The control of western juniper (Juniperus occidentalis Hook.) trees released shrub and herbaceous vegetation from dominance of the tree overstory. Revegetation of the woodland with desirable browse and forage species required weed control of both shrub and herbaceous species. The method of tree control (herbicidal, mechanical, or wood harvesting) influenced subsequent weed control-revegetation options in the understory. Physical restraints imposed by standing dead trees resulting from picloram (4-amino-3,5,6-trichloropicolinic acid) treatment limited the use of weed control and seeding equipment. Accumulation of litter interfered with the use of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] for annual grass control on both the picloram and wood harvest plots. Establishment of forage species was most successful on the mechanically cleared plots. Attempts to revegetate with browse species were largely foiled by native herbivores, and selective predation occurred on herbaceous species.

Two field experiments were conducted from 1993–1994 through 1995–1996 growing seasons in Harney County, OR, to determine the relative competitive abilities of Elymus elymoides (squirreltail) a native perennial range grass, and Taeniatherum caput-medusae (medusahead), an exotic annual grass weed. The 1993–1994 growing season was very dry, 1994–1995 was dry, and 1995–1996 was wetter than average. One experiment tested seedlings vs. seedlings in each of three seasons. The second experiment tested seedlings plus second- and third-year established E. elymoides plants vs. 77 caput-medusae over 2 yr. Biomass, seed production, and soil moisture utilization 15, 30, 45, and 60 cm deep by the two species were measured. A randomized block design with factorial arrangement was used, with 25 2.25-m2 plots per block. Initial seeding densities of each species were 0, 10, 74, 550, and 4,074 seeds m−2 in all combinations of density. In the seedling vs. seedling experiment, intraspecific competition by 77 caput-medusae on itself was always significant (P ≤ 0.10) for both biomass and seed production. Interspecific competition by E. elymoides seedlings on T. caput-medusae biomass and seed production was not significant (P ≥ 0.10) in 2 of 3 yr and was always less than intraspecific competition by 77 caput-medusae. Only 0.4% of E. elymoides seed germinated, and no seed was produced in the very dry first year, but 84% of remaining seed was viable for the next year, which had better moisture conditions for germination and establishment. Interspecific competition affected (P ≤ 0.10) E. elymoides seedling biomass and seed production throughout the study. Intraspecific competition affected (P ≤ 0.10) seedling E. elymoides seed production in the dry year but not in the wetter than average year. In the mature E. elymoides experiment, intraspecific competition by T. caput-medusae on weight and seed production per plant was greater than interspecific competition from E. elymoides. Seedling/mature E. elymoides reduced T. caput-medusae weight per plant in the dry year but the effect was not biologically significant. Larger, mature Eelymoides plants produced 600 to 3,000 seeds per plant during the wet year; neither intra- nor interspecific competition was a factor. Taeniatherum caput-medusae was better able to access deeper soil moisture and was more aggressive at extracting soil moisture than were E. elymoides seedlings in the wet year. Cold soils and low oxygen due to wet soils may have restricted E. elymoides seedling root activity. Mature E. elymoides plants did not appear restricted by cold soils or low oxygen. Established second- and third-year E. elymoides plants were able to compete for soil moisture down to 45 cm. The generally greater interspecific competitive effects of T. caput-medusae on E. elymoides than vice versa suggested that it will be difficult to establish an E. elymoides stand in an existing T. caput-medusae community without first suppressing T. caput-medusae. Individual E. elymoides plants did establish and were productive with and without T. caput-medusae competition.

Medusahead is an invasive annual grass that, once established, severely affects range-land productivity and stability. Medusahead builds large seedbanks in the litter and on the soil surface. Effective weed control of medusahead involves either inhibiting germination from the seedbank, eliminating the seedbank, or enhancing germination so that plants are available for control. The purpose of this study was to determine the influence of nitrogen enrichment, immobilization, and nitrification inhibition treatments in the field on the size and germination status of medusahead seedbanks. The germination status of medusahead seeds in seedbanks was determined by periodically collecting field samples of surface soil and litter and bioassaying them in greenhouse emergence tests. Control seedbanks had increased seedling emergence with KNO3 or GA3 enrichment of the bioassay substrate. The combination of these two materials increased emergence. Nitrogen enrichment increased seedling establishment in the field. Carbon enrichment in the field decreased seedling establishment and increased medusahead seeds in seedbanks. Nitrapyrin treatment decreased medusahead in the field similar to carbon enrichment. In comparison to the control or other treatments, GA3 enrichment was not as effective in increasing emergence from nitrapyrin-treated bioassay samples. The combination of carbon and nitrapyrin treatments was very effective in eliminating medusahead emergence in the field, but in wetter years, it never completely eliminated medusahead seedling recruitment and subsequent reproduction. These treatments have promise for influencing succession in medusahead infestations if an adapted perennial species, capable of competing under low nitrogen levels, becomes available.

