How to Manage Pests

Pests in Gardens and Landscapes

Invasive Plants

Published 11/07

In this Guideline:

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Mature plant of yellow starthistle.

Mature plant of yellow starthistle.

Pampasgrass, Cortaderia selloana, male plant.

Pampasgrass, Cortaderia selloana, male plant.

Hottentot fig, Carpobrotus edulis, infestation in a sand dune.

Hottentot fig, Carpobrotus edulis, infestation in a sand dune.

A flowering fennel plant, Foeniculum vulgare.

A flowering fennel plant, Foeniculum vulgare.

INTRODUCTION

Invasive plants are a distinct group of weeds that occur in natural habitats. The purpose of this Pest Note is to clarify how invasive plants differ from common garden and agricultural weeds, to describe the occurrence and impact of invasives in California, to discuss how invasives can be spread through sales or movement of ornamental plants, and to identify approaches for managing invasive plants.

DEFINITIONS AND CLASSIFICATION

In most cases we think of weeds as native and non-native plants that impact crop production, either in commercial settings or in home fruit and vegetable gardens; cause health problems in livestock, pets, and humans; or are aesthetically unpleasing in turf and urban landscapes. In contrast, invasive plants are generally non-natives that infest natural ecosystems, including wildlands, rangelands, and pastures. Table 1 shows the differences between agricultural or garden weeds and invasive plants.

Table 1. Comparison of common weedy plants and invasive plants.
  Agricultural or garden weeds Invasive plants
Introduction Usually accidental by people, animals, equipment, or seed contamination. Can be accidental, but more often the original intention was for ornamental or aquarium use, or for forage, food, fiber, medicinal, or soil stabilization purposes.
Disturbance Require human disturbance to establish and persist. Benefit from human disturbance, but disturbance not required.
Persistence Will not persist without human disturbance, usually soil tillage or irrigation. Once introduced, plants survive and spread on their own without further human assistance.
Life form Primarily terrestrial annuals or herbaceous perennial species. All growth forms, including aquatic, climbing vines, parasitic, herbaceous, or woody plants. Can have annual, biennial, or perennial life cycles.

The important biological difference between invasive plants and garden weeds is the ability of invasive plants to disperse, establish, and spread without human assistance or disturbance. Because of this, they are much more problematic in natural environments than are typical weeds.

Naturalized Defined

“Invasive” and “naturalized” are terms used frequently in reference to both non-native plants in wildland areas and to garden plants. The term”naturalized” is used to describe a non-native plant that is capable of surviving and reproducing without human intervention for an indefinite period. Naturalized plants that do not spread away from where they were introduced are not generally a significant problem either in a garden or in a natural habitat. However, naturalized species that do spread and survive in new areas are called invasive plants.

Invasive Defined

Invasive plants cause ecological disruption to natural ecosystems, but the severity of the impact varies considerably based upon the plant species and the area being invaded. The worst invasive species, such as saltcedar (Tamarix ramosissima) or yellow starthistle (Centaurea solstitialis), have caused substantial changes to the character, condition, form, and nature of the invaded habitat. In scientific literature, these species are sometimes referred to as landscape transformers.

Noxious Defined

“Noxious” is a legal term, used by regulatory agencies, such as the California Department of Food and Agriculture (CDFA) and the U. S. Department of Agriculture Animal Plant Health Inspection Service (USDA-APHIS). To be considered noxious, a plant has to be listed on a noxious weed list maintained by one or both of these agencies. Listing is typically based upon the threat of this weed to agriculture or noncrop areas and allows these agencies, along with the county agricultural commissioner, to ban, quarantine, or eradicate these plants. In California, CDFA has started to list invasive plants based on their threat or impact to wildlands.

INVASIVE PLANTS OF CALIFORNIA

California boasts the greatest amount of natural botanical diversity of any state in the U.S., with nearly 5000 native plant species. In addition to native species, there are about 1300 non-native species that have become established in the state. About 200 to 300 of these are weeds of agricultural crops, turf, or gardens. The remaining 1000 or so are naturalized plants of wildlands or disturbed non-crop areas, some of which are important invasive plants.

