Alyssum Desertorum – Desert Madwort

Alyssum desertorum is a species of flowering in the mustard family known by the common name desert madwort. Known also as the lepidium fremontii, Fremont’s peppergrass and desert pepperweed, it features individual yellow-centered yellow to cream tiny flowers, often fading to white which are about 1/4 inch in diameter. They form numerous dense, spherical clusters one inch or more across, on a shrub-like plant which is unremarkable for much of the year when not in bloom but quite distinctive in April, May, June and July. They are prevalent in the Mojave Desert northwards across most of Nevada, up to 6,500 feet in elevation. Alyssum desertorum has spread into many other states, where it is considered an invasive weed. Plants branch profusely, generally staying quite close to the ground but reaching heights of 3 feet in favorable locations.
Alyssum desertorum is native to northern Africa, Eastern Europe, and western Asia. It was brought to North America for medicinal reasons, as the plant was purported to cure hiccups, mental illness, and rabies. Desert alyssum, sometimes also called dwarf alyssum, was first reported in the U.S. in 1941 and Canada in 1955. Together with a closely related species called yellow alyssum, it infests 37 U.S. states and seven Canadian provinces.
A hairy annual herb producing upright stems up to about eight inches tall, the desert alyssum produces small round, notched fruits a few millimeters long. The seeds are winged and have embryonic roots.
Desert alyssum is an annual plant that grows from a seed, then flowers, sets seed, and dies every year. Seeds sometimes germinate in spring, but alyssum often grows as a “winter annual,” with seeds germinating in autumn, plants overwintering as rosettes and resuming growth in spring, then producing seeds before the plants die in summer. Desert alyssum seeds must after-ripen on the soil surface for several weeks before they can germinate. Seed longevity in the soil seed bank is unknown, but most seeds of these weeds germinate every year. Seed pods contain two chambers, each capable of producing two seeds.
They grow from three to 10 inches tall, and each plant is usually multi-stemmed, branching from the base. Leaves (0.25- to 1-inch long) and stems are covered with tiny, flat, star-like shaped hairs that are visible with a magnifying glass. The hairs help the plants retain moisture and give the plants a pale to grayish appearance. The hairs of desert alyssum have eight to 20 rays. The desert alyssum often occupies harsh and rocky sites at lower elevations, sandy flats, hillsides, and among sagebrush.
Desert alyssum has flourished as serious droughts have weakened other plants. Mild winters favor seedling establishment of desert alyssum; and reduced tillage in cropland farming systems have most likely contributed to its establishment in non-desert areas.
Desert alyssum is a shallow-rooted plant that accelerates soil erosion. It extracts surface soil moisture which impedes seedling establishment of other plant species. Outside the desert, desert alyssum is a weak invader that requires sites be disturbed before it can establish, but it can persist and dominate disturbed areas, making it a highly undesreable plant in many places.

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