If you live in an inland climate with warm summers, you may also try a process called solarization for preemptive weed abatement. This will require initial removal of as much surface weed material as possible, together with sharp rocks and other debris that may puncture the plastic sheeting used in the process. This technique is only practical for broad, continuous areas free of twists, turns and other obstacles. Moreover, the selected area should be tilled lightly and smoothed of any lumps prior to laying the plastic. If tilled too deeply, weed seeds and regenerative parts may be placed beyond the reach of lethal temperatures.
The plastic should be clear - don't use black plastic as this does not trap as much of the sun's energy as the former. The plastic should form a tight seal against the soil - any air gaps will create a thermal buffer zone before the soil. Having it tight will also keep the plastic motionless in the wind, thereby preventing air movement underneath. The edges of the plastic can be sealed by piling soil on top of the edges. Another tip: irrigate the soil heavily prior to laying the plastic. The moisture will allow more heat to be conducted deeper into the soil profile, and throughout air spaces. It will also help maintain the high temperatures once they are achieved.
If done correctly, and performed during an extended period of 90+ degree weather conditions, lethal soil temperatures of 130 - 150 degrees F can occur on enough days, down to a depth of up to 8 inches. To do enough damage, these temperatures should occur for a least 7 cumulative days. This may mean leaving the plastic intact for a few weeks or more, depending on the weather.
No herbicide will kill weed seeds directly, but this method can, in addition to killing rhizomes and stolons. Such is important since weeds withhold germination of the entire seed bank by nature's design. Solarization, however, has its limitations. Most mortality will occur in the top several inches of soil, and things may be missed deeper down. A round or two of spraying may be necessary afterwards, not only to test the efficacy of the solarization treatment, but also to kill any stragglers. Weeds on the edges of the plastic are mostly likely to escape.
In addition, some fresh compost and/or mycorrhizal inoculum should be incorporated post-solarization and before planting to discourage pathogenic fungi from rapidly recolonizing the newly sterilized soil.
As far as Dymondia, all I can add is that it requires a lot of water during establishment, but quickly puts out deep roots and becomes quite drought-hardy in time.
Matt
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