The California Invasive Plant Council (Cal-IPC), a nonprofit organization, has created a useful inventory of invasive plants. Using a process based upon 13 criteria, they have listed about 200 species as threats to California’s wildlands. These 13 criteria fall into three groups:

  • ecological impact
  • invasive potential
  • current distribution

This list does not have legal standing, but is based upon the best available published literature and knowledge of invasive plant experts from California. It is a good guide to the invasive plants that can cause the greatest amount of damage to the environment and provides a wealth of background information on each plant on the list. The list notes the types of ecosystems invaded, the regions of the state invaded, and a general ranking (High, Moderate, or Limited) of the plants according to their statewide ecological impact. This ranking is a useful guide to the overall severity of a species, but it does not mean that a plant listed as Limited is not a significant problem in only one area of the state or that a High ranking means the plant is present everywhere in California.

Impacts

Invasive plants can cause significant economic and ecological damage in natural areas. From an economic standpoint, invasive species can reduce livestock forage quality and quantity, jeopardize animal and human health, increase the threat of fire or flooding, interfere with recreational activities, or lower land value. In addition, aquatic weeds can also impact the movement and navigation of private and commercial vessels, block irrigation systems, and impede livestock access to water.

Ecological Change

Invasive plants can also cause dramatic ecological changes that impact both plant and animal communities. This is often due to landscape transformations that reduce the adaptability and competitiveness of more desired native species. Such transformation can be caused by the excessive use of resources by invasive plants. This includes an increased ability to capture light, consume water or nutrients, or deplete gases (oxygen and carbon dioxide) in aquatic systems. For example, a 10,000 acre infestation of giant reed (Arundo donax) on the Santa Ana River in Orange County is estimated to use 57,000 acre feet more water per year than native vegetation.

Invasive plants can also transform environments in many ways:

  • Changing the soil fertility of the ecosystem.
  • Promoting a shorter interval (or in some cases longer) fire frequency that is not conducive to the survival of native species.
  • Stabilizing sand dunes in areas adapted to constant fluxes.
  • Promoting soil erosion by increasing water runoff down slopes.
  • Colonizing intertidal mudflats used for shorebird feeding grounds.
  • Accumulating leaf litter that acts as a suppressive mulch, which prevents the establishment of more desirable species.
  • Creating a saline environment as roots absorb salts from deep in the soil and redistribute them from the foliage to the soil surface.

These mechanisms create a more suitable environment for invasive species, at the expense of native plants, leading to a reduction in desirable plant diversity. Such impacts change the biological structure and relationships with other organisms in an area. For example, insects are often the source of nutrition for birds and reptiles. Many native insects are able to feed only on specific native plant species. If their preferred plants are crowded out by invasive plants, insect numbers can be dramatically reduced by the lack of sufficient food. This can subsequently cause a decrease in the animals that feed on them.

It has been reported that sections of rivers heavily infested with giant reed plants are nearly devoid of songbirds due to a lack of food. Often native birds do not nest in the branching pattern that is characterized by many invasive plants.

In addition, dense stands of invading saltcedar near desert rivers and oases cause another situation with unforeseen consequences, creating ideal cover for large carnivores such as cougars and coyotes. This makes it very risky for deer, bighorn sheep, and other prey animals to approach these water sources.

California has limited and diminishing untouched natural habitats, especially in regions of the state highly populated by humans. Infestations of invasive plants severely degrade the value of these sensitive sites. Many of these remaining natural areas are home to plants and animals listed as “Threatened and Endangered” by the U. S. Fish and Wildlife Service or the California Department of Fish and Game. Based on data from the California Natural Diversity Database, the California Department of Fish and Game estimates that 181 rare plant species are currently threatened by invasive plants.

Invasive Plants Originating from the Horticultural Industry

Of the species listed on the California Invasive Plant Council Inventory, about 37% were accidentally introduced to the state as contaminants of seed, clothing, equipment, vehicles, soil, ballast, animals, or packing materials. The remaining 63%, however, were intentionally introduced as landscape, pond or indoor ornamentals, aquarium plants, soil stabilization species, animal forage species, or human food, fiber, or medicinal plants. The majority of these intentionally introduced plant species came through the nursery industry as ornamental landscape species. Fourteen of these plants are listed on the California Department of Food and Agriculture noxious weed list, allowing them to be regulated by this agency and the local agricultural commissioner. The rest of the invasive plants that came from nursery introductions can still be imported and traded in California without restriction.

