Mexican Fireweed

Kochia scoparia

Kochia seedling
Kochia seedling. Photo courtesy of Matt Lavin. Licensed under CC BY-SA 2.0.
Kochia is a large annual herb known to be highly invasive and slightly toxic. It is characterized by by its pyramidal or rounded bushes up to 7 feet tall as well as ability to grow in harsh environments.
Kochia emerge in early spring due to its ability to tolerate extreme cold, germinating in temperatures from 39 to 106 degrees F. Germination has been known to occur several times throughout the growing season, as it takes advantage of moisture when it's available. Still, seedlings of Kochia are able to germinate under high stress conditions, such as lack of moisture, high salinity, and extremes in pH. They are very tiny as seedlings and tend to congregate in high density, often "carpeting" an area. The seedling as a whole is very pubescent.
Kochia seedling
Kochia seedling. Photo courtesy of Phil Westra. Licensed under CC BY 3.0.
The leaves of the Kochia are arranged as simple and alternate with a linear to narrowly ovate shape. The petioles are short, and the edge of each leaf is fringed with hairs. Kochia is generally very bushy and known to exceed 7 feet in heighth. Its ability to survive in such harsh weather conditions is owed to its extensive tap root system, sometimes reaching depths of 15 feet and widths of 20 feet. In the late fall, the foliage turns red. At the end of its life, the base of the plant breaks off at ground level and the dead plant becomes a tumbleweed. This is its method of seed delivery and dispersion.
Kochia branch
Kochia branch and leaves. Photo courtesy of Bruce Ackley. Licensed under CC BY 3.0.
Kochia flower bracts are surrounded by tufts of hair and the entire flower head is a elongated. The flowers themselves are green and without petals. They are perfect with 3-5 stamens or pistillate, each with two stigmas. Kochia flowers produce single brown, flattened seeds that mature within a star-shaped fruit.
Kochia flowers
Kochia flowers. Photo courtesy of Phil Westra. Licensed under CC BY 3.0.
Kochia is very invasive in the areas where it is distributed, with such a high tolerance for extreme conditions. It competes strongly for light, nutrients, and soil moisture, often reducing crop yield in affected areas. Contamination and suppression of nearby plants is caused by a chemicals the Kochia releases into the soil. The plant itself can be toxic to grazing animals in large amounts of consumption.
Effects of Kochia can be greatly reduced or eliminated by tilling seedlings in early spring. Its effect can be minimized for subsequent years if mature plants are slashed or mowed before they're able to produce seed.
Due to Kochia's leaf characteristics (pubescence and wax), absorbtion of herbicides can be inhibited. Effectiveness is highly dependent on the maturity of the plant and the dosage of the herbicide, with the effectiveness lessening as the plant matures. Certain populations of Kochia have shown resistance to 2, 4-D, auxinic herbicides, sufonylurea, dicamba, and triazine. Please contact your local weed specialist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.