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Search Results for ' Invasive plants'

PAL Questions: 13 - Garden Tools: 3 - Recommended Websites: 11

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Keywords: Paulownia tomentosa, Woody plant propagation, Invasive plants

PAL Question:

How can I propagate a Paulownia tree?

View Answer:

Something to consider before propagating this tree is its invasive potential. Depending on your location, increasing the population of Paulownia trees may not be wise. The Plant Conservation Alliance includes Paulownia tomentosa on their "Least Wanted" list. If you are in King County in Washington State, you may be interested to know that the Center for Invasive Species shows this tree in its Early Detection and Distribution map.

Nevertheless, directions for propagation are available. Peter Thompson's book, Creative Propagation (2nd edition, Timber Press, 2005), states that Paulownia is best propagated by seed in the spring, or by semi-mature root cuttings laid horizontally just below the surface of the soil. I suggest that you think twice before propagating this tree.

Season All Season
Date 2007-12-13
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Keywords: Ranunculus, Invasive plants

PAL Question:

Can you tell me the spread of Ranunculus ficaria 'Green Petal'?

View Answer:

The Royal Horticultural Society's A-Z Encyclopedia of Garden Plants (ed. Christopher Brickell; DK Publishing, 1996) indicates that Ranunculus ficaria (now known as Ficaria verna) spreads a foot or more, but the following information, from a local gardener (Paghat) who grows several cultivars of this plant, suggests that "more" may be quite a bit more. Excerpt: Due to their being potentially invasive, we placed them where it would not matter, but might even be rather nice, if they spread a great deal.

The species is considered invasive, as these sites indicate:

From the Plant Conservation Alliance's "Least Wanted" list

From the USDA site.

It is possible the cultivar is less invasive than the species, but I would certainly keep an eye on it.

Season All Season
Date 2007-04-13
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Keywords: Helianthemum, Helleborus, Heuchera, Propagation, Euphorbia, Invasive plants, Invasive plants

PAL Question:

I am wondering if the following plants can be divided or propagated successfully: Heuchera, Donkey Tail Spurge (Euphorbia), Corsican Hellebore, and Helianthemum.

View Answer:

I consulted The American Horticultural Society Plant Propagation book, edited by Alan Toogood (DK Publishing, 1999), and it says the following:

  1. Heuchera: by division or by seed in spring. Since cultivars may not come true from seed, I would recommend dividing your plants. Once spring growth has begun, lift the plant from the ground and remove small sections from around the edge (look for good roots, and 2-3 shoots).
  2. Euphorbia myrsinites: (Just a note: based on the USDA information that this plant is invasive in Oregon and banned in Colorado, I would think twice before propagating it. This species does a fine job of propagating itself, apparently. In general, the genus Euphorbia can be propagated by division in early spring, or from spring to summer, by seeds in fall or spring, and by cuttings in summer or fall, but if you were to propagate by cuttings, you would need to protect your skin from the sap.
  3. Helleborus argutifolius can be propagated by division after flowering, or by seeds in summer. Test seed capsules for readiness by gently squeezing. If the seed capsule splits to reveal dark seeds, it is ready for harvest. Wear gloves! H. argutifolius (Corsican hellebore) often self-seeds. Check around the base of the plant in spring. When each seedling has at least one true leaf, gently lift and transplant to moist, fertile soil in light shade.
  4. Helianthemum can be started from greenwood cuttings rooted in summer and fall, and by seeds sown in spring in a frost-free location.

If you would like further information on the relative ease or difficulty of each of these methods for each of these plants, I recommend coming to the Miller Library and looking at our books and other resources on propagation. Here is a link to a booklist.

Season All Season
Date 2007-04-11
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Keywords: Miscanthus, Ornamental grasses, Invasive plants

PAL Question:

I am looking for an ornamental grass that doesn't get over 5 feet tall and am wondering what are the growing conditions for Miscanthus sinensis (Gracillimus)? How much sun does it need, will it spread and invade my other plants, is it invasive in our area (Seattle)?

View Answer:

I found several cultivars of Miscanthus listed on the local web site Great Plant Picks. Miscanthus sinensis 'Morning Light' will reach about 5 feet tall by 4 feet wide. The following article from Colorado State University Extension may give you additional ideas on grasses for your garden. Although the following link is for southwest Washington gardens, this Washington State University list of ornamental grasses may be of use. It includes Miscanthus sinensis 'Gracillimus,' and indicates it does not have a problem with self-seeding.


