How to Manage Pests

Pests in Gardens and Landscapes

Invasive Plants

Published 6/17

In this Guideline:

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Mature plant of yellow starthistle, Centaurea solstitialis.

Mature plant of yellow starthistle, Centaurea solstitialis.

Saltcedar, Tamarix ramosissima.

Saltcedar, Tamarix ramosissima.

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

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

Big periwinkle, Vinca major.

Big periwinkle, Vinca major.

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. Weeds may cause health problems in livestock, pets, and humans and 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.

The important biological difference between invasive plants and garden or agricultural 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.
Table 1. Difference between designation of a plant as a general weed or an invasive.
  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, fertilization, 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.
Naturalized and Invasive Defined

"Invasive" and "naturalized" are terms used frequently in reference for both non-native plants in wildland areas and 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 and 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 plants often 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 and 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 more recently listed invasive plants based on their threat to or impact on wildlands.

INVASIVE PLANTS OF CALIFORNIA

California boasts the greatest natural botanical diversity of any state in the United States, with nearly 5,000 native plant species. In addition to native species, there are about 1,500 non-native species that have become established in the state. About 250 to 300 of these are weeds of agricultural crops, turf, or gardens. The remaining 1,200 or so are naturalized plants of wildlands or disturbed noncrop areas, some of which are important invasive plants.

The California Invasive Plant Council (Cal-IPC), a nonprofit organization, has produced a useful inventory of invasive plants. Using a process based upon 13 criteria or questions, they have listed about 215 species as threats to California's wildlands (see www.cal-ipc.org). 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. Within the inventory, there are currently 42 plants listed as highly invasive, 93 as moderately invasive, and 80 as limited invasiveness. Of these invasive plants, the majority are biennial or perennials (46%), winter annuals (25%), or woody plants (22%).

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 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 transformations can be caused by the excessive use of resources by invasive plants.

These include 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 transform environments in many ways:

  • Changing the soil fertility of the ecosystem.
  • Promoting a shorter (or in some cases longer) interval between fires 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 characteristic of 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 Wildlife. The U. S. Fish and Wildlife Service reports that more than 400 of the over 1,300 species currently protected under the Endangered Species Act are considered to be at risk at least partly due to displacement by, competition with, and predation by invasive species, including 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.

Almost 80% of these intentionally introduced plant species came through the nursery industry as ornamental landscape species. Twenty-nine 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.

Between 2011 and 2016, UC Master Gardener volunteers visited between 223 and 265 nurseries in 35 to 38 California counties 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 invasive species are available in the majority of garden centers and other retail outlets, some are readily available, and some are uncommon.

In addition, many invasive species that are widespread in California and were introduced through the nursery industry are no longer being sold or are rarely for sale. This includes bridal broom, capeweed, crystalline iceplant, giant reed, jubatagrass, myoporum, red sesbania, Russian-olive, saltcedar, Tasmanian blue gum, and bridal, French, Portuguese, and Spanish broom.

Most encouraging is that the sales of many invasive plants, such as big periwinkle, Chinese tallowtree, green fountaingrass, and Scotch broom have begun to decrease over the period of these surveys. Only the most recently surveyed invasive plants, including Mexican feathergrass and water hyacinth, have not shown reduction over the past three years of the survey.

Overall, there has been a significant reduction in the percentage of nurseries selling invasive plants. In 2012, 30% of nurseries sold at least one invasive plant; but in 2015 only13% of nurseries sold the same invasive plants surveyed in 2012. Thus, it appears that the efforts to reduce the sale of invasive plants in California are succeeding, although not yet completely successful.

