be good to understand all this better.