WEEDS

Identification and Control for Master Gardeners

Linda R McMahanFaculty for Community HorticultureOregon State University Extension ServiceYamhill Countylinda.mcmahan@oregonstate.edu Photos by the author

Weeds

Characteristics Identification Some Examples When just plain weeds become Invasive Control Strategies

What next. . . . In this case, our weed is really quite lovely, so what then? This particular “weed” is Jimson weed, Datura stramonium, a toxic plant that is considered to be an agricultural pest. However, some people grow it for its ornamental value.

So you have a weed

Is it a weed or native plant? This is a good question to ask. The plant is purple loosestrife, Lythrum salicaria, which is considered to be highly invasive throughout most of the United States.

This one looks so pretty and it is growing in a wetland along a nature trail in Beaverton, Oregon

English ivy for example. How do we go about doing such a task? Can the removed ivy be composted? What are the best methods of removal and control?

Removing some weeds can be very hard work . .

First Let’s Look at Weed Characteristics

A Weed is sometimes defined as “a plant out of place,” but perhaps that is too simple

Colonizers Reproduce successfully— some have

numerous seeds, sometimes genetically identical, for rapid establishment

Others quickly reproduce by vegetative reproduction

Have become successful hitchhikers in bird seed, garden seeds, plant pots, and on anything that moves including boots, tires, water, and wind

First Let’s Look at Weed Characteristics

Weeds can indeed be “crafty” and it is sometimes quite difficult to “outsmart” them

May engage in chemical warfare by suppressing the growth of surrounding plants

Many survive in nutrient poor soil—some are even nitrogen fixers to gain a competitive advantage in poor soils

On the following few slides, I have referred to helpful web resources on the left panel of each page

This weed is spreading rapidly throughout Oregon. It is a biennial and difficult to hand-pull. It helps maintain its weed status through chemical warfare by suppressing growth of other plants or interfering with their ability to form beneficial relationships with soil fungi.

Now here’s a nasty weed known as garlic mustard, Alliaria petiolata. It is introduced from Asia and Europe.

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Some Examples

As gardeners, you are perhaps more familiar with this plant. It is a winter annual, sometimes blooming and producing seeds before we emerge in late winter to check on our gardens.

This plant, Cardamine hirsuta, is sometimes called snapweed or shotweed for its habit of explosive release of its seeds when touched.

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Some Examples

Here’s another familiar lawn weed. Each flower stalk reproduces over a long period of time, producing hundreds of seeds.

Common plantain, Plantago major

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Some Examples

Annual bluegrass is extremely difficult to control because it seems to thrive with our common garden practices. Even though these plants have been sprayed with herbicide, they do not seem to be responding and even these small plants have already produced seeds.

Yet another familiar weed is annual bluegrass, Poa annua, a common lawn pest.

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Some Examples

Weed Categories

Different groups require different control methods

Winter Annuals – some of our most challenging garden pests

Summer Annuals Biennial weeds Perennials

Some have deep and persistent taproots

The “nastiest” create underground runners or storage systems

Shrubs, Vines and Trees

Another Distinction

These two major groups of flowering plants can require different kinds of control

Some weeds are Monocotyledons, including most notably weedy grasses and sedges

Other weeds, often referred to as broadleaf, are Dicotyledons

If you are controlling broadleaf weeds such as dandelions in lawns using herbicides, be careful to choose appropriate products so as not to kill the grass as well

Native Plants as Weeds?

You may be confused to find native plants listed as weeds in some of the resources

Examples are yarrow (shown left), wild cucumber, poison oak, native irises, and horsetail

This is because some of these plants are considered to be agricultural pests, garden pests, or cause human or animal health issues-so control methods are available and may be appropriate in some cases

Weed Examples and Controls

The following slides show examples of various categories of weeds

In each case, control methods are mentioned

Please note that we will cover controls more thoroughly near the end of this presentation

Winter Annuals

Because they bloom and produce seed in the winter, these weeds tend to catch us by surprise if we are not watching

Typically germinate in the fall and flower in the winter or early spring

Examples include snapweed, some mustards, chickweed, and some weedy geraniums

They spread through seed production

The best control for this small winter annual is persistent hand weeding. It has a relatively short seed life of approximately 3 years so persistent control can effectively reduce and even eliminate the population. You have to be fast, however, this one can bloom in mid-winter! Pre-emergent treatments can also be effective.

Herb Robert, Geranium robertianum, sometimes called “stinky bob” is a newly listed invasive species in Oregon.

Winter Annuals

Summer Annuals

At least, summer annuals are more visible, but that does not mean they are always easy to control

Seeds of summer annuals germinate in the spring, then bloom and set seed before fall frosts

Examples include lambsquarters, pigweed, mallows, ragweed, and spurge

This weed often appears as a contaminant in bird seed. It has small but attractive flowers and a distinctive post-flower appearance shown here. it is a summer annual and can be controlled through hand weeding.