UC Master Gardener volunteers visited 125 nurseries in 19 California counties in 2003 to conduct a survey of the retail availability of a selection of landscape ornamentals that are also described as invasive species (Table 2). The results indicate that a few species are available in the majority of garden centers and other retail outlets, many are readily available, and some are uncommon. Additional species that were introduced to California through the nursery industry and are on the California Invasive Plant Council Inventory are listed in Table 3. Some of these species are also common in the nursery trade, while others may be hard to find. However, even though some of these species may be difficult to find locally, virtually any plant can be purchased interstate via catalogs and the Internet.

Table 2. Example invasive plant species available at 125 retail nurseries in 2003 in California, frequencies, and the regions invaded in the state.*
Species Common name Number of nurseries % of nurseries Number of counties Regions invaded
Arctotheca calendula capeweed 9 7 4 NC,CC
Carpobrotus edulis Hottentot fig, iceplant 15 12 9 NC,CC,SC
Cortaderia selloana pampasgrass 35 28 9 CC, SC
Cotoneaster spp. cotoneasters 58 46 13 NC,CC
Crateagus monogyna English hawthorn 2 2 2 NC,CC,SC
Cytisus scoparius Scotch broom 38 30 9 CAL
Cytisus striatus Portuguese broom 4 3 2 NC,CC,SC
Delairea odorata Cape-ivy 1 1 1 CC,SC
Elaeagnus angustifolia Russian-olive 13 10 4 CV,CC,D
Eucalyptus globulus Tasmanian blue gum 12 10 6 NC,CC,SC,CV
Ficus carica edible fig 48 38 11 CC,SC, CV
Foeniculum vulgare fennel 34 27 10 CAL
Genista monspessulana French broom 15 12 6 NC,CC,SC
Hedera helix English ivy 88 70 16 CAL
Helichrysum petiolare licoriceplant 47 38 11 NC,CC
Leucanthemum vulgare ox-eye daisy 10 8 7 NC,CC,SC,SN
Mesembryanthemum crystallinum crystalline iceplant 13 10 6 NC,CC,SC
Myoporum laetum myoporum 44 35 9 CC,SC
Pennisetum setaceum crimson fountaingrass 81 65 16 NC,CC,SC,SN
Ricinus communis castorbean 3 2 2 CC,SC,CV
Robinia pseudoacacia black locust 12 10 7 CAL,D
Schinus terebinthifolius Brazilian peppertree 11 9 5 SC
Spartium junceum Spanish broom 5 4 2 NC,CC,SC
Vinca major Big periwinkle 92 74 16 SC,CV,SN

*Data from UC Master Gardener survey conducted in 2003.

 

Table 3. Other invasive plant species of horticultural origin listed in the Cal-IPC Invasive Plant Inventory.
Species Common name Regions invaded (Key)
Acacia melanoxylon Black acacia NC,CC,SC
Ageratina adenophora Croftonweed CC,SC
Agrostis stolonifera Creeping bentgrass NC,CC,SC,CV,SN
Ailanthus altissima Tree-of-heaven CAL
Asparagus asparagoides Bridal creeper CC,SC
Chrysanthemum coronarium Crown daisy CC,SC
Conicosia pugioniformis Narrowleaf iceplant CC
Cordyline australis Giant dracaena NC,CC
Cynodon dactylon Bermudagrass SC,D
Digitalis purpurea Foxglove NC,CC,SN
Echium candicans Pride-of-Madeira NC,CC,SC
Eichhornia crassipes Water hyacinth CC,SC,CV
Eucalyptus camaldulensis Red gum NC,CC,SC,CV
Festuca arundinacea Tall fescue CAL
Hypericum canariense Canary Island hypericum CC,SC
Ilex aquifolium English holly NC,CC
Iris pseudacorus Yellowflag iris CC,SC,CV,SN
Linaria genistifolia ssp. dalmatica Dalmatian toadflax CAL
Lobularia maritima Sweet alyssum NC,CC,SC
Lythrum salicaria Purple loosestrife NC,CV,D
Myosotis latifolia Common forget-me-not CAL
Myriophyllum aquaticum Parrotfeather NC,CC,SC,SN
Olea europaea Olive NC,CC,SC,CV
Oxalis pes-caprae Buttercup oxalis NC,CC,SC
Pennisetum clandestinum Kikuyugrass NC,CC,SC
Phoenix canariensis Canary Island date palm CC,SC
Poa pratensis Kentucky bluegrass CAL,D
Prunus cerasifera Cherry plum NC,CC
Pyracantha spp. ZPyracantha NC,CC,SC
Retama monosperma Bridal broom SC
Salvinia auriculata complex (including S. molesta) Giant salvinia CC,D
Sapium sebiferum Chinese tallowtree CV
Saponaria officinalis Bouncingbet NC,CC,SC,CV,D
Schinus molle Peruvian peppertree CC,SC,CV,SN
Sesbania punicea Red sesbania CV
Tamarix aphylla Athel tamarix CV,SC,D
Tanacetum vulgare Common tansy NC,SN
Washingtonia robusta Mexican fan palm SC
Watsonia meriana Bulbil watsonia NC
Zantedeschia aethiopica Calla lily NC,CC,SC
What makes nursery species more invasive than other introduced species?

