Answer: It is daunting to begin a new garden on a weed infested site, and you are wise to start well in advance. First off, and no matter what else you do, remove every seed head you see. That will save you some work with seedling weeds next year.
Without knowing for sure what the weeds are, it is difficult to predict which methods will work best. Assuming the worst, and since you would like to work on the soil this fall, your quickest method would be to apply glyphosate according to the label instructions. This chemical must be absorbed by the leaves and translocated to the roots which can take considerable time even after the plant appears to be dead. So be sure to read and follow the instructions, including the waiting period after the application, or it may not be effective.
Another option would be to dig up and grub out each large weed by hand, taking as much root as you can. Also dig and sift out as many of the myrtle roots and runners as you can. Then cover the area with several layers of newspaper or cardboard, topped by a thick layer of organic mulch. This method works by smothering the weeds, but it takes quite a while, especially in cool weather.
In either case, next spring, if you see weeds reappearing you will need to spot treat them again either by pulling, smothering or spraying. The weed problem is always worst the first year or two in a garden, after that it gets much easier. Good luck with your project!
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