BE CAREFUL! Many are self-seeding.

M. sinensis 'Gracillimus' Maiden grass 4.5' FS Most popular. Seldom self-seeds.

M. sinensis var. purpurascens Purple maiden grass, Flame grass 3 - 5' FS Gorgeous red-orange fall foliage. One of the earliest flowering varieties of maiden grass.

M. sinensis 'Silberpfeil' Eulalia 4 - 5' FS One of the hardiest varieties of maiden grass.

M. s. 'Morning Light' Dwarf maiden grass 4 - 5' S, LSh Arguably best all-around plant of the Miscanthus group. Blooms late with reddish flowers.

M. s. 'Adagio' Japanese silver grass to 3.5' S, LSh Compact with silver-gray foliage. Two- to three-feet long panicles emerge pink, fade to white.

M. s. 'Flamingo' Japanese silver grass to 6' Large, loosely open, pink-tinted inflorescences. Slightly pendant blooms appear late summer.

M. s. 'Sarabande' to 6' Similar to Gracillimus, but finer textured. Golden copper colored inflorescences in August.

M. s. 'Strictus' Porcupine grass 4 - 6' FS One of the hardier Miscanthus cultivars. Tolerates wet soils.

M. s. 'Variegatus' Variegated silver grass 4 - 6' S, PSh Prefers moist, fertile, well-drained soil.

M. s. 'Zebrinus' Zebra grass 4 - 8' S, PSh

Update from 2012 on the invasive potential of Miscanthus cultivars:

Wendy DesCamp of the Washington State Noxious Weed Control Board reports the following:
"There is now a record of the plant growing in eastern Washington by the Columbia River in Benton County. [described as follows:] Shallow backwater on N shore of Columbia River . . . below McNary Dam, elev. 85 m, 45 degrees, 55.9 minutes N, 119 degrees 21.4 minutes west. Collected by Peter Zika, 17 June 2011.
From what I can find, this is the first collection of naturalized Miscanthus sinensis collected in Washington."

The State Noxious Weed Board is considering whether it should be added to the monitor list or not. The monitor list is a list of plants the Board is keeping track of to collect information and to see if the plants are occurring or spreading in Washington.

UW Botanic Gardens Director, Professor Sarah Reichard had this to say about Miscanthus sinensis:
"We have had it in the Soest Garden for years and I have not seen it invade and I am looking for seedlings. However, not invading in the artificial environment of a garden, with water and nutrient inputs means little for invasion in the wild. I have not heard of it being invasive here, and I have been paying attention to both this species and Imperata cylindrica.It might be a good addition to the [noxious weed] monitor list."

Season All Season
Date 2007-05-16
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Keywords: Hedera, Invasive plants, Climbing plants

PAL Question:

My question is about ivy for growing up a brick wall. What would you recommend? How do Boston ivy and English ivy compare for this purpose? We live in New Jersey.

View Answer:

First of all, it is important to know that clinging plants, such as Boston ivy and English ivy have the potential to "damage old, soft mortar and strip off pebbledash". (Gardening with Climbers by Christopher Grey-Wilson and Victoria Matthews) It is also suggested that these vines have a "structurally sound surface and must be prevented from reaching under house eaves and roof tiles and into window casements." (The New Royal Horticultural Society Dictionary Manual of Climbers and Wall Plants edited by JK Burras and Mark Griffiths)

The New Jersey Invasive Species Strike Team has a factsheet on both English ivy (Hedera helix)and Boston ivy (Parthenocissus).

In addition to taking this information into consideration, it would also be important to identify the amount of sunlight and the extent to which the side of the house will be exposed to harsh winter winds and temperatures. Neither Boston nor English ivy is recommended for full sunlight. Boston ivy will give you more fall color and interest and will withstand cold winters. (Simon & Schuster's Guide to Climbing Plants by Enrico Banfi and Francesca Consolino)

If you want to consider an alternative vining plant, you might want to install a trellis. That way you will not have to rely solely on vines which cling to the brick. You could try Clematis or some the honeysuckle species that are native to the northeastern U.S. There are several listed in this article by William Cullina, "Alternatives to invasive or potentially invasive exotic species," from the New England Wildflower Society:

  • Lonicera ciliosa (Orange Honeysuckle)
  • Lonicera dioica (Limber Honeysuckle)
  • Lonicera flava (Yellow Honeysuckle)
  • Lonicera sempervirens (Trumpet Honeysuckle)

Season All Season
Date 2007-07-02
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Keywords: Noxious weeds--Washington, Noxious weeds, Invasive plants

PAL Question:

Can you provide me with an extensive list of resources for checking whether a plant is invasive or a noxious weed?

