Can Plants Be Invasive Pests?

The broom shrub is attractive with bright yellow flowers, but the dense impenetrable growth of the plants crowd out native plants and impedes movement by people or animals. (Iva / Adobe Stock)
Tulare-Kings Counties Master Gardener

In a garden, insects would come to mind as pests, but plants? Yet plants can become invasive pests. According to California Department of Fish and Wildlife, approximately 1100 species, subspecies and varieties of plants not native to California have naturalized and continue to reproduce in the wild. There are over 180 plants listed as noxious weed species. This designation results from their ability to crowd out agricultural crops, degrade rangeland, increase the potential for wildfire and flooding, consume valuable water, degrade recreational opportunities, and threaten California native plant species.

In a garden setting, a plant may seem “weedy” and crowd out other plants. Unfortunately, many weedy plants can spread by the wind or animals to areas outside your garden. This can become a serious problem if waterways, pasture, or natural open spaces are nearby.

An example of an invasive plant pest

One of the plants UCANR lists as a pest in the urban landscape is Brooms, a 3-10 foot tall shrub in the legume family. They flower from mid-spring to summer and can produce 2,000 to 12,000 seeds in pods on a single plant. The shrub is attractive with bright yellow flowers, but the dense impenetrable growth of the plants crowd out native plants and impedes movement by people or animals. The broom plants are highly flammable, increasing wildfire risk. Broom species include Scotch broom, French broom, Spanish broom, and Portuguese broom. The plants can live up to 20 years and the seeds can remain viable in the soil for over 30 years. The plant was brought to California in the 1850s from their native western and central Europe to reduce erosion and stabilize hillsides and sand dunes. They tolerate poor soil, have no dormancy period and are not readily grazed by animals. Their name comes from the use of their long whip-like stems which were cut and bundled to create brooms for household use.

It might appear broom plants in an urban backyard wouldn’t be a problem. However, the huge number of seeds produced by even one plant can easily spread beyond your yard. The numerous seed pods burst and scatter seed up to 20 feet from the plant. The seeds have a hard coating and this protects them as they float in water or settle in the gravel in a river. The plants are most rapidly spread by waterways, which would include rivers, streams, and irrigation ditches. They can also collect in the mud on machinery or vehicles and are spread elsewhere. For an additional negative, all parts of the Spanish broom plant are considered toxic.

Nurseries usually do not currently sell the common invasive brooms, but may sell hybrids which potentially could become invasive. If you have broom plants on your property, the options for removal are listed in the UC IPM Pest Notes: Publication 74147. The pest note has detailed application instructions for use of an herbicide to prevent drift with damage to desirable plants or into rivers or streams. Mowing is not recommended as the root remains and the plant will grow back, in addition to the potential for increased seed dispersal and fire risk while mowing. After removal of established shrubs, maintain a healthy cover of a desirable plant and avoid disturbing the soil, as seeds will remain in the soil for years. Monitor for new seedlings erupting and remove them while they are small.

Invasive plants often start out sounding like the perfect plant—they need little attention, tolerate poor soil, regrow if damaged and are self-spreading. Unfortunately, they can use these qualities to become a major pest in your yard and in natural areas. It is always wise to research any plant before adding it to your garden. Many times it will save you money, as the plant may never thrive in our climate. Learning about the plant’s habits can save hours of work if the plant has the potential to become an invasive pest.

Gardeners will be live to answer your questions at the World Ag Expo in Tulare on Feb. 14 and 15. You can also contact them at 559-684-3325, or visit their web site at ucanr.edu/sites/UC_Master_Gardeners.

This column is not a news article but the opinion of the writer and does not reflect the views of The Sun-Gazette newspaper.

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