Rapid germination and root growth have been identified as important plant characteristics for establishment in arid environments. This study evaluates germination, radicle elongation, and root growth rates of two annual grasses, two forage species, and yellow starthistle. Yellow starthistle germination, radicle elongation, and root growth were faster than those of medusahead and hedgehog dogtailgrass. Subterranean clover possessed rapid seedling growth characteristics similar to those of yellow starthistle, whereas Palestine orchardgrass was most delayed.

Seeds of plants naturalized in the United States were examined for the presence of elaiosomes. Seeds of 47 species belonging to 13 families (Asteraceae, Boraginaceae, Dipsacaceae, Euphorbiaceae, Fabaceae, Fumariaceae, Lamiaceae, Liliaceae, Poaceae, Polygonaceae, Resedaceae, Rosaceae, and Solanaceae) were found to have elaiosomes, indicating that these species are probably mymecochorous, i.e., dispersed by ants. These include important rangeland weeds such as bull thistle, Canada thistle, musk thistle, diffuse knapweed, spotted knapweed, and leafy spurge. Myrmecochory in naturalized species may enhance their weediness in areas where they are established and assist their colonization of new and relatively closed communities.

This study documented an undescribed aspect of medusahead germination behavior. Fibrous hairs were consistently observed growing from the coleorhizal surface of germinating medusahead seeds. Three medusahead accessions were investigated to determine temperature effects on coleorhizal hair growth and germination. Coleorhizal hair emergence at 8 C was first noted at 72 h. By 24 h at 18 C, 15 to 74% of seeds had hairs, and at 28 C, hairs were visible on 75 to 94% of seeds. Germination at 18 and 28 C was 80% at 96 h; germination at 8 C did not exceed 80% until 168 h. Less than 4% of germinated seeds were without coleorhizal hairs. Appearance of coleorhizal hairs confirmed that germinating medusahead embryos were active at least 72 h before germination was complete. These data affirm coleorhizal hairs are a normal part of medusahead germination biology. Coleorhizal hairs probably aid early phases of medusahead seedling establishment.

Weed germination studies often quantify only radicle emergence to indicate germinated seeds. Four categories were used to investigate root and shoot emergence from germinating seeds including: root (seeds with radicles emerged); shoot (seeds with plumules or cotyledons emerged); complete (both roots and plumules emerged); and total (any emergence: roots, shoots, or both). Medusahead and yellow starthistle germination tests illustrated that emergence was equal in the root and total categories. Shoot and complete categories had identical counts, but those counts were often different from the total and root categories. Emergence of medusahead shoots at 8 C was 3 d later than roots, but at 18 C roots and shoots emerged nearly simultaneously. Yellow starthistle roots were not affected by picloram, but emergence of cotyledons (shoot category) was reduced by 80 %. Yellow starthistle seeds stored for 74 and 90 mo produced more radicles than cotyledons after 28 d at 28 C. Medusahead and yellow starthistle root emergence in the laboratory sometimes exceeded seedling emergence from a greenhouse soil mixture. The presence of both roots and shoots (complete category) was the most accurate criterion for documenting yellow starthistle and medusahead seed response to laboratory test conditions.

The spread of medusahead across the western United States has severe implications for a wide range of ecosystem services. Medusahead invasion reduces biodiversity, wildlife habitat and forage production, and often leads to increased fire frequency and restoration costs. Medusahead is problematic in the Intermountain West and California Annual Grasslands. The last review of medusahead ecology and management was completed 20 years ago. Since the last review, there have been scientific advances in medusahead management suggesting a significant need to develop an up-to-date synthesis. Medusahead continues to pose a serious threat to rangeland ecosystems. In this synthesis, we present new information regarding the ecology of medusahead, suggest a framework for managing medusahead based on invasion level, and identify research needs to further improve management of this invasive annual grass. Success of different management practices varies between the Intermountain West and California Annual Grasslands, signifying that the best management practices are those specifically tailored with consideration of climate, soil, plant community characteristics, and management objectives. Prevention and control treatments that are useful in the Intermountain West may not be practical or effective in the California Annual Grasslands and vice-versa.