will it be a weed? - c.ymcdn.comc.ymcdn.com/sites/ · by asking some simple questions, you can...

By asking some simple questions,

you can evaluate the invasive

potential of any plant

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By Daniel J. Hilburn, Ph.D.Luther Burbank, perhaps America’s

best-known plant breeder, introduced Himalayan blackberry (a.k.a. Armenian blackberry) to this country. Big mistake. It is now one of the worst weeds we have in Oregon.

You don’t want to make the same mistake, but you want to try something new. How do you know if a plant will become a weed? Is there anything a grower or plant collector can do without a lot of scientific knowledge and a back-ground in plant risk assessment?

Yes, there is! Read on.Of course, not all noxious weeds

are escaped ornamentals. Many arrived as seed contaminants or “hitch hikers,” but the list of Oregon weeds that were introduced on purpose is longer than it should be.

The list includes Scotch broom, Japanese knotweed, Portuguese broom,

Jubata grass, yellow flag iris, giant hogweed, orange hawkweed, kudzu, Spanish heath, yellow floating heart, Himalayan blackberry, seeded butterfly bush, gorse, spurge laurel, old man’s beard, English ivy and many others.

We’d be doing our descendents a huge favor by slowing down the rate of introduction of plants that become weeds. You can make a difference by asking a few simple questions when choosing plants.

Let’s start with perhaps the most obvious question: Is it a weed any-where else? I once heard an expert say that 90 percent of the uncertainty about potential weediness of non-native plants could be determined by answering this simple question.

If the plant in question is considered a weed somewhere else, beware. The characteristics that make it weedy in one place will likely cause it to be weedy in any place with favorable conditions.

Weed problems can be quite region-al, sometimes for no obvious reason, but it is also true that many of the world’s worst weeds are problems in many plac-es, sometimes on several continents.

Himalayan blackberry, for instance, is also weedy in Europe, South America,

Will it be a weed?

and Australia. Japanese knotweed has emerged as a major weed across North America. Ditto for purple loosestrife, giant hogweed, and yellowflag iris.

Kudzu, “the weed that ate the South” after being intentionally intro-duced, gave us a scare a few years back when it was discovered doing quite well at sites in Aurora and Portland, Ore.

So the first thing you can do when contemplating trying a new plant is to go online and “Google it” and see if there are any references to invasiveness or weediness in other places where it has been introduced. Synonyms like “spreads rapidly” and “aggressive” are red flags. Dig a little deeper if you see those clues.

Quick and dirty risk assessmentSometimes there won’t be any infor-

mation on invasiveness. Then what? Is it really non-invasive, or has it just never been tried before? At this point you should ask four additional questions:

1. How does it spread?2. Where does it thrive?3. Would it be difficult to control?4. Does it have a down side?

The seeded butterfly bush (Buddleja davidii) has been considered an attractive landscape plant, but was declared invasive in Oregon due to its prolific ability to crowd out native plants. The Oregon Department of Agriculture is individually approving seedless varieties for propagation and sale in the state.

Peter clarke, licensed under the creative commons

attribution-share alike 3.0 unPorted license.

AUGUST 2010 ▲ DIGGER FARWEST EDITION 121

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Plants likely to become invasive generally produce lots of seeds that spread by wind or birds. Typically, they thrive in a variety of habitats with or without competition from other plants. Quite often, they are difficult to control and have a downside quality, such as thorns, poisonous sap, or a tendency to contaminate crops. Beware of plants that fit this profile.

Let’s try this “quick and dirty” assessment method with a few plants you are probably familiar with:

1. Daffodil — Not considered a weed anywhere; produces very few seeds; naturalizes but doesn’t out-com-pete other plants; control isn’t an issue; no drawbacks, plus they are beautiful. Good plant.

2. Himalayan blackberry — Weedy in many places in the world; produces lots of seeds which are spread by birds and other wildlife; lives just about any-where and out-competes native vegeta-tion; difficult to control; and their thorns are nasty. Noxious weed.

3. Blue spruce — Not a weed anywhere; not a lot of seeds; grows where it’s planted, if you take care of it; no control issues; and no dark side. Good plant.

4. Seeded butterfly bush — Zillions of tiny seeds; competes with native plants; weedy in New Zealand and Europe. (Note that the new seedless varieties coming on the market now should eliminate the invasiveness of this plant. Yeah!). Noxious weed.

5. Tree of heaven — prolific seeder; grows just about anywhere; considered a weed elsewhere (though not yet an official State noxious weed in Oregon); difficult to control. Warning! Warning! Warning! Not a good choice.

Oregon’s weed rating systemThese same questions form the basis

of Oregon Department of Agriculture’s Weed Rating System. Eric Coombs, our biocontrol entomologist, developed this system some years back. Members of

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our staff have improved it and vali-dated it with a variety of plants in the years since. It is not the only risk assessment tool ODA uses, but it is the simplest. Thankfully, it relies on basic knowledge of plants that is available to non-scientists.

