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Are we talking like 12" high weeds or 4 foot high weeds? If its not too high and thick you should be ok with round up and tilling. What is the soil like? Good top soil, or lots of clay or something? Does it stay moist, or really wet, or dry out real bad? Do you plan on watering it if need be? I would try to plant something that does well in your soil conditions. You will get the best results if you test the ph level and add lime if need be to reach the ph levels that are best suited for what you are planting. At the very least I would add fertilizer when planting.