Many of the characteristics required for a plant species to be successful as a landscape ornamental are also qualities that can lead to invasiveness in natural settings (Table 4). These shared characteristics are the reasons that 48% of the invasive plant species had their origins in the horticultural trade.

Table 4. Characteristics that make nursery and invasive species successful.
Characteristic Horticultural plants Invasive plants
Germination and planting Easy to propagate and establish Few germination requirements and easy to establish
Growth Grow rapidly Grow rapidly
Reproduction Produce abundant flowers Prolific seed producer with successful dispersal mechanism
Environmental fitness Ability to grow in many regions and remain hardy Ability to adapt to a variety of environmental conditions, including drought and salt stress, and able to spread to many regions in the state
Pest resistance Free of insect pests and diseases Free of natural enemies and diseases

WHAT CAN BE DONE ABOUT INVASIVE PLANTS?

There are three basic aspects of weed control that also apply to invasive plants: prevention, eradication, and management. Each of these is discussed below.

Prevention

Rather than waiting for an invasive plant to become a problem, it is always better to prevent potential invasives from entering an area and becoming established or naturalized. With accidentally introduced invasive plants, such as yellow starthistle, prevention includes many familiar principles used to prevent or manage agricultural weeds. Yellow starthistle seed, for example, typically arrives in a new area as a contaminant in soil or a crop, particularly forage or hay; on or in a grazing animal; or on a vehicle or piece of equipment. When leaving an area where there are invasive plants, it is important not to transport any reproductive structures to areas where that plant has not established.

Managing the spread of invasive plants introduced through the horticultural trade is more challenging. Gardeners, as well as others in the landscape and nursery profession, often prefer “exotic” plants because they add new and different species to landscapes and gardens. Therefore, new non-native plant species are constantly being sought and introduced to California, but only a small percentage is likely to become invasive. Because the undesirable impacts of invasive plants on natural habitats is becoming more widely recognized, especially by conservation organizations and government agencies, the nursery industry has begun to understand the need to manage new plant introductions to minimize this problem. Some nurseries and botanic gardens have instituted voluntary programs to eliminate known invasive plants and to recommend non-invasive alternatives.

A series of regional brochures called Keep It in the Garden are available that highlight invasive horticultural plants established in wildlands in each region of the state. The brochures also include a general discussion of the issue and the important role that the gardening public can play in preventing new invasive plant problems. Keep It in the Garden brochures are available from the UC Master Gardener volunteers in most counties in California. Cal-IPC also has promoted horticultural alternatives to invasive plants in a series of brochures entitled Don’t Plant a Pest. These brochures target specific regions of the state or growth forms of invasive plants (e. g., woody plants, aquatic plants) and recommend native and non-native plants that have the same form or function as the undesirable species, but are not invasive in wildland or natural areas. These brochures can be obtained via the Cal-IPC website, along with more information on the plants listed in Tables 2 and 3.

The key element is to know what horticultural plants are invasive in your area of the state and to avoid planting them in your garden. If a plant listed in these two tables is invasive in your region of the state, it should be avoided for landscape use, especially for locations near natural areas. It may be safe to use in other regions, but sometimes the plant is not listed as invasive in an area merely because it has not yet become a presence.

