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Article and photos by:
Jon P. Rebman,
Curator of Botany,
San Diego Natural History Museum
jrebman@sdnhm.org

 

Please remember, do not take plants out of their native habitats. Purchase only plants that are nursery grown and propagated, never accept field collected specimens.

Beautiful But Bad: Pretty Plant Pests

Introduction

Raphanus sativus (Wild Radish).  Photo by Jon Rebman
Raphanus sativus
(Wild Radish)

What are those beautiful fields of yellow and orange flowers that are along most of our local freeways and roadsides? Well, it is probably a plant that is one of a whole scourge of beautiful weeds that are invading and thriving here in San Diego County.

Beauty is not an indicator of whether a plant is native or not. In fact, in San Diego County we have some very attractive weeds that are major nuisances for us and for our native flora. We are fortunate to live in one of the most diverse counties in USA with almost 2200 different kinds of plants occurring here. Unfortunately, a large part of our "newer" diversity is due to the introduction and naturalization of almost 500 non-native plants. That is a staggering number of weedy species when you compare it to other locally diverse groups like birds in the county because the number of weedy plants alone about equals all of the different birds (488) that have ever been reported for San Diego County.

Gazania linearis (Gazania), photo by Jon Rebman.
Gazania linearis
(Gazania)

Two subjective terms that I am using in this article are "beauty" and "weedy." We all have different tastes in beauty and what one person may find attractive another may not. You will have to trust me in this paper on my choice of a few "beautiful" plant species, but all of these species do have either very showy flowers, leaves, or inflorescences that most people would typically find attractive. In fact, many invasive exotics were first introduced to the area as ornamental plantings.


Chrysanthemum coronarium, Crown Daisy is another member of the sunflower family Asteraceae, photo by Jon Rebman.
Chrysanthemum coronarium,
Crown Daisy is another member of the sunflower family (Asteraceae) and well represented locally.

What Is A Weed?

This is a more difficult term to define than most people realize. Webster's Ninth New Collegiate Dictionary (1988) defines a weed as "a plant that is not valued where it is growing and is usually of excessively vigorous growth, especially one that tends to overgrow or choke out more desirable plants." In relation to San Diego County, this definition encompasses two of the most detrimental aspects of weedy species, which are their exotic origin (introduced to our region) and their competitive abilities in local habitats. However, other applications of the word "weed" can be associated with their rapid establishment in disturbed habitats such as along roadsides and recently cleared areas. In this context, a weed is a pioneering species that can obviously disperse well to new locations, establish itself in one of the first stages of plant community succession, and populate the area quickly by either seeds or vegetative means. Although this characteristic is very common in many exotic and invasive plants, we do have a few native species that can also act in this manner like Heterotheca grandiflora (Telegraph Weed), Calandrinia ciliata (Red Maids), or Eucrypta chrysanthemifolia. In respect to weed diversity in our county, all of the weedy plant species that I am including in this article are not only non-native, but have also become naturalized in our area. Thus, these weeds exist and reproduce on their own in native habitats, without the aid direct human interaction.

When it all comes down to the basics, a weed is really any plant that grows in a place where we do not want it to grow; a very subjective term.

Cortaderia selloana (Pampas Grass), photo by Jon Rebman.
Cortaderia selloana
(Pampas Grass)


Parkinsonia aculeata (Mexican Palo Verde), photo by Jon Rebman.
Parkinsonia aculeata
(Mexican Palo Verde)

San Diego Weed Diversity

How many weeds do we have in the county? Well, that number is constantly changing. Unfortunately, the number is usually increasing because we are always finding new weed records in the county such as recent horticultural escapees or new introductions along roadsides, paths or ditches that are just beginning to take hold due to human activities. The good news is that rarely that number goes down because of the efforts of devoted individuals from organizations such as the California Native Plant Society or California Exotic Plant Pest Council who get out there and fight off some of the weed invasions by hand until they actually extirpate the weed from the county, or at least get it into a more controllable situation.

San Diego County’s current diversity is about 469 exotic, naturalized plant taxa out of a 2147 total. This represents about 22 percent of our known flora that are not supposed to be here. This diversity is especially evident in a few plant groups like the grasses and the mustards. In the Grass family (Poaceae), 88 out of 174 species (51%) are not native to our county and in the Mustard family (Brassicaceae), 32 of 80 species (40%) are of exotic origin. The four genera with the greatest number of exotic species in our county are Acacia (9), Chenopodium (8), Bromus (7), and Eucalyptus (7).

Lonicera japonica (Mexican Palo Verde)
Lonicera japonica
(Japanese Honeysuckle)

Weed Impacts

Some people may say "so what" about a few different weeds in our area, but the real problem is not their mere presence but how they adversely impact our local ecology and offset various natural balances and associations. For example, weeds are in direct competition with native plant species for space, water, nutrients, and biological resources such as pollinators and seed dispersers. This constant competition can replace, displace, or even drive natives to extinction due to niche and resource theft. We have over 200 sensitive plant species in our San Diego County and they are already in a rather fragile state so it does not take much to cause them a lot of harm.