Common mallow, Hibiscus trionumSummer Annuals

Biennials

Since they form rosettes the first year, they may escape our attention until the second year flowering stem suddenly bolts

Biennials typically have a 2-year life cycle. The first year, the seed germinates and the plants produce a rosette, a round “circle” of leaves that remains flat to the ground. The following year, the stem “bolts” to produce flowers and seeds

Examples include bull thistle, foxglove, and common mullein

One control for bull thistle is removal of all flowers before they set seed. Hoeing while plants are small is also effective. Seed are prolific. Repeated tilling or mowing and control with contact herbicides can also be effective.

Bull thistle, Cirsium vulgare, is a biennial forming a prickly rosette the first year, than a tall blooming stalk

Biennial Weed

This biennial produces hairy felt-like leaves in a rosette (shown above) the first year. The second year, it sends up a tall stalk of attractive yellow flowers. A single plant can produce more than 100,000 seeds. Control is through competition with other plants (it likes bare ground), hand removal when the soil is loose, and deadheading to remove flowers and seeds before dispersal.

Mullein, Verbascum thapsusBiennial Weed

This common roadside weed seems to be increasing in Oregon. Recommendations for control include deadheading flowers or using herbicide at the rosette stage because some are not effective when the plant begins to bolt. Some say the “hooks” on the seed stage of the flowering head may have been the inspiration for Velcro.

Teasel, Dipsacus fullonumBiennial Weed

Another “pretty weed”. This one has a tap root—it is a wild carrot after all. Its habit is usually biennial, but plants sometimes persist for additional years. This common roadside weed is resistant to many herbicides. When young, they often can be hand-pulled from soft or moist soil.

Wild carrot or Queen Anne’s lace, Daucus carota

Biennial Weed

Perennial Weeds

Perennials are herbaceous plants that die back and regrow from underground roots or stems each year

Dandelions are simple perennials spreading by seed

Canada thistle, quackgrass, field bindweed, yellow nutsedge and others create underground networks as part of their reproductive strategy

Each plant can produce hundreds of seeds which take off on the wind to infest the neighbor’s lawn as well as yours. Can be controlled through persistent hand-weeding with a weeding tool or selective broadleaf herbicide applications. Young leaves of dandelion are often available for use as salad greens.

Ahh. Our old favorite the dandelion, Taraxacum officianale

Simple Perennial

Unlike the biennial bull thistle, even persistent hand weeding or deadheading makes control difficult because underground pieces readily regenerate into new plants. Can sometimes be effectively controlled with weed barriers and with repeated 2-4 D or other contact herbicides beginning in September.

Canada thistle, Cirsium arvense, is difficult to control –it is a perennial and spreads underground

Spreading Perennial

The PNW Weed Handbook recommends 2,4 D or glyphosate applied in the fall while plants are still actively growing. Most of the other chemicals listed are not available to homeowners. Persistent clipping and covering with landscape cloth are sometimes effective controls in smaller landscapes.

Wild morning glory aka field bindweed, Convolvulus arvensis, is another perennial weed requiring persistence

Spreading Perennial

Weedy Woody Plants

Many woody plants can become pests, even invasive ones

Examples of invasive woody plants are Himalayan blackberry, honey locust, Norway maple, English ivy, and Scotch broom—all are problems in Oregon

Less invasive but common ones include the nightshades and poison oak

These vine-like woody plants with poisonous black or red berries are of concern both as pests and for their toxicity. Seeds readily germinate throughout the year and they are tolerant to many herbicides, making control more difficult. Manual removal is often effective. If you choose herbicides, use during strong summer growth or early fall when plant resources are being sent to the roots for winter storage.

Nightshades, Solanum nigrum and others

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Scot’s broom is a shrub first introduced into Astoria, Oregon as an ornamental. Now it “ornaments” our landscape, displacing native species and causing allergic reactions for many allergy sufferers. Biological controls have been introduced and are sometimes available. Other methods of control include pulling with a “weed wrench,” burning, and herbicides.

Scotch broom, Cytisus scoparius

The issue of weed control is tough!

Let’s begin to find some answers

Identification is important ID helps us learn if the plant is

a weed or not ID will tell us how important the

weed is to control ID will give us the information

we need to recommend appropriate weed control

Weeds of the West

Published by OSU Press and available in the master gardener library

A picture-based book, with entries grouped by plant families

Most of the common weeds of Oregon are in this book

This is your first place to look!