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. Results of University of California Master Gardener survey of invasive plants for sale in California nurseries. Hyphens indicate that plant was not (or is no longer) surveyed.
Common name Species Percent of nurseries where plant was for sale
2011 2012 2013 2014 2015 2016
Species rarely or no longer sold in California nurseries
Bridal broom Retama monosperma 0 0 - - - -
Capeweed Arctotheca calendula 0 1 1 - - -
Crystalline iceplant Mesembryanthemum crystallinum 0 0 <1 - - -
French broom Genista monspessulana <1 0 0 0 <1 0
Giant reed Arundo donax 0 0 - - - -
Jubatagrass Cortaderia jubata 0 0 - - - -
Myoporum Myoporum laetum 1 0 0 - - -
Portuguese broom Cytisus striatus 0 0 - - - -
Red sesbania Sesbania punicea 0 0 - - - -
Russian-olive Elaeagnus angustifolia <1 0 0 - - -
Saltcedar Tamarix spp. 0 0 - - - -
Spanish broom Spartium junceum 1 1 <1 - - -
Tasmanian blue gum Eucalyptus globulus <1 0 - - - -
Species declining or removed from sale or new to survey
Big periwinkle Vinca major 16 19 19 9 5 3
Chinese tallowtree Triadica sebifera (=Sapium sebiferum) 2 3 1 1 <1 0
Green fountaingrass Pennisetum setaceum 8 8 10 6 4 4
Hottentot fig, highway iceplant Carpobrotus edulis 3 2 2 3 1 4
Mexican feathergrass Nassella tenuissima (=Stipa tenuissima) - - 27 39 25 23
Pampasgrass Cortaderia selloana 9 6 5 5 4 6
Scotch broom Cytisus scoparius 2 3 3 0 0 0
Water hyacinth Eichhornia crassipes - - 6 6 5 6
Table 3. Other invasive plant species of horticultural origin in the Cal-IPC Invasive Plant Inventory. Species listed include their ranking on the Cal-IPC inventory.
Species Common name Regions invaded Cal-IPC rating
Acacia dealbata Silver wattle NC, CC, SC Moderate
Acacia melanoxylon Black acacia NC, CC, SC Limited
Ageratina adenophora Croftonweed, thoroughwort CC, SC Moderate
Agrostis stolonifera Creeping bentgrass NC, CC, SC, GV, SN Limited
Ailanthus altissima Tree-of-heaven CAL Moderate
Asparagus asparagoides Bridal creeper CC, SC Moderate Alert*
Asphodelus fistulosus onionweed SC Moderate Alert*
Carpobrotus chilensis Sea-fig, iceplant CC, SC Moderate
Centaurea debeauxii Meadow knapweed NC Moderate Alert*
Chrysanthemum coronarium Crown daisy CC, SC Moderate
Conicosia pugioniformis Narrowleaf iceplant CC Limited
Cordyline australis Giant dracaena NC, CC Limited
Cotoneaster franchetii, C. lacteus, C. pannosus Cotoneasters CC Moderate
Crataegus monogyna English hawthorn NC, CC, SC Limited
Cynodon dactylon Bermudagrass SC, D Moderate
Delairea odorata Cape-ivy CC, SC High
Digitalis purpurea Foxglove NC, CC, SN Limited
Echium candicans Pride-of-Madeira NC, CC, SC Limited
Erica lusitanica Spanish heath NC Limited
Eucalyptus camaldulensis Red gum NC, CC, SC, GV Limited
Festuca arundinacea Tall fescue CAL Moderate
Ficus carica Edible fig CC, SC, CV Moderate
Foeniculum vulgare Fennel CAL High
Gazania linearis Gazania CC Moderate Alert*
Hedera canariensis, H. helix Algerian and English ivy CAL High
Helichrysum petiolare Licorice plant NC, CC Limited
Hypericum canariense Canary Island hypericum CC, SC Moderate Alert*
Ilex aquifolium English holly NC, CC Moderate Alert*
Leucanthemum vulgare Ox-eye daisy NC, CC, SC, SN Moderate
Limonium ramosissimum Sea lavender CC, SC Limited
Linaria genistifolia ssp. dalmatica Dalmatian toadflax CAL Moderate
Linaria vulgaris Yellow toadflax NC Moderate
Lobularia maritima Sweet alyssum NC, CC, SC Limited
Lythrum salicaria Purple loosestrife NC, GV, D High
Mentha pulegium Pennyroyal CAL Moderate
Myosotis latifolia Common forget-me-not CAL Limited
Myriophyllum aquaticum Parrotfeather NC, CC, SC, SN High Alert*
Olea europaea Olive NC, CC, SC, GV Limited
Oxalis pes-caprae Buttercup oxalis NC, CC, SC Moderate
Pennisetum clandestinum Kikuyugrass NC, CC, SC Limited
Phoenix canariensis Canary Island date palm CC, SC Limited
Poa pratensis Kentucky bluegrass CAL, D Limited
Polygonum cuspidatum, P. sachallinense Japanese and giant knotweed NC, CC Moderate
Prunus cerasifera Cherry plum NC, CC Limited
Pyracantha spp. Pyracantha NC, CC, SC Limited
Ricinus communis Castorbean CC, SC, CV Limited
Robinia pseudoacacia Black locust CAL, D Limited
Saccharum ravennae Ravennagrass GV Moderate Alert*
Salvia aethiopis Mediterranean sage SN Limited
Salvinia auriculata complex (including S. molesta) Giant salvinia CC, D High Alert*
Saponaria officinalis Bouncingbet NC, CC, SC, GV, D Limited
Schinus molle Peruvian peppertree CC, SC, GV, SN Limited
Schinus terebinthifolius Brazilian peppertree SC Limited
Tamarix aphylla Athel tamarix GV, SC, D Limited
Tanacetum vulgare Common tansy NC, SN Moderate
Tetragonia tetragonioides New Zealand spinach CC, SC Limited
Ulex europaeus Gorse NC, SC High
Washingtonia robusta Mexican fan palm SC Moderate Alert*
Watsonia meriana Bulbil watsonia NC Limited
Zantedeschia aethiopica Calla lily NC, CC, SC Limited
Key to regions invaded:
CAL – all of the state except the deserts
NC – north coast CV – Central Valley
CC – central coast SN = Sierra Nevada and Cascade mountains
SC – south coast D – Great Basin and Sonoran deserts

