Round up is mostly gone from the soil after seven days, when other things can be planted.

Digging out grass is likely to be ineffective as well as hard work because if you leave any, even tiny bits, of the roots and rhizomes behind, the grass will grow again.

A chemical free method is to cover the grass with 6 to 10 layers of newspaper, wet it and then cover with 4 to 6 inches of fine shredded mulch or straw. Wait about two months and the grass will be dead and you can plant through the newspaper and mulch.

No, it is not persistant. As herbicides go, it is one of the best ones because it sticks to the soil and breaks down quickly. It will kill just about any green plant if it gets on the leaves, though you may have to apply it several times depending on the kind of grass. I used roundup last fall and then 2 more times this spring (I had bermuda grass to kill off). I seeded a short time after the last spray and the grass grew like crazy.

The only thing I would recommend, that I didn't do, is to water the area daily for 2-4 weeks before you use roundup to really germinate stuff. THEN get it all with the roundup. The area was only sort of a lawn area and was pretty weedy.

CJ

Title: Initial and residual activity of glyphosate and SC-0224 in a sandy soil.
Authors: Devlin, Robert M.; Karczmarczyk, Stanislaw J.; Zbiec, Irena I.; Koszanski, Zdzislaw K.

Authors affiliation: Lab. Exp. Biol., Univ. Massachusetts, East Wareham, MA, USA.

Published in: Crop Protection, volumn 5, pages 293-296, (1986).

Abstract: "Initial and residual herbicide activity of glyphosate [1071-83-6] and SC-0224 (trimethylsulfonium carboxymethylaminomethylphosphonate) [81591-81-3] when applied to a sandy soil was investigated. A bioassay employing wheat (Triticum vulgare cv Mericopa) was used to det. the residual activity of the herbicides on different characteristics of plant growth. At 5 kg/ha, both herbicides significantly reduced shoot length. This was obsd. in wheat planted immediately as well as 10 days after application. SC-0224 was more active than glyphosate, significantly reducing shoot length at the 2.5 kg/ha rate 10 days after application. Root length was reduced by both herbicides at 2.5 and 5 kg/ha in wheat planted 10 days after application. Redns. in fresh and dry wt. were also obsd. for both shoots and roots. Thus, SC-0224 and glyphosate have residual herbicide activity in sandy soil, at least £10 days after application, and SC-0224 has more herbicidal activity than glyphosate in this respect."
----------------------

I wouldn't let the above reference scare you off. In practical terms, meaning in normal (not sandy) soils and with normal delays before re-seeding (months, not days), you will not have any problems. Roundup is definitely the most safe and effective method to eliminate turf grasses, IMHO.

It also depends on the pH of your soil (and possibly other things, like the amount of phosphate that you use).

Title: Influence of soil composition on adsorption of glyphosate and phosphate by contrasting Danish surface soils.
Authors: Gimsing, A. L.; Borggaard, O. K.; Bang, M.

Authors affiliation: Chemistry Department, Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871, Frederiksberg C, Denmark.

Published in: European Journal of Soil Science, volumn 55, pages 183-191, (March 2004).

Abstract: "The herbicide glyphosate and inorganic phosphate are strongly adsorbed by inorganic soil components, especially aluminium and iron oxides, where they seem to compete for the same adsorption sites. Consequently, heavy phosphate application may exhaust soil's capacity to bind glyphosate, which may lead to pollution of drain- and groundwater. Adsorption of phosphate and glyphosate to five contrasting Danish surface soils was investigated by batch adsorption experiments. The different soils adsorbed different amounts of glyphosate and phosphate, and there was some competition between glyphosate and phosphate for adsorption sites, but the adsorption of glyphosate and phosphate