mature black locust fully in bloom with white flowers, in the midst of a pine forest
Black locust, though beautiful at first glance, can become quite aggressive, crowding out native plants

Nobody likes weeds in our gardens or on our land. We also don’t want invasive plants that crowd out native plants. Wait — what’s the difference?

Invasive Plants

An invasive plant is “a plant that is non-native to the ecosystem under consideration, and whose introduction is likely to cause economic or environmental harm, or harm to human health” (Cal-IPC). In terms of the Redbud-CNPS focus on native plants, we focus on invasive plant species that are undesirable because they crowd out locally native plants; this reduction in natives, in turn, harms wildlife because it eliminates plants wildlife need for food and shelter. An invasive plant may start as a cultivated plant, then “escape” more broadly across our property or into wildland. Certain species are more likely to do this, because they have no natural predators here, and, often, because of their prolific reproduction mechanisms. Nationally, invasive species are the second-greatest threat to endangered species, after habitat destruction (Source: Cal-IPC).

Our state hosts about 1,100 species, subspecies, and varieties of plants that didn’t naturally occur in the state but became naturalized and now reproduce in the wild. (Source: https://wildlife.ca.gov/Conservation/Plants/Invasives).

Invasives – Plants to Never Plant. Some ornamental non-native plants crowd out native plants, create severe fire danger, and/or destroy natural habitat and native ecosystems. This CNPS handout identifies invasive plants you should never purchase or plant — and should remove if they are in your garden or on your property, to stop further spread.

An ongoing alliance of nurseries, scientists, environmental organizations, and governmental agencies, Plant Right, evaluate plants’ invasive “risk,” educates California’s retail nurseries about invasive species, and promotes the sale of only non-invasive species. Their invasive plant list features plants still available for sale; it doesn’t include plants no longer sold but invasive on wildland.

Weeds

My mother’s sole gardening advice was, “A weed is but a flower out of place.” As folks know who’ve had to pull out overly enthusiastic California poppies or California goldenrod, in a garden setting, there’s a grain of truth there. Some weeds, such as Scotch broom, have lovely flowers and bright the landscape — though we know they drive out local natives. Overall, they simply don’t belong here. On the flip side, even certain native plant species, like Chilean aster, also a beauty, are so assertive that they can also become weeds!

grey-green hairy vetch, with a few purple flowers, thoroughly wound through a rush
Hairy vetch (Vicia villosa) is a weed with attractive lavender flowers, but it can overwhelm adjacent plants

Here are a few tools to help you tackle problematic plant species where you live:

  • Weeds — The Good, The Bad, The Ugly. This recorded Zoom presentation from Master Gardeners of Nevada County includes characteristics of weeds, identifying common weeds, and ways to reduce weeds on your property.
  • The California Invasive Plant Council (Cal-IPC)‘s website is full of tools and information to help land managers, landscapers, homeowners and all Californians make the best choices for California. For instance, their Plants A-Z list includes links to species-specific weed management information.
Reducing and Eradicating Invasive Plants and Weeds

Selecting an appropriate technique for handling invasive plants and weeds requires considering both the species and the extent of its coverage. Do consult the Cal-IPC website (and others they recommend) for species-specific guidance.

Timing is also important in weed reduction. Certain techniques work best soon when annual grasses first emerge; other techniques are most effective when vines are in flower but before they fruit.

  • The Weed Workers’ Handbook. How to control 35 of the most invasive weeds in the SF Bay Area, many of which are also problematic in our counties, such as brooms, Himalayan blackberries, and star thistle.
  • Non-Chemical Weed Control Training. Presentations from trainings on burning, large-machine mowing, whole plant removal, grazing, competitive planting, and biological control.
  • Mulching. For low-growing invasive or weedy annuals and grasses, applying a four-inch layer of mulch when plants are low (or cut low) can be very effective in a home landscape setting. This technique may require waiting 6-12 months before planting. More on mulch in the future…