The increase in weeds can also alter natural fire occurrences and frequencies in our region. Although fires are a normal event in many of our plant communities, like chaparral, an increased frequency of fire events can be detrimental to native plant establishment and reproduction. Abundant weedy species, especially introduced annual grasses and forbs, produce so much dry, flammable biomass that they can increase the rate of fire recurrence. This elevated fire frequency can detrimentally impact many native cacti and other perennials that may not have time to recover or get to reproductive maturity between fire episodes.

The introduction of weeds that are closely related to native species can cause negative effects as well. Hybridization between weeds and natives can possibly lead to “impure” genetic resources and reduce the wild gene pool of traits that have allowed the species to succeed. Plant groups such as native prickly-pears (Opuntia spp.) that hybridize easily can be impacted quickly by local horticultural plantings and escapees such as the non-native Opuntia ficus-indica (Mission Prickly-Pear). Even native desert species transplanted to the coastal side of our county can have impacts on our local flora. Encelia farinosa (Brittlebush), native to the Anza-Borrego Desert area, has been planted along many of our coastal roadways and interspecific hybrids have now been discovered between this species and the native, coastal species, Encelia californica (California Encelia). It is yet to be determined what kind of genetic effects this may have on the natural populations of E. californica.

Nicotiana glauca (Tree Tobacco)
Nicotiana glauca
(Tree Tobacco)

The presence of weeds can also impact our native fauna. Exotic plant species can decrease the quality of food and habitat for local animals. For example, the invasive tamarisks/salt-cedars (Tamarix spp.) proliferate and take over riparian habitats in our county but because this plant is weak-branched it does not provide good nesting places for many bird species dependent upon native riparian plants. Other introduced plant species i.e. Ricinus communis (Castor Bean) are poisonous, if ingested, and can kill the native fauna. In most cases, weedy plant species are unknown entities to our native animals. Consequently, weed populations are often not used by the local fauna, which is adapted to our native plant communities.


An example of a Ricinus communis (Castor Bean). The plants are poisonous, if ingested, and can kill the native fauna.
An example of a Ricinus communis (Castor Bean). The plants are poisonous, if ingested, and can kill the native fauna.

Wicked Weeds To Watch Out For

Here is a list of some attractive, but aggressive weedy plant species that occur commonly in San Diego County. In the Sunflower family (Asteraceae), our most well represented family in the county, a few weedy members include: Centaurea spp. (Star-thistles), Chrysanthemum coronarium (Crown Daisy or Garland), Dimorphotheca sinuata (Blue-eye Cape-Marigold or African Daisy), and Gazania linearis (Gazania). From the Mustard family (Brassicaceae) commonly encountered invasive species that are attractive due to their high numbers in almost monotypic populations include: Brassica nigra (Black Mustard), Hirschfeldia incana (Short-Pod Mustard), Lepidium latifolium (Broad-leaf Peppergrass), and Lobularia maritima (Sweet Alyssum), Raphanus sativus (Wild Radish), and Sisymbrium spp. (i.e., London Rocket). Other families with a few good-looking weedy species are: Fig-Marigold family (Aizoaceae) with Carpobrotus edulis (Hottentot-fig or Iceplant), Malephora crocea (Croceum Iceplant, Coppery Mesemb), and Mesembryanthemum crystallinum (Crystalline Iceplant); Legume family (Fabaceae) with Acacia melanoxylon (Black Acacia), Lathyrus tingitanus (Tangier Pea), Senna didymobotrya (Senna), and Spartium junceum (Spanish Broom); Grass family (Poaceae) with Cortaderia selloana (Pampas Grass), Pennisetum setaceum (African Fountain Grass), and Rhynchelytrum repens (Natal Grass). Other beautiful weeds from various families include: Amaryllis belladonna (Naked Lady, Belladonna-Lily), Chasmanthe floribunda (Chasmanthe), Cistus creticus (Purple Rock-Rose), Commelina benghalensis (Bengal Dayflower), Eucalyptus spp. (Gums), Geranium anemonifolium, Ipomoea purpurea (Common Morning Glory), Lantana spp. (Lantana), Limonium perezii (Perez’s Marsh-Rosemary), Lonicera japonica (Japanese Honeysuckle), Mirabilis jalapa (Four O’Clock, Marvel of Peru), Nerium oleander (Oleander), Oxalis pes-caprae (Bermuda-Buttercup), Ricinus communis (Castor Bean), Tamarix spp. (Tamarisks, Salt-Cedars), and Tropaeolum majus (Nasturtium).