ODA’s Weed Rating System is designed to distinguish truly noxious weeds, such as yellow starthistle, from nuisance weeds such as the dande-lion. State resources — yours and my tax dollars — are spent excluding or containing noxious weeds because they threaten the health of watersheds, wildlife habitat, forests, or agriculture. Management of nuisance weeds is left up to landowners because those weeds don’t have the same risks.

Even though it wasn’t designed to predict invasiveness of ornamental

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plants, the elements of this rating system are the same ones that plant importers, nurserymen, and collectors ought to consider for screening new plants. After all, what you are trying to do is to deter-mine whether plants being considered now could become weeds in the future.

See the rating form on Pages 126–127 for the latest version of the ODA Weed Rating System. Note that the ques-tions don’t require exact answers or spe-cialized knowledge.

Question 4 needs a little explana-tion. Weeds that are widespread rate lower on this scale, because there is less chance they could be eradicated.

What if there is no information?If you don’t know how many seeds

a plant produces, you don’t know where it thrives, and you don’t know if it is a

weed elsewhere, how can know if it’s a good choice or not ?

Answer: you can’t. Just as scientists and government

regulators can’t guess which plants will be weeds, neither can you. You need information to base these decisions on. Luckily, with the advent of the Internet, this information is much easier to come by than it was just a few years back. In most cases, you should be able to answer the basic questions right from your computer.

Oregon is lucky to have a proac-tive nursery association that supports noxious weed regulation. We also have many nursery and weed experts at OSU and ODA.

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Oregon Department of AgricultureNoxious Weed Rating System

1) Detrimental Effects (circle all that apply) 1. Health: causes poisoning or injury to humans or animals 2. Competition: strongly competitive with crops, forage, or native flora 3. Host: host of pathogens and/or pests of crops or forage 4. Contamination: causes economic loss as a contaminate in seeds and/ or feeds 5. Interference: interferes with recreation, transportation, harvest, land value, or wildlife and livestock movement

Total items circled: ____

2) Reproduction, capacity for spread (Circle the best description) 1. Few seeds, not wind blown, spreads slowly 2. Many seeds, slow spread 3. Many seeds, spreads quickly by vehicles or animals 4. Windblown seed, or spreading rhizomes, or water borne 5. Many wind-blown seeds, high seed longevity, spreading rhizomes, perennials

Rating (number you circled): ____

3) Difficulty to control (Circle the best description) 1. Easily controlled with tillage or by competitive plants 2. Requires moderate control, tillage, competition or herbicides 3. Herbicides generally required, or intensive management practices 4. Intensive management generally gives marginal control 5. No management works well, spreading out of control



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4) Distribution (Circle the best description) 1. Widely distributed throughout the state in susceptible habitat 2. Regionally abundant in part of the state, 5 or more counties, more than 1/2 of a county 3. Abundant throughout 1- 4 counties, or 1/4 of a county, or several watersheds 4. Contained in only 1 watershed, or less than 5 square miles gross infestation 5. Isolated infestation less than 640 acres, more than 10 acres 6. Occurs in less than 10 acres, or not present, but imminent from adjacent state


5) Ecological impact (Circle the best description) 1. Occurs in most disturbed habitats with little competition 2. Occurs in disturbed habitats with competition 3. Invades undisturbed habitats and crowds out native species 4. Invades restricted habitats (i.e., riparian) and crowds out native species


6) Final score (Total of all numbers)

TOTAL POINTS: ____

How to scoreNoxious weeds are those non-native plants with total scores of 11 points or greater. All noxious weeds are prohibited from importation, propaga-tion, and sale. “A” weeds are targeted for eradication.

Ratings: 11–15 = “B” 16+ = “A”

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breed out invasive tendencies. Dr. Ryan Contreras has already been played a critical role in helping ODA design a butterfly bush policy that will allow nurseries and their customers to con-tinue enjoying seedless varieties of this ornamental plant without contributing to the State’s noxious weed problems. If you have questions about whether a plant has weedy tendencies, ask Dr. Contreras or your ODA Horticulturist.

There is a growing body of science behind plant risk assessment. James Altland covered the topic well in an article called “Predicting Plant Invasions” in the May 2006 issue of Digger. He dis-cussed how scientists assess the potential for plants to become invasive.

Dr. Altland also pointed out that cur-rent regulations in this country related to new plant introductions do not include an assessment of invasive potential. Until they do, it is up to us to responsibly manage the process.

If we self-screen plant introductions, the government won’t have to step in with new regulations. If we don’t, they will. Please do your part. Your descen-dents will be glad you did!

Daniel J. Hilburn, Ph.D. is the admin-istrator of the Oregon Department of Agriculture Plant Division. He can be reached at [email protected].

The invasive Scotch broom (Cytisus scoparius) costs Oregon some $47 million per year, mostly because it makes tree planting in forests more difficult.

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