What Every Gardener Should Know About Weeds and Invasives

Kristine Hanson of The Dirt on Gardening talks with Robert Norris, professor emeritus of plant sciences at University of California Davis, about weeds and invasive plants.

some plants are invasive and cause problems

Common nursery plants that can cause problems: morning glory, water hyacinth and Vinca.

Related To:

  1. Plants
  2. Weeds
  3. Gardening

Weeds and Invasives

Every spring, there's a nagging word that creeps into the back of every gardener's mind, and that word is weeds -- a single and unpleasant syllable that brings with it thoughts of stooping among the tomato vines or behind the azalea bushes, until your back hurts, plucking hard-to-reach space invaders while enduring summer heat and buzzing insects.

Making matters worse, it's possible that you've brought "weeds" home from your local nursery without even knowing it. Some plants sold as groundcover or ornamentals can, in fact, become unwelcome residents of your garden as they spread beyond your control. Vinca, for instsance, is a common groundcover that commonly outgrows its welcome in the backyard setting.

Worst of all, certain kinds of weeds can go well beyond a simple nuisance to gardeners. Personifying, as some do, that key phrase "vigorous growth," some plant species, if allowed to grow unchecked, can cause serious and even devastating environmental problems.

A Weed By Any Other Name

Even the term "weed" is a bit problematic and ambiguous since one person's weed might, for example, be another person's ornamental grass or flowering vine. Merriam-Webster defines weed (n) as: "a plant that is not valued where it is growing and is usually of vigorous growth; especially one that tends to overgrow or choke out more desirable plants." So if we think of "weed" in a general sense as any undesirable or nuisance plant, it's a disturbing thought that some of those can be introduced -- either by accident or with good intention -- by the gardener.

An even more disturbing thought is the fact that some exotic plants species can cause problems far beyond the boundaries of your backyard, actually causing serious and widespread damage to natural habitats and ecosystems. Plant specialist Robert Norris explains how many plants, innocently purchased from nurseries or plant suppliers, can become invasive and profoundly problematic when introduced in certain settings or under the "wrong" set of circumstances.

"When a plant gets into an ecosystem," says Norris, "it's sometimes possible that it will literally take over that ecosystem, displacing everything that was there -- or nearly everything that was there -- before."

Water Hyacinth

common nursery plants that can cause problems

Water hyacinth is considered one of the most serious aquatic weed problems on a worldwide basis.

Here are just a few examples of plants that have been deliberately introduced, but have outgrown their welcome on a huge scale:

Water hyacinth (Eichhornia crassipes), widely sold as an aquatic plant for backyard water features, first became identified as a problem in Florida, but has now found its way into water ecosystems across the southern US. The plant is capable of extremely rapid growth and has overtaken streams, lakes and parts of rivers, virtually clogged commercial waterways, actually halted boat traffic in some areas and generally caused a host of environmental problems. According to Norris, water hyacinth is now considered one of the most serious aquatic weed problems on a worldwide basis. If you have water hyacinth growing in a backyard pond or water feature, never dispose of it by simply dumping it out where it might inadvertently be introduced into nearby bodies of water. Make sure that it is thoroughly dried out and completely dead before disposing of it. Or you can take Dr. Norris's advice: "I suggest cremating it!"

Kudzu (Pueraria montana), native to China and Japan, was introduced to the US in the early 1900s as a forage plant for livestock, a soil stabilizer and ornamental vine. It is a trailing perennial with lush foliage, purple flowers and is sometimes employed for erosion control. But as anyone living in the southeastern US knows, it has a growth rate that is astonishingly rapid (more than a foot a day in peak growing season!), and gives new meaning to the term "biomass." It's dense growth pattern can mean tens of thousands of plants occupying a single acre of land, and its root systems may extend to a depth of twelve feet, making it particularly difficult to eradicate once established. The species has become hugely problematic for a variety of reasons -- able to crop up on roadsides, hillsides, empty lots and nearly any plot of disturbed soil; quickly covering wide expanses of acreage with its dense growth; choking out virtually all varieties of plant life in its path; killing even mature trees by growing over them and blocking out sunlight, and even covering over and growing into buildings. Some small towns in the rural South have virtually been "swallowed up" by kudzu.

Morning Glory

morning glory has attractive flower but is pesty

Morning glory has the potential to grow into a backyard pest.

