Devils weed is an invasive plant that is native to tropical America. Devil weed (A.K.A Siam weed, bitter bush) is a well-documented pest and is considered one of the world’s worst 100 invasive species. However, it is highly invasive and is on the Hawai’i State Noxious Weed List. Also, it is considered invasive in 46 states.
Devils weed in O’ahu
Devils weed small seed size enables it to move around as a contaminant. It’s unknown how it came to Hawaii. Likely it arrived hidden on muddy tires, tools, or shoes. No one in their right mind would have intentionally introduced this vile plant. First detected in 2011 at the Kahuku Training Area, Oahu, devils weed is now found in Kahana Valley, Pūpūkea, and most recently in’ Aiea (2015).
Kahuku Training Area is also the weekend home of the Kahuku Motocross Track, popular with motocross and dirt bike enthusiasts and hunters. Decontamination of bikes and hunting gear after use is necessary to prevent it from spreading across O’ahu and other islands.
Devils weed in Hawaii Island
Devil weed was found on the Big Island for the first time in 2021, growing at Hilo’s dragstrip and motocross area and just beginning to spread into nearby agricultural fields. This is no surprise, considering it was previously found growing on a motocross track on Oʻahu. The tiny, sticky seeds were likely transported by accident from Oʻahu, potentially on clothing, equipment, gear, or off-road recreational vehicles.
Sometimes described as the ‘perfect weed,’ devil weed (sometimes written as devilweed) reaches reproductive maturity in as little as 6 months. Each plant can generate as many as 800,000 seeds! The lightweight seeds are easily windborne, spreading up to 250 feet from the parent plant. Seeds remain dormant yet viable in the soil for more than a year.
By far, the greatest spread is by humans through the accidental movement of seed. Contaminated tools, shoes, clothes, fur, and hair contribute to seed movement. Seeds remain dormant yet viable in the soil for more than a year. Devil weed reproduces vegetatively from the crown buds, stem pieces, and root fragments. It thrives in full sun, both in disturbed and natural areas.
Do NOT pull or cut down the plant. We will properly remove and dispose of the plant to prevent spreading seeds and re-sprouting.
Description and Dispersal:
- Herb or shrub with long rambling branches
- Leaves are triangular shaped, opposite, with a toothed leaf edge. Leaves are also limp with velvety hair and have a distinct turpentine smell when crushed.
- The leaves have three thick veins shaped like a pitchfork, hence the common name “devil weed.”
- Flowers are held in small clusters—pale purple to off-white, usually with distinct long stamens.
- Fruits are tiny and have soft white hairs, which allow it to be spread on the wind or water. The hairs also act like Velcro, attaching themselves to anything passing by.
- Wind-dispersed sand-grained sized seeds are also easily spread by birds and other animals when they eat the fruit and by people when dirt or mud containing the seeds stick to shoes, clothing, equipment, or vehicles.