If the plant already exists in your garden, what is the best thing to do? Again, if your garden is near natural habitat areas, or near roads, flood channels, or waterways that might be corridors that these plants can use to get to open space, then removal should be considered. At the very least, the plant should be kept in a vegetative state so it does not reproduce. If you choose to remove these plants, please dispose of them carefully. Make sure any reproductive parts, such as fruit, seed, or root pieces, do not escape during the removal process. In locations that have mandatory green-waste programs, cut off any reproductive parts and bag them separately for disposal, then send or take the rest of the green waste to the compost system.

Eradication: Early Detection and Rapid Response

Eradication refers to the complete elimination of a pest. The principle behind eradication is to kill the plant before it reproduces or spreads. After prevention, eradicating a small population of an invasive plant is the most cost effective pest control tactic. The California Department of Food and Agriculture, assisted locally by the county agriculture commissioner, has a long history of eradicating noxious weeds throughout the state. Many of their eradication efforts have been very successful and have kept small weed incursions from becoming widespread problems. Eradication has two components: early detection and rapid response. The gardening public can assist by learning to recognize when a new plant is expanding beyond where it was planted and either remove it right away or report it to the local county agriculture commissioner.

Invasive Plant Management

The control of invasive plants uses many of the same tools and tactics used for control of other weeds, including mechanical, chemical, cultural, and biological controls. Some management options used to control invasive species in rangelands or wildlands are not generally available in urban or agricultural landscapes. Examples are prescribed burning, grazing, revegetation programs, and much more extensive use of biological control agents. In most cases, integrated approaches using combinations of these methods are more effective for long-term suppression of invasive species and for recovery of the land to a more functional and productive ecosystem.

Invasive plants are those that can invade natural areas, particularly when those areas are surrounded or adjacent to urban environments. These natural areas are typically owned by a public entity, including parks and open space districts or privately by a non-profit organization, such as The Nature Conservancy. It is the responsibility of these organizations to manage invasive plants on their property. Many of these organizations have volunteer programs to remove invasive plants and can always use more help. Often these agencies or non-profit organizations have information on their invasive plant programs at their visitor centers or on their Web sites. Most state and federal agencies that manage land, such as California Department of Fish and Game, State Parks, the National Park Service, the U. S. Bureau of Land Management, have extensive information on invasive plants and animals on their Web sites. A good place to start is at the USDA Agricultural Library National Invasive Species Information Center. It is also important to recognize that management of invasive plants needs to consider sensitive plants and animals. For example, in southern California, public agencies that manage or regulate rivers and streams restrict invasive plant control activities during spring to fall because threatened and endangered birds, such as the Least Bell’s Vireo or Southwestern Willow Flycatcher, nest during this period. There are often rare species of native plants that also need to be protected in many of these infested sites.

WARNING ON THE USE OF CHEMICALS


REFERENCES

Bell, C. E., C. A. Wilen, and A. E. Stanton. 2003. Invasive plants of horticultural origin. HortScience 38(1).

Bossard, C. C., J. M. Randall, and M. C. Hoshovsky. 2000. Invasive Plants of California’s Wildlands. UC Press, Berkeley, CA.

California Invasive Plant Council (Cal-IPC). 2006. California Invasive Plant Inventory. Cal-IPC Publ. 2006-02, Berkeley, CA.

DiTomaso, J. M. and E. A. Healy. 2003. Aquatic and Riparian Weeds of the West. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3421.

DiTomaso, J. M. and E. A. Healy. 2007. Weeds of California and other Western States. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3488.

Hickman, J. C. (Ed.). 1993. The Jepson Manual. Higher Plants of California. UC Press, Berkeley, CA.

Richardson, D. M., P. Pysek, M. Rejmanek, M. G. Barbour, F. D. Panetta, C. J. West. 2000. Naturalization and invasion of alien plants: concepts and definitions. Diversity and Distributions 6:93-107.

PUBLICATION INFORMATION

[UC Peer Reviewed]

Pest Notes: Invasive Plants
UC ANR Publication 74139         PDF to Print

Authors: C. E. Bell, UC Cooperative Extension, San Diego Co.; J. M. DiTomaso, Plant Sciences, UC Davis; C. A. Wilen, UC Statewide IPM Program, San Diego Co.
Produced by UC Statewide IPM Program, University of California, Davis

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