Weeds of the San Luis Valley

Canada thistle (Cirsium arvense) is a non-native, deep-rooted perennial that spreads by seeds and aggressive, creeping, horizontal roots (rhizomes) that can grow 15 to 30+ foot in depth. Canada thistle can produce about 1,000 to 5,000 seeds per plant that can survive in the soil for 20 years or more. Typically, it reproduces vegetatively about 6 foot in a season through a creeping root system, and can quickly form dense monotypic stands. Every piece of root, from 2 cm in length, is capable of forming new plants.

Hoary Cress is a non-native deep rooted perennial that thrives in a wide range of soil types. They grow to two foot tall and are extremely invasive. Hoary Cress reproduces via both seed and root, with root being the primary reproduction source. One plant can produce from 1,200 to 4,800 seeds per plant with a soil seed bank life of three or four years. Within three weeks of germination the seed produces new rosettes which emerge within 3 ½ weeks and root shoots that have been documented to grow as fast as approximately 4 ft in a season. A single plant can produce 450 new shoots in a single growing season.

Russian Knapweed develops on many soil types, is very drought tolerant and is known to have allelopathic properties. Russian Knapweed is reported to have lethal effects on livestock due to its uptake of Zinc in the soil. The plant reproduces primarily through root system and fragments of one inch or less can reproduce into a new plant from depths of six inches. Russian Knapweed also reproduces via seed in small quantities, survival of seeds are from 3 to 5 years in soil bank, and new growth by seed is uncommon.

Black Henbane is an annual / Biennial with a long tap root and reproduces only by seed, but produces approximately 100,000 to 500,000 seeds per plant. Seeds are known to last approximately 5 years in the soil. Black henbane is an opiate and all parts of the plant are poisonous andpotentially lethal to humans and livestock. The plant and seeds have been used as a hallucinogenic and theflowers and give off an odor that causes a kind of “high”.

Perennial Pepperweed can grow to a height of six foot tall and reproduces via both seed and root. Each plant produces over 4,800 seeds per plant but seeds have been reported to not have the reproductive capabilities as the root. Most of the pepperweed’s root system grows within the first 2 foot of the soil but have been known to penetrate to a depth of 10 foot or greater and expand at a rate of 10 feet per year. Root segments produce adventitious buds; pieces as small as 1 inch in length are capable of generating new shoots. The Perennial Pepperweed is also known to have Allelopathic properties which prevent propagation of desired plant life.

Tamarisk or Salt Cedar is beautiful, but it uses about 200 gallons of water a day which is very dangerous in our arid climate. A bulldozer or prescribed fire can be used to open up large stands of salt cedar. These methods must be followed up with an herbicide treatment of the re-sprouts when they are 1 to 2 meters tall. Chainsaws, or loppers for smaller plants, are effective for cut-stump treatments to smaller infestations or in environmentally-sensitive management areas.

The Common Tansy is often found in flower beds, this perennial plant reproduces by both seed and creeping rootstocks. Control this plant through repeated mowing or cutting. Tilling will cause common tansy to spread. Wear gloves and goggles, because tansy can be toxic.

The showy snapdragon-like flowers are bright yellow with a deep orange center. It develops an extensive root system, making control options varied. Pulling is not recommended due to the extensive root system, but several chemical options are available.

With silvery-green leaves and a scent like heaven, these trees are beautiful but demand a lot of water. Replace Russian olive trees with native trees and remove seedlings and saplings before they mature. Stumps respond well to a long, hot burning fire or herbicide treatment.

Diffuse Knapweed is a non-native biennial forb that reproduces solely by seed. A biennial is a plant that completes its life cycle within two years. During the first year of growth, Diffuse knapweed appears as a rosette in spring or fall. During the second year in mid to late spring – the stem bolts, flowers, sets seed, and the plant dies. Once the plant dries up, it breaks off at ground level and becomes a tumbleweed which disperses the still viable seeds over long distances. A prolific seed producer, Diffuse knapweed can produce up to 18,000 seeds per plant. Therefore, the key to managing this plant is to prevent seed production. Diffuse knapweed can grow 1 to 3 feet tall, and is diffusely branched above ground. This gives the plant a ball-shaped appearance and tumble-weed mobility when broken off. Leaves are small, and are reduced in size near the flowering heads. Flowers are mostly white, sometimes purple, urn-shaped, and are located on each branch tip. Bracts that enclose the flowerheads are divided like the teeth of a comb, and are tipped with a distinct slender spine. Upon drying, the bracts become rough, rendering them injurious to the touch. Flowers bloom July through August. Seed set usually occurs by mid-August.

Houndstongue is a short-lived perennial or biennial forb. It produces rosettes in the first year and bolts a stout, erect stem, that is 1 to 4 feet tall by mid-summer of the second year. Then it flowers and produces fruit. Flowers are reddish-purple (occasionally white) and droop slightly from densely clustered panicles. The five rounded petals are cupped by five sepals covered with long, soft white hairs. Flowering occurs from May to July. The simple leaves are lance or oblong shaped, with a smooth edge and no teeth or lobes. Leaves are alternate, 1 to 12 inches long and 1 to 3 inches wide. The leaf tip is sharply pointed, like a hound’s tongue, yet are covered with long-soft white hairs. Leaves often appear dusty and insect-ridden. A thick, dark, woody taproot can reach 3 to 4 feet deep.

Leafy Spurge is a non-native deep-rooted perennial that spreads by seed and extensive, creeping roots. The roots can extend as deep as 30 feet into the soil and are extremely wide-spreading. The roots are brown and contain numerous pink buds that generally produce new shoots or roots. Leafy spurge can grow from 1 to 3 feet in height. The stems are smooth, pale green, and thickly clustered. Leaves are alternate, narrow, linear, and 1 to 4 inches long. The flowers are very small and yellowish-green. They are enclosed by very visible yellowish-green, heart-shaped bracts. The entire plant contains white, milky sap that exudes readily upon a stem or leaf breakage. This sap can damage eyes and sensitive skin. Leafy spurge is one of the earliest plants to emerge in the spring. Flower clusters develop 1 to 2 weeks after stem emergence which is from mid-April to late May. One large leafy spurge plant can produce up to 130,000 seeds. Three-sided seed capsules explode when ripe and project the seeds up to 15 feet away from the parent plant.

Spotted and hybrid knapweed is a non-native, short-lived perennial forb that reproduces mainly by seed. Spotted knapweed can grow up to 900 seeds per plant annually that are viable for up to 8 years. The key to distinguishing spotted from other knapweeds is the black-tipped, spiny, involucral bracts (phyllaries) at the base of the flower. Unlike diffuse knapweed, it does not have a long, distinct terminal spine at the tip of the bracts. Spotted & hybrid knapweed can grow up to 3 feet tall on rigid stems that are openly branched on the upper half of the plant. Urn-shaped flowers are solitary on the tip of each branch. Flowers are pink to purple and rarely white. Leaves on the stem are alternate, deeply lobed, and become smaller and simple near the tips of the stem. Basal rosette leaves are deeply lobed and up to 6 inches long. Flowers bloom from June to October and seed-set usually occurs by mid-August. Spotted knapweed can also reproduce vegetatively from lateral roots. Spotted knapweed can cross with diffuse knapweed to form the hybrid knapweed.