Invasive Plant Species Threatens Arroyo Seco

The city’s Recreation and Parks Commission will hear a presentation Tuesday on invasive plant species threatening the Arroyo Seco.

The information-only item is scheduled for the commission’s regular meeting at 6 p.m. at City Yards Public Works Building.

Department of Parks, Recreation and Community Services (PRCS) staff will present findings on major invasive plants in the Arroyo Seco. The presentation will cover environmental and public safety impacts.

Invasive plants are non-native species that establish and spread easily in new environments. They were introduced through landscape trade, agriculture, accidental contamination and global trade.

In a preliminary report, the PRCS Department said six major invasive species have been identified in the Arroyo Seco.

Tree of Heaven, native to China, was introduced as a shade tree. It grows extremely fast and forms dense stands along canyon slopes and stream banks.

Black Mustard, native to Eurasia, was brought by colonists. It germinates rapidly and produces prolific seeds along trails and flood channels.

Milk Thistle produces more than 10,000 seeds from a single plant. Seeds remain viable for up to 10 years in disturbed soils.

English Ivy, introduced as an ornamental plant, grows in dense mats. Its trailing vines reach more than 30 feet and are toxic to wildlife and humans.

Tree Tobacco and Castorbean both pose toxicity risks. Castorbean seeds are highly toxic to people and animals.

The invasive plants create multiple environmental and public safety concerns. These include biodiversity loss, disruption of ecological relationships and increased fire risk.

The species also alter soil chemistry and affect hydrology and erosion patterns. They impact human and animal health and limit recreation activities.

Pasadena manages invasive species through contracted weed abatement services. Greenfield Management provides brush clearance in the arroyo.

The California Conservation Corps conducted fuel reduction projects in 2014 and 2022. Another project is scheduled for 2026.

Restoration projects from 2020 to 2025 included habitat restoration and invasive plant removal. These occurred at Berkshire Creek, Oak Grove and La Loma Bridge.

The city coordinates volunteer efforts with One Arroyo. Activities include wildfire defensible landscaping and ecological awareness programs.

On Tuesday, the Recreation and Parks Commission meeting will be held at 233 West Mountain Street on the building’s second floor.

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