Seed Germination and Seedling Emergence of Scotch Broom (Cytisus scoparius) Timothy B. Harrington* Scotch broom is a large, leguminous shrub that has invaded 27 U.S. states. The species produces seeds with a hard coat that remain viable in the soil for years. Growth-chamber studies were conducted to determine effects of temperature regime and cold-stratification period on seed germination. Seedling emergence, mortality, and biomass also were studied in response to sulfometuron and metsulfuron herbicides and variation in soil texture and watering regime. Germination was greatest for a dark/light temperature regime of 15/20 C. Initial rates of germination increased as stratification period was varied from 0 to 60 d, but final germination after 90 d did not differ significantly among periods. Applied alone or in combination, sulfometuron and metsulfuron decreased biomass and increased mortality of seedlings. Mortality from simulated soil drougbt was greater in the presence versus absence of sulfometuron (20 and 6% mortality, respectively) probably because the herbicide reduced root biomass by 58 to 95%. Invasiveness of Scotch broom is facilitated by a prolonged period of germination across a broad temperature range. Increased control of Scotch broom seedlings with sulfometuron is likely if application is timed to expose recently emerged seedlings to developing conditions of soil drought. Nomenclature: Metsulfuron; sulfometuron; Scotch broom, Cytisus scoparius (L.) Link CYSC4. Key words: Temperature, stratification, soil texture, watering regime, metsulfuron, sulfometuron. Scotch broom is a large, leguminous shrub that has invaded foliar applications of clopyralid, fluroxypyr, and triclopyr 20 eastern and 7 western U.S. states (U.S. Department of herbicides reduced Scotch broom cover to 10% or less by 40 Agriculture Natural Resources Conservation Service [USDA wk after treatment (Blair and Zedaker 2005). Preemergent NRCS] 2009). A native to the Mediterranean, the specieswas (PRE) applications of hexazinone and metsulfuron herbicides first introduced in California in the 1850s as an ornamental were effective at reducing survival of emerging seedlings; (Gilkey 1957). Scotch broom seedlings grow vigorously to however, applications of sulfometuron herbicide reduced leaf form dense stands quickly, excluding native plants and development and biomass of seedlings but did not affect their altering community structure of prairies, woodlands, and survival (Ketchum and Rose 2003). Those authors speculated young forests (Bossard and Rejmanek 1994; Wearne and that stunted development of Scotch broom seedlings from Morgan 2004). Although Scotch broom typically invades sulfometuron would render them more vulnerable to roadsides and other areas of soil disturbance, it has the mortality from soil drought-conditions typical of summer potential to establish and survive in conditions of low light in the Mediterranean climate of the Pacific Northwest. availability (10% of full sun; Williams 1981), such as those of To improve understanding of Scotch broom invasibility in a forest understory (Harrington 2007). Scotch broom is a the Pacific Northwest, growth-chamber studies were conduct- prolific seed producer with individual shrubs producing an ed to determine effects of temperature regime and cold- average of 9,650 viable seeds per year (Bossard and Rejmanek stratification period on seed germination. Seedling emergence, 1994). Once buried, seeds are capable of germinating from mortality, and biomass responses also were studied to identify depths of 1 to 6 cm (Bossard 1993; Williams 1981). Seeds potential interactions of the soil-active herbicides, sulfome- have an impervious coat that prevents germination (Young turon and metsulfuron, with soil texture and watering regime. and Young 1992), but rapid alternating immersion in boiling The research included testing the null hypothesis that seedling water and liquid nitrogen dramatically improved germination mortality from soil drought was not affected by PRE 3.5-fold (Abdallah et al. 1989). Bossard (1993) found that application of sulfometuron. some seeds germinated without further treatment when kept moist at temperatures of 5 to 18 C (< 23% of seeds sown); however, addition of the pregermination treatment of Materials and Methods Abdallah et al. (1989) resulted in germination rates of 60 to Seed Sources. Scotch broom seeds were collected annually in 98%. Seeds buried in the soil had delayed germination for at 2004 to 2006 from the Olympia and Matlock, WA, areas. least 5 yr, enabling development of a large seed bank (Bossard Collections were made beginning in mid-July, as soon as seed 1993; Bossard and Rejmanek 1994). In Australia, seed banks pods began to dehisce, and continued into