Information for regions invaded adapted from the Cal-IPC Inventory. These designations are simplified versions of the designations used in the Cal-IPC Inventory.

*An Alert is listed on species with High or Moderate impacts that have limited distribution in California, but may have the potential to spread much further.

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 reason that 48% of the invasive plant species had their origins in the horticultural trade.

Table 4. Characteristics that make horticultural 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

MANAGEMENT

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

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.

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 avoid weed introduction or to manage agricultural weeds. Yellow starthistle seed, for example, typically arrives in a new area as a contaminant in soil or crop seed (particularly alfalfa 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 parts, such as fruit, seed, or root pieces, to areas where that plant has not established.

Sale of invasive plants in nurseries. 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 of them are likely to become invasive.

Because the undesirable impacts of invasive plants on natural habitats are 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. Scientists at the University of California, Davis have recently developed a 20-question Plant Risk Evaluation tool that can accurately predict whether a newly introduced plant has a high potential for escape from cultivation.

The PlantRight program, administered through Sustainable Conservation, promotes horticultural alternatives to invasive plants on their website. The site targets specific regions of the state or growth forms of invasive plants (e. g. woody plants, aquatic plants) and recommends 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.

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 on their website 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.

Disposing of invasive plant material. 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 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, it is important to dispose of them carefully. Make sure any reproductive parts 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 (often referred to as EDRR). 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.

Management in Natural Areas

Invasive plants can invade natural areas, particularly when those areas are surrounded by 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 websites. Most state and federal agencies that manage land, such as the California Department of Fish and Wildlife, State Parks, the National Park Service, and the U. S. Bureau of Land Management, have extensive information on invasive plants and animals on their websites. 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 from 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. pp. 360.

Cal-IPC. 2006. California Invasive Plant Inventory. Cal-IPC Publ. 2006-02, Berkeley, CA. 39pp.

DiTomaso, J.M. and E.A. Healy. 2003. Aquatic and Riparian Weeds of the West. Oakland: UC ANR Publication 3421. 442 pp.

DiTomaso, J.M. and E.A. Healy. 2007. Weeds of California and other Western States. Oakland: UC ANR Publication 3488. 1808 pp.

DiTomaso, J.M., G.B. Kyser et al. (13 other authors). 2013. Weed Control in Natural Areas in the Western United States. Univ. Calif. Weed Research and Information Center. Davis, CA. 544 pp.

Baldwin, G.G., D.H. Goldman, D.J. Keil, R. Patterson, T.J. Rosatti, and D.H. Wilken (eds.). 2012. The Jepson Manual Vascular Plants of California, Second Edition. University of California Press. Berkeley, CA. 1600 pp.

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

Produced by University of California Statewide IPM Program

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