View Answer:

Here is a list of helpful resources:

Washington State Noxious Weed List from the US Department of Agriculture

State noxious weed list and schedule of monetary penalties from the Washington State Legislature

Class A, Class B, and Class C

Washington Department of Ecology (aquatic plants)

Washington Invasive Species Coalition and their GardenWise handbook

King County Noxious Weed Lists

National Invasive Species Lists

Plant Conservation Alliance Alien Plant Invaders list

Invasives in British Columbia

The lists which are national in scope are useful too, as some plants not yet officially listed as invasive here may still be plants to watch out for.

There are a great many books on this subject. A recent one, co-authored by a faculty member here, is Invasive Species in the Pacific Northwest edited by P.D. Boersma, S.H. Reichard, and A.N. Van Buren; Rebecca L. Gamboa, photo editor. University of Washington Press, c2006.

Season All Season
Date 2007-10-10
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Keywords: Acanthus mollis, Weed control, Invasive plants

PAL Question:

I am wondering how invasive bear's breeches is? I have heard it can be invasive in the Northwest. Will I be battling roots or suckers constantly? Can it take over any plants near it?

View Answer:

Acanthus mollis, or Bear's breeches, is not listed as noxious in King County, Washington State, or on the federal list of noxious plants. This is not the same as saying it isn't potentially aggressive, although I've never heard about it being a serious problem here. It is considered invasive in parts of Australia, though.

The Plants for a Future database offers the following information on this plant and its growing habits:
"Plants can become invasive, spreading by suckers, and they are difficult to eradicate due to their deep roots."

According to the Pacific Northwest site, Rainyside Gardeners, it is sometimes difficult to get this plant to bloom. A Washington State University Extension site says that Acanthus mollis is potentially invasive in climates warmer than ours.
"This species is classified as a groundcover in that any pieces of root cut from the original plant can easily contribute to further plant spread."

If you want to grow it but are concerned about it spreading, you could try containing the roots with an 8-inch root barrier (similar to what is used to keep running bamboo in check). On the other hand, if you have this plant and decide that you wish to be rid of it, the book Wildly Successful Plants: Northern California Gardens by Pam Peirce and David Goldberg (Sasquatch Books, 2004) says that removing every bit of root over two or three seasons of growth should get rid of the plant. If you cannot eradicate it by continually digging up each new shoot, you may have some luck using a flame weeder (with due caution and appropriate protection). Northwest Center for Alternatives to Pesticides describes how to use this tool.

Season All Season
Date 2008-09-25
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Keywords: Cytisus scoparius, Noxious weeds--Washington, Plant quarantine, Invasive plants

PAL Question:

Is it safe to plant Cytisus scoparius 'Moonlight' here on Whidbey Island? I know that Scotch broom is thought to be invasive, but I wonder if maybe this variety is less of a problem.

View Answer:

Some sources (such as the State of Oregon's noxious weed control board) have said that "sterile cultivars" of Cytisus scoparius are exempt from regulations governing noxious weeds. However, the Center for Urban Horticulture's Professor Sarah Reichard, an expert on invasive species, says the following:

"The 'sterile cultivar' issue is huge worldwide. The reality is that sterile cultivars depend on the type of sterility: there are many reasons a plant might be sterile. Only a few of them can be considered to be stable under varying environmental conditions.

Regardless of what is done in Oregon, in Washington it is illegal to sell or grow any cultivars of Cytisus scoparius. Moonlight is less aggressive, but I have definitely seen it seeding out. But it does not matter how aggressive it is: it is still on the quarantine list in this state because that is the way the state law is worded. Island County may not have it on their high profile noxious weed list because it is only a B non-designate there because it is widespread. But our noxious weed (control) and our quarantine lists are two different things in this state and it is quarantined here."

For future reference, here are links to Washington State Plant Quarantine and Noxious Weed lists.

Season All Season
Date 2009-05-16
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Keywords: Buddleia, Asclepias, Invasive plants, Endangered plants

PAL Question:

Is butterfly weed invasive in the Northwest?