September. Prior were estimated to store from 4,000 to 21,000 seeds m-2 to being removed from their pods, seeds were dried (Sheppard et al. 2002). thoroughly at room temperature and then stored in sealed Scotch broom is considered a noxious weed in California, glass containers at 5 C. Seeds from the previous growing Hawaii, Idaho, Oregon, and Washington. Tactics for season were used in each study. controlling Scotch broom in forest and rangelands include cutting, prescribed fire, and herbicide treatments. Manual Temperature Regime. Seed germination was studied from cutting of broom in August prevented sprouting on greater January to March 2005 in five dark/light (14/10 h) than 90% of shrubs (Bossard and Reimanek 1994). Individual temperature regimes: 5/10, 10/15, 15/20, 20/25, and 25/ 30 C. Seeds were counted into 15 samples of 100 each, imbibed by soaking each sample for 24 h in deionized water, and cold stratified for 45 d at 5 C. Seeds were then placed on moistened paper trays in covered Petri dishes, and the dishes were placed in a controlled-environment growth chamberl high spray volume-three to six times that recommended on with daytime light conditions averaging 87 umole m-2 s-1 the product label for aerial and ground applications-was photosynthetic photon flux density (PPFD). Germination was used to increase uniformity of herbicide distribution within recorded when the hypocoryl had elongated at least 2 mm. the container. Containers were placed in a controlled- Germination counts were conducted daily for 24 d after environment growth charnber3 with light conditions of sowing, and paper trays were remoistened with 1 to 2 ml of approximately 50 umole m-2 s--1 PPFD and a dark/light deionized water at each count. The experimental design is temperature regime (14 hl10 h) of 15/20 C. Soils were completely randomized with three replications of the five watered uniformly with deionized water periodically during temperature regimes. the study. Emergence was recorded when the epicotyl had elongated at least 2 mm. Emergence and mortality counts Stratification Period. Seed germination was studied from were conducted through 90 d after sowing. Count frequency May to July 2005 after five cold-stratification periods: 0, 15, was every day during the first 45 d and every 2 to 3 d 30,45, and 60 d. Seeds were counted into 15 samples of 100 thereafter. At the end of the study, living seedlings from each each. Methods for imbibing and stratifying seeds were container were removed, separated into shoot and root identical to those of the temperature study except where components pooled by container, and dried in a forced-draft noted. The dark/light temperature regime (14/10 h) was 15/ oven at 65 C to a constant weight. The experimental design is 20 C- the optimum egime identified in the temperature completely randomized with three replications of 10 study. Germination counts were conducted through 90 d treatments arranged as a factorial of the five herbicide after sowing. Count frequency was evety day during the first treatments and two soil textures. 45 d and every other day thereafter. The experimental design is completely randomized with three replications of the five Sulfometuron and Watering Regime. Seedling emergence, stratification periods. mortality, and biomass responses to sulfometuron level and contrasting watering regimes were studied from January to Herbicides and Soil Texture. Seedling emergence, mortality, April 2007. Study methods were identical to those of the and biomass responses to two herbicides were studied from herbicides and soil texture study except where noted. The February to May 2006 after seed was sown in clear plastic following treatments were randomly assigned to three containers (dimensions: 12 cm long by 17 cm wide by 6 cm containers per soil texture: deep) filled with forest soils of contrasting texture. The first soil, collected at a site neat Matlock, WA (47.206°N, 123.442°W), is a very gravelly loamy sand of the Grove series (Dystric Xerorthent) formed in glacial outwash. The second soil, collected at a site near Molalla, OR (45.196°N, 122.285°W), is a cobbly loam of the Kinney series (Andie Dystrudepr). Each soil was sieved to remove coarse fragments greater than 2 mm. Fifteen containers were filled with a fixed Container dimensions were 11 cm long by 11 cm wide by weight of dry soil from each source, soil was tamped level, 3 cm deep. To initiate germination, soils for both watering resulting in a 3-cm depth, and deionized water was applied to regimes were kept at field capacity (35 and 50% volumetric achieve field capacity based on measured weight (volumetric moisture contents for Grove and Kinney soil series, moisture contents of 35 and 50% for Grove and Kinney soil respectively) until the ninth