Grass weeds such as Panicum miliaceum L. (broom corn millet) are a persistent problem for agriculture, causing significant crop losses. A weedy biotype of broom corn millet is already a troublesome weed on North American arable farms, and has recently emerged as a threat to New Zealand corn and maize crops. This thesis describes aspects of the biology of broom corn millet under New Zealand conditions. Experiments were designed to understand under what conditions broom corn millet is mostly likely to affect crop growth. Experiments were conducted in a glass house and a laboratory to observe the effect of temperature on germination and early growth of broom corn millet. The pot-based glasshouse experiment compared germination and growth between a range of controlled substrate temperatures. The response of broom corn millet to temperature was typical of that expected for a C₄ plant. At 10°C seeds germinated later and in lower proportions compared to 15°C, 20°C and 25°C. Growth and above ground dry biomass accumulation also increased with increasing substrate temperature, with the highest dry biomass accumulated at 25°C, primarily because of increases in germination rate. In a laboratory experiment conducted at temperatures ranging from 5°C and 34°C, temperatures greater than or equal to 20°C were more favourable for germination of broom corn millet seeds. The optimum temperatures for germination were 27°C to 34°C. The threshold germination temperature for the seed lot used was 7.4°C. Broom corn millet seeds were tested for their ability to germinate and emerge from a range of planting depths when planted in pots containing 16 soil types from around New Zealand. Seedlings emerged from 120 mm depth in all soil types. In six soil types seeds of broom corn millet were able to emerge from the greatest depth tested of 170 mm, very deep compared to most herbaceous weeds. In general, seedling emergence reduced with increasing depth, whereas suicidal germination increased. Step-wise binomial regression of emergence against various soil physical properties did not reveal any significant relationship between soil physical properties and seedling emergence. To observe the affect of competition on both the weed (broom corn millet) and the crop (sweet corn), plants of both species were grown together in pots at a range of planting ratios. Plants were also grown in monoculture to observe growth without competition. In the competition trial broom corn millet emerged after sweet corn and affected sweet corn above ground biomass during the first four weeks. However, this effect did not persist as sweet corn biomass increased irrespective of the level of competition from broom corn millet plants. The monoculture experiment indicated that sweet corn grew better without competition whereas growth of broom corn millet was stimulated while growing in competition. The poor competition by broom corn millet plants was assumed to be the result of unseasonal low temperatures during the period immediately after sewing and demonstrated that broom corn millet plants emerging after the crop may not affect crop growth. The likely persistence of New Zealand broom corn millet seeds in soil is unknown. A laboratory based Controlled Ageing Test (CAT) was therefore evaluated for its ability to predict the persistence of seeds. The test was conducted using seeds of nodding thistle (Carduus nutans), for which real time persistence data is available. In two additional experiments, the CAT was used to estimate the potential persistence of New Zealand sourced broom corn millet seeds. The CAT derived half life time (P₅₀) of nodding thistle seeds did not compare well with the field derived P₅₀ for nodding thistle seeds, with the CAT results suggesting less persistence compared to actual persistence. Examination of the CAT results for broom corn millet showed a decline in seed viability from 30 to 50 days, followed by a sharp decline at 75 days. The midpoint of the initial decline (40 days) was taken as the P50 for broom corn millet. This value is similar to existing information for broom corn millet in North America, and indicates that broom corn millet will form a moderately persistent seed bank in New Zealand. In conclusion, results suggest that higher temperatures favour the growth of broom corn millet. Planting of crops earlier in the season may reduce competition by this weed. However, increasing temperatures as a result of global climate change will enhance conditions for broom corn millet and may increase future crop losses caused by this weed. The ability of broom corn millet’s relatively large seeds to germinate and emerge from depth will limit the efficiency of conventional weed control practices, such as ploughing and stale seed beds. The ability to form a moderately persistent seed bank suggests that once introduced, broom corn millet will be challenging to eradicate because of its prolific seed production. Significant changes in weed control practices will therefore be required to manage broom corn millet in the future.