View Answer:

First, I have a plant name question for you: did you mean the shrub Buddleia davidii which is also called butterfly bush, or the herbaceous perennial Asclepias tuberosa, the plant most often referred to by the common names butterfly weed or butterfly milkweed?

In King County, Buddleia davidii is a Class B noxious weed, which means that its control is recommended but not required by law. Many gardeners grow this shrub as well as other less aggressive species. If you absolutely cannot live without this plant in your garden, you can go a long way toward preventing its spread by keeping the blooms deadheaded as soon as they fade (before they set seed).

Asclepias tuberosa can seed itself around, but it is classified as an endangered plant in New Hampshire, a threatened plant in Vermont, exploitably vulnerable in New York, of special concern in Rhode Island, and possibly extirpated in Maine. It attracts butterflies and hummingbirds. Here is more information so you can determine which plant you have in your garden:
Lady Bird Johnson Wildflower Center
USDA Plants Database

Season All Season
Date 2010-02-04
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Keywords: Cuscuta, Noxious weeds, Invasive plants

PAL Question:

I'm doing a science fair project on dodder plant and I'm seeking information about the plant, and a source of seeds or plants for the project.

View Answer:

Dodder is a parasitic plant that lives on crops, ornamentals, native plants, and weeds. Because it has limited chlorophyll, it can't make enough food to support itself, and so relies on the plants it colonizes for nourishment. It belongs to the genus Cuscuta, in the family Convolvulaceae (same family as morning glory). It was formerly referred to as Grammica.

Perhaps the reason that seeds and plants are not readily available is that dodder causes great damage to the plants it parasitizes. The U.S. Department of Agriculture's plant protection and quarantine office states that "products, including foods, containing whole dodder seeds (Cuscuta spp.) are prohibited entry into the United States. APHIS regulates whole dodder seeds, both as a parasitic plant pest under Title 7 Code of Federal Regulations (CFR), Part 330 and as a Federal noxious weed under Title 7 CFR, Part 360."

Here is additional information from University of California, Davis's Integrated Pest Management website. Dodder is sometimes referred to as the "Vampire Plant," as this University of Florida Extension document explains. Although your project, safe within the confines of a lab or classroom, might pose no threat, it is not legal to sell Cuscuta seeds or plants in the U.S.

Season All Season
Date 2010-05-05
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Keywords: Alstroemeria, Invasive plants

PAL Question:

I've just been given two pots of Alstroemeria psittacina 'Variegata,' a lovely red variety. Is this particular variety invasive here in the Pacific Northwest? I've grown the orange ones and then they took over--very hard to eradicate from beds. Does anyone know if the red ones are as invasive?

View Answer:

The Pacific Bulb Society lists this species under its previous name, Alstroemeria pulchella, and says it is weedy in some gardens and barely survives in others. Alstroemeria psittacina may be officially listed as invasive in some areas (in Australia, for example), but even if not officially designated as such, it may grow aggressively. This listing on the Floridata website, describes it as follows:

"This is a seductive plant. It is colorful, unusual, and exotic looking and effortless to grow once you get it going. Every gardener I know who has seen it has wanted it, begged a start, then nurtured it and delighted in it - for a few years. Then every one of them has come to curse the way it spreads and taken to ripping it out with a vengeance. Perhaps its best use is as breeding stock for developing more spectacular and less troublesome varieties of Alstroemeria."

What you could do is grow your plants in a container, to avoid potential problems with weediness. I think aggressive spreading should be assumed with this genus unless otherwise specified.

Season All Season
Date 2010-08-12
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Keywords: Ailanthus altissima, Noxious weeds, Invasive plants

PAL Question:

There seems to be a plant invasion in my Seattle neighborhood. I think what I've been seeing are larger specimens of Ailanthus altissima (Tree-of-Heaven) surrounded for many blocks by smaller seedlings of this same tree. It sprouts up through the middle of landscape plants and lawns, and right up against concrete foundations. I feel I should be warning people, but I'd like to know what the local status of this tree is, and I want to be sure I have identified it correctly. I already know it's aggressive here, and I know it's been designated invasive in other parts of the U.S. and the world.

View Answer:

You are correct that Ailanthus altissima has quite a track record for invasiveness. As you say, the U.S. Department of Agriculture's National Invasive Species Information Center lists it. If you want to show people in your neighborhood some clear images of the tree in various stages, the Invasive Plants Atlas of the United States has good information.

