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Plant Answer Line Question

Keywords: Silybum marianum, Noxious weeds--Washington

Silybum marianum: Should I give away baby plants? I bought the original "rare" plant from a local Perennial Society (can't remember exact name)15 years ago. 4 feet tall, and prickly. I decided to get rid of it, but baby plants still pop up every year. Variegated, dramatic, but too prickly for a small garden!


I'm glad you asked about your Silybum marianum (milk thistle) starts. This plant is considered an invasive species in many places, including Washington State. King County lists it as a Class A noxious weed, meaning that eradication of the plant is required by law. Below is information excerpted from King County's site:

This Class A noxious weed has a very limited distribution in Washington State, and eradication is required. The largest infestations in the state are in pastures in the southeastern section of King County but infestations are occasionally found elsewhere. Early detection and rapid, effective response is of the highest priority for this noxious weed.

Although occasionally found in gardens, it is illegal to sell or buy milk thistle in Washington State and all existing plantings should be removed in order to prevent accidental spread.

Milk thistle is toxic to livestock when consumed in large quantities, and it forms dense stands in pastures and rangelands. California reports up to 4 tons per acre in heavily infested areas. The leaves are very distinctive, with white marbling on the shiny green leaves.
Control Methods:
Manual: For small sites with few plants, pull or dig up rosettes or the bolted plants before seed heads form. Use a shovel to cut the plant off about one inch below the ground so the plant will not re-sprout. Chopping the leaves from one side of a rosette can provide access to the central growing point. Wear protective clothing. To be fully effective, all mature seed heads need to be bagged and removed so no new seeds are left on the site. Immature seeds can continue to develop in cut plants, and the less stem that remains attached to the flower head, the faster the seed head will dry out.

I highly recommend you do what you can to eradicate this plant and prevent its spread. Do not put it into the compost but instead bag and dispose of it as trash.

Date 2016-12-22
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August 01 2017 12:36:01