Weeds of the Northern US and Canada

Published by Lone Pine Press – also available in the master gardener library

Includes many weeds and relatives

Also arranged by family

Includes many weeds of wetlands and waterways

Plant Identification Tools

To find out how it works, go to the site and look at the picture of bull thistle. Make sure you look at both the mature plant and seedling to get idea of this resource

http://weeds.ippc.orst.edu/pnw/weeds?weeds/id/index.html

This is the picture part of the Pacific Northwest Weed Handbook

Unfortunately, it does not include a key, but it can be used to help verify names and identities you find elsewhere

This source includes photographs of seedlings which can be very useful

Oregon Small Farms Website The OSU Small Farms website offers another

resource to learn about weeds It includes information on pasture weeds,

management practices, weed toxicity, and guides to controlling some of the weeds (see field bindweed, for example)

Remember that toxicity questions need the involvement of an OSU faculty member

Pasture and commercial questions also need to involve an OSU faculty member

http://smallfarms.oregonstate.edu/weedsplants%2526pests

Some examples of common weeds and recommended treatments

The online version of the PNW Handbook is at http://weeds.ippc.orst.edu/pnw/weeds

Once you have an identification, solutions for control may be found in the PNW Weed Control Handbook, which is also available online

This resource was developed primarily for commerical users and has limited information in available on weed control for home gardeners

You may need to use approved web resources to find solutions

When do “Just Plain Weeds” Become Invasive?

Some weeds are so aggressive that they have the ability to take over natural ecosystems, competing successfully with native plants

These have crossed the line into “invasive plants” and are costing billions of dollars annually to control

Examples already considered are herb Robert (shown left), purple loosestrife, English ivy (shown left), and garlic mustard

More are shown in the next few slides, many were introduced intentionally to the US as herbs or ornamental plants

This species is spreading rapidly throughout the state. Oregon Department of Agriculture notes that no biological control methods are available http://www.oregon.gov/ODA/PLANT/WEEDS/profile_shinygeranium.shtml The Western Invasives Network recommends hand weeding, weeding with a flame torch, or selected broadleaf herbicide treatments. http://www.westerninvasivesnetwork.org/phpBB3/viewtopic.php?f=11&p=178

Shining geranium, Geranium lucidum, is a newly listed noxious weed in Oregon. Usually grows as a winter annual.

A vigorous grower introduced as a berry crop which subsequently escaped to the wild. Control is usually mechanical (mowing, cutting) or chemical (typical applications in the fall).

Himalayan or Armenian blackberry, Rubus armeniacus

This introduced species is also considered to be invasive. Control methods are generally the same as for Himalayan blackberry

The evergreen blackberry, Rubus laciniatus

Despite the symbolism—this species was inspiration for the fleur-de-lis symbol, yellow flag iris is invasive in many parts of the world. It grows in streams and is spread through waterways. Control is difficult and expensive.

Yellow flag iris, Iris pseudacorus

Weed Control – checking the arsenal

Fast action can save a lot of effort. If you can keep a weed from flowering or spreading, you have eliminated untold hours of future control!

Mechanical and Cultural–preferred methods Deadheading or hand weeding Weed whackers, mowers, flame

torches Mulching or covering – most weeds

need sunlight to germinate Biological control if available Tolerance of minor weeds Chemical control if it is effective –

the last choice – always read the label

Become Familiar with the Home Garden Section of the PNW Weed Handbookh

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guidelines Chemical tools are limited to

only a few – glyphosate, triclopyr, 2,4 D, and dichlobenil are the most common

Check the table on page 2 to see which practices and chemicals are effective on certain weeds and which are not

Pre-emergent Herbicides

Check out this site made available through Washington State University Extension on the various kinds of herbicides available to the homeowner

If using materials from other states, the most beneficial and relevant information can usually be found in neighboring states like Washington, Idaho, and California

http://cru.cahe.wsu.edu/CEPublications/eb1214/eb1214.html

Some answers just seem too good to be true – Corn Gluten, unfortunately, fits into this category

Corn gluten meal appears to work in other parts of the United States, but not in most of Oregon—instead it may make your weeds grow even more quickly. In our climates, it kind of acts like a fertilizer.

Corn Gluten Meal to control weeds?

Vinegar as Weed Control

Acetic Acid was first introduced as an herbicide in Oregon around 2002

People began using it because “concentrated vinegar” seemed like a good idea – Vinegar is 5% acetic acid

The 7% solutions generally found in these products are not concentrated enough to be effective

More concentrated products are possibly hazardous and may not be legal for use as herbicides in Oregon

See this factsheet for more information: http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/acetic_acid_factsheet.pdf

Thank You for Viewing this Presentation

You may use this presentation and the photographs freely for educational purposes without express permission.

If you would like to use photographs for commercial purposes, please contact the author at linda.mcmahan@oregonstate.edu