Non-Native and Invasive Plants in the Lower Arroyo Seco

PlantDescription
Red GumThe Red Gum Eucalyptus (Eucalyptus camaldulensis) is native to Australia where it has an extensive range and grows primarily in riparian habitats consuming large amounts of water. Mussolini used Eucalyptus trees to drain swamps and wetlands in Italy. In California, the tree is allelopathic, meaning that it inhibits the growth of native plans nearby. The tree contains several toxins, including terpenes, and is fire-prone due to its high oil content. The Redgum lerp psyllid is a plant-juice sucking pest insect that damages the tree and infests two dozen other Eucalyptus species.
Chinese ElmChinese Elm (Ulmus parvifolia) is a fast-growing tree that is often invasive because of its aggressive growth habit. The tree is also susceptible to a variety of pests and diseases. The trees can be stricken by Dutch Elm disease where beetles tunneling beneath the tree's bark spread a fungus that turns the leaves yellow and eventually they die. Wood rot, antracnose and other fungal diseases that often occur in the tree can spread to other nearby native plant species.
Tree of HeavenTree of Heaven (Ailanthus altissima) is a wide-spread invasive species in Southern California. The trees change soil chemistry through allelopathy, negatively affecting nearby trees and plants. Toxic levels are maintained through the growing season. Within a short period of time one tree can scatter seed over a large area and create a thicket of sprouts from its wide-spreading roots. It was introduced as a landscape ornamental but escapes gardens and spreads by seeds and creeping roots that produce many suckers. It is widely distributed in California, primarily in wastelands and disturbed, semi-natural habitats.
Tree TobaccoTree tobacco (Nicotiana glauca) is native to Argentina but is now widespread as an introduced species in California. The plant was introduced to California about one hundred years ago and has now spread in vacant lots, along roadsides, near streams and other riparian areas. It is a common roadside weed throughout the southwestern United States, and an invasive plant species in California native plant habitats. The shrub or small tree grows to be about 15 feet high. The leaves of tree tobacco contain a chemical called anabasine that makes tree tobacco leaves poisonous.
Pampas GrassInvasive plants such as pampas grass (Cortaderia selloana) displace native plants and create habitats that are lower in biodiversity. Pampas grass has leaf blades that are highly undesirable as food or shelter to birds and other wildlife and can actually cause physical harm to those animals, including humans, because the leaves are extremely sharp. The plants often spread from nearby gardens. There are a several native plants, such as deer grass, that are good substitutes and will promote biodiversity.
Shamel AshShamel Ash (Fraxinus uhdei) is a fast-growing, medium to large tree (up to 100 feet tall and one foot wide) that grows naturally in mixed mountain forests from west-central Mexico to Costa Rica. It has been introduced to California where landscapers sometimes use it as a street and shade tree. Shamel Ash is a prolific seed producer and has become highly invasive in Hawaii where it has spread from cultivation into disturbed forest areas and is now considered one of the alien species most disruptive to native ecosystems. Invasions are more intense on fertile soils and along streams. In some stands of Shamel Ash, understory vegetation is almost completely absent, but sparse stands can harbor a profuse understory of non-native introduced species.
Date PalmCanary Island date palm (Phoenix canariensis) is a large tree that has escaped cultivation in southern California to invade stream corridors as well as orchards and even landscaped areas. This palm is native to the Canary Islands off the coast of Africa. Growing to 80 feet tall, Canary Island date palms tend to grow in clusters that form a dense canopy that excludes light from reaching beneath them, leading to a loss of native plants. It has escaped from landscape plantings via dispersal in water or by birds. Seeds carried by winter rains can be washed into storm drains and then to creeks and rivers. The tree is susceptible to a disease killing these palms called fusarium wilt and has large hanging frowns that can be dangerous particularly during wind storms.
Golden WattleGolden Wattle (Acacia pycnantha) is a shrub with golden yellow flowers and narrow leaves that is now sometimes found in the coastal regions of California. It is native to southeastern Australia where it is the most weedy of a variety of Australian wattles. It favors woodland habitats. It spreads via abundant, long-lived seeds and is dispersed by ants, birds, wind, water and garden waste. It has naturalized and is considered a Category 1 invasive in South Africa (invader plants must be removed and destroyed immediately). There is a high risk of this plant becoming invasive in California according to Cal-IPC California Invasive Plants Council.

Why Remove Invasive and Non-Native Plants?

Some local residents have expressed concern about the removal of non-native plants in the Van de Kamp restoration program. They wonder whether it is really necessary. And why so many, they ask.

The Lower Arroyo Seco is a nature preserve as defined by the Arroyo Seco Public Lands Ordinance, which reflects the city's policy to protect and enhance the natural character of the Arroyo. "Planting shall be limited to native plants," the ordinance states, "that may be planted with material appropriate to the Arroyo Seco and the semi-arid South California climate."

The Van de Kamp restoration program is not a landscaping program for a standard urban park. It is a habitat restoration program that is intended to protect nature and upgrade the quality of plants there. Habitat restoration is a science that consists of grouping appropriate trees and shrubs in zones that will enhance the natural character and functions of special environmental regions. It has tremendous benefits for the ecosystem health of the area affected, for the birds, wildlife and people.

Grant programs that fund important projects like this generally specify appropriate native habitat in environmentally important areas like the Lower Arroyo because that is good science and important for the long-term ecosystem health and sustainability of the area.

The plant list for the Van de Kamp restoration program has been professionally prepared by habitat experts. The trees and shrubs to be removed are not part of the historic character of the Arroyo and do not contribute positively to the ecosystem health of the Lower Arroyo. They are like weeds in a garden. Invasive and inappropriate trees and shrubs like this interfere with the re-establishment of native habitat. They often create create dead zones where native plants cannot live and can introduce exotic pests, bugs and diseases that will harm the habitat, the natural insects and even wildlife.

The number of trees and plants involved is another important concern. The critics note that more than 100 trees will be removed, but most of these are weeds and non-native trees, defined as not mature as per the Pasadena City Trees and Tree Protection Ordinance. According to the plans, twenty-two mature trees (as defined by the ordinance) will be removed, which is quite appropriate in an area as large as the restoration zone.

The areas to be replanted in the Lower Arroyo near the Van de Kamp bridge are mostly barren areas that have been degraded and neglected over the years and need some care.

There is now significant momentum for the restoration of a living stream in the Lower Arroyo, a long-held dream of many Pasadenans. Los Angeles County is pushing the US Army Corps of Engineers to complete their long-delayed ecosystem restoration study of the Arroyo Seco, including the Lower Arroyo. The state-sponsored Upper Los Angeles Rivers and Tributaries Working Group recently enthusiastically endorsed the Lower Arroyo for stream restoration. Pasadena will lose credibility with resource agencies and funders if it does not implement carefully-designed habitat restoration programs.

Isn't that a wonderful goal! Let's restore nature in the Arroyo Seco.

Non-native Trees to be Removed

NumberTypeBotanical Name
11Canary Island PinePinus canarienses
26Red GumEucalyptus camaldulensis
7Mexican Fan PalmWashingtonia robusta
12Chinese ElmUlmus parvifolia

13 snags and grasses as well as 48 other non-native trees will also be removed.

Arroyo Seco Foundation, PO Box 1622, Pasadena, CA 91109-1622 www.arroyoseco.org