Ipomoea spp
Though certainly not so environmentally or economically damaging as the two previous examples, morning glory is another flowering vine that has attractive blooms but which may ultimately grow into a backyard pest. Morning glory seeds are available at most garden centers, and the plant is often introduced deliberately -- and is even sought after--by gardeners for its beautiful, trumpet-shaped blossoms. The vine's rapid growth and spread, however, can eventually turn it into something of a problem. Once this plant is established, it's quite difficult to eliminate since its hard seeds can stay dormant in the soil for long periods and may germinate even five or 10 years after the parent plant has died.

Other common species "masquerading" as desirable landscape plants, but that are considered invasives, include honeysuckle (Lonicera spp.), English ivy (Hedera helix), privet hedge (Ligustrum spp.), tree-of-heaven (Ailanthus altissima), mimosa (Albizia julibrissin) and Japanese wisteria (Wisteria floribunda).

These are just a few of the most common plant species that have been categorized as invasive. A great deal more information about damaging invasive plants species can be found online simply by searching on the word "invasives." And, according to Robert Norris, perhaps the best place to go for information on problem plants in your area is your local cooperative extension. They're likely to know about the most common pest plants in your region of the country and what you might be able to do to help remove or control them. They'll likely also have recommendations for some desirable native plants that you can grow as an alternative to exotic and potentially invasive species.

Weed Control Tip: As the case of the morning glory seeds amply illustrates, one year of seed production can result in a weed problem that may last for many years. The best way to eradicate weeds is, whenever possible, eliminate them early -- before they've gone to seed.

Green Earth Factoid: As a growing body of scientific evidence indicates, the earth is undergoing a global rise in temperatures that is likely tied to increasing proportions of greenhouse gasses like CO2 building up in the atmosphere. A number of carefully controlled scientific studies have shown that this climatic shift is having significant impact on plant and animal species in many locations all over the globe -- including, in some cases, how and where certain plants grow. And in fact, recent studies indicate that it is frequently the opportunistic and invasive types plants, such as some woody vines, that benefit most readily by this global climate change -- possibly at the expense of mature trees and forests. One study at Duke University showed that increases in carbon dioxide gasses is particularly beneficial to one particularly noxious plant -- poison ivy. The study demonstrated that increased levels of carbon dioxide not only boosted poison ivy growth by as much as 150 percent, but it also caused the plant to produce more of the compound urushiol, the toxin responsible for skin rashes in humans.

Weed Control Basics

basic tools for weed removal are varied

Basic tools for dealing with weeds: physical barriers like landscape cloth and mulch, hand-tools to assist in removal and chemical herbicides.

For ordinary backyard or garden weeds, there are three basic ways of dealing with the problem:

Mechanical removal is about what it sounds like: manually pulling up and removing weeds by hand -- aka "weeding." A variety of hand tools like hoes, weed-pullers and a popular one known as a "hula hoe" can help make this tedious job somewhat less so.

Cultural weed control involves controlling weeds "before they start" using such physical control mechanisms as weed barriers, landscaping cloth and mulch.

Chemical weed controls, the use of commercial herbicides, may provide the fastest and easiest solution in some instances, but they carry with them obvious (and some not-so-obvious) environmental and health-related risks. Using them can introduce risk of harm to pets, children and others. They can also enter the water table through a variety of means and cause environmental harm in areas nearby or even far "downstream." Organic gardeners avoid the use of chemical herbicides altogether. If you do opt to use them, be certain to read and follow manufacturer's directions and safety warnings carefully, and be sure to use only currently available chemicals. Some older herbicides have been discontinued because of serious and newly discovered health risks.

Southern Home-Turf Tip: Rust

According to master gardener "Farmer Fred" Hoffman, dandelions aren't the only invasive pest you'll encounter on southern lawns. Another of the most common is actually not a plant but a fungus -- rust. This damaging fungus grows on grasses during the spring, summer and fall, and can be seen in the form of yellowing or brown blades of grass. The orange-colored pustules can be spread throughout your lawn simply by ordinary foot traffic.

To help control rust on your lawn, water and fertilize regularly. Most importantly, mow your lawn weekly. Bag the clippings and put them in the trash rather than spreading them back across the live turf. In that way, you'll help your lawn stay green -- not orange -- throughout the growing season.

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