BROOM SNAKEWEED (GUTIERREZIA-SAROTHRAE) DISPERSAL, VIABILITY, AND GERMINATION

Broom snakeweed achene dispersal was monitored by placing surface-leve l traps outwards in the cardinal directions from 12 plants and collect ing the achenes weekly or bi-weekly from September 1993 until seeds we re no longer retained by the plants after 42 wk. About 50% of the ache nes dispersed between October and December. Especially high numbers of achenes were dislodged during periods of incense winter winds and rai ns, with 78% of the seed placed into the east tray and 86% falling wit hin 50 cm of the parent plant. Achene production averaged 3,928 (+/-1, 146) per plant in 1993 and 2,036 (+/-987) per plant in 1994. Achenes c ollected over time directly from the inflorescence and achenes stored in nylon packets on the soil surface averaged 82% viability during fal l and winter. Achene viability declined rapidly in late spring, and fe w remained viable before the next seed crop. Greenhouse experiments co mpared the influence of water application interval and water amount on broom snakeweed germination and seedling survival. Treatments consist ed of 4 water intervals: daily, 5-d, 10-d, and 15-d intervals; and 4 w ater amounts: field capacity (1/1 fc), 3/4 fc, 1/2 fc, and 1/4 fc. Ger mination was 52% at daily 1/1 fc, and no seed germinated at daily 1/4 fc. Data suggest that optimum germination occurs when soils are mainta ined at a minimum soil matric potential (Psi m) > -180 kPa for at leas t 4 d. Optimum Psi m for seedling survival appears to range between -3 00 and -900 kPa, while seedling mortality would generally be expected with a Psi m of > -1800 kPa.