Answer: Preparing the soil prior to building your rock garden will result in years of carefree enjoyment of your new landscape feature. I'd dig the weeds out and then amend the soil with compost or other organic matter to help it drain well. You could use a vegetation killer but you'll still need to dig out the debris, and there will be a waiting period prior to planting so the chemicals can degrade. Why not just work the soil and remove the weeds without the chemicals? After removing the debris and amending the soil, place your rocks. At this point you can cover the entire area with several thicknesses of newspapers to suppress weeds or you can purchase a weed block material. Be sure to use weed block (it will allow water to penetrate but will block sunlight which will keep weed seeds from sprouting) rather than black plastic. Or, you can plant and then spread a pre-emergent weed killer (such as Preen) over the bare soil to keep weeds from sprouting. If you use weed block, you'll probably want to disguise it with a layer of gravel or small rocks.
The rocks shouldn't have any effect on the plants in terms of leaching minerals, but rocks do absorb and radiate heat so some plants will be sensitive to the heat generated. I'd use succulents or other heat loving plants in close proximity to the rocks. Termites shouldn't be attracted to the area, but ants sometimes are because the soil is warm and dry. You can avoid an infestation by running the sprinkler periodically to moisten things up, or you can spot treat if ant hills become apparent.
Best wishes with your new rock garden!
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