If you aren't completely certain what the plant is, you can bring samples to the Herbarium here at the Center for Urban Horticulture, or you can compare and contrast what you have observed with some close look-alikes:

Tree-of-Heaven is mentioned in a pamphlet on alternatives to invasives for Eastern Washington gardeners, but it has not yet achieved official invasive status. The Washington State Noxious Weed Control Board is considering a nomination to designate it as a Class B noxious weed (second highest priority) for the 2012 state weed list.

You could encourage your neighbors to eradicate it when possible. It spreads by seed (which can be dispersed by birds but especially by wind), and by root sprouts. It is a very fast grower, and it is important, when digging it up, to get every last bit of root, or you will soon find more of it sprouting. The California Invasive Plant Council has excellent, detailed information on its history, its growth and reproductive habits, and several methods of controlling its spread.

Season All Season
Date 2011-08-03
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Keywords: Trachystemon orientalis, Invasive plants, Ground cover plants

PAL Question:

A mail-order nursery catalog I recently browsed is touting Trachystemon orientalis as the plant for problem areas of the garden where other plants would struggle. They say it will rapidly cover blank or weedy areas in the garden, whether they are sunny, shady, wet, or dry. What can you tell me about it? It sounds intriguing, but I'm worried it might be invasive!

View Answer:

Trachystemon orientalis is in the same family as borage (Boraginaceae), with similar blue flowers and rough leaves (to my eye, it's a plant for a wild garden, with its resemblance to borage and comfrey). According to Perennials by Roger Phillips and Martyn Rix (Firefly Books, 2002), it is a native of eastern Bulgaria, northern Turkey, and the western Caucasus region. It is naturalized in parts of England. The authors say it thrives "in wet beech forest on shady river banks and on damp rocks at up to 1000 meters, flowering in March-May." It spreads by rhizomes and is tolerant of neglect. Its preferred site is moist shade, but it adapts to a wide variety of conditions, and excels at smothering weeds. There is additional information from Missouri Botanical Garden.

You may be right to wonder when a plant seems to be just the thing for almost any spot, and when it is promoted for its ability to spread easily. Some ground cover plants hover on the brink between covering the ground you want covered, and exceeding those boundaries. TrachystemonIt's common name is Abraham-Isaac-Jacob. The origin of that name is unclear, but my interpretation is that a plant named for the patriarchs of Genesis has a connection to generational continuity, a fitting name for a plant which is skilled at self-propagation!

See garden writer Margaret Roach's blog post entitled "A Plant I'd Order," and notice what she says about the likely behavior of this plant in the Pacific Northwest.

Trachystemon orientalis is included in the Manual of the Alien Plants of Belgium and is listed in the Invasive Species Compendium. You could test its behavior in your garden for a season or two, and if it shows signs of aggression, you should still be able to eradicate it. If your aim is to be cautious, you may want to avoid planting it.

Season All Season
Date 2015-01-24
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Keywords: Caryopteris, Buddleja, Invasive plants, Butterflies

Garden Tool:

The popular Butterfly Bush, Buddleja davidii, has proven to be a weed. Found growing in natural areas far from any garden, Buddleja's seeds are dispersed by the wind. Oregon and Washington both list it as a noxious weed.

What is a butterfly loving gardener to do? Try Blue Mist shrub (Caryopteris x clandonensis 'Black Knight' ) as a blue/purple, summer flowing shrub substitute or at least only grow the named Buddleja cultivars or the species Buddleja globosa which may be less of a threat.

Season: All Season
Date: 2007-04-03
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Keywords: Convolvulus arvensis, Hedera helix, Weeds, Invasive plants, Holly

Garden Tool: In late spring watch out for seedlings of invasive plants bindweed (perennial morning glory), English holly and English ivy. Birds love to eat ivy berries, which are only produced by mature plants that have stopped climbing. The berries ripen in late winter, just in time for birds to "sow" the seeds in your garden. These three weeds are easy to pull up when their root systems are still undeveloped.

Season: Spring
Date: 2007-05-17
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Keywords: Hedera helix, Noxious weeds, Invasive plants

Garden Tool:

Did you know that one English ivy plant removed from a tree in the Olympic National Park weighed an estimated 2,100 pounds? The King County Noxious Weed Control Program has a great deal of information on how to control ivy. If you would like to receive the information in other formats, call them at 206-296-0290.

Season: All Season
Date: 2002-09-18
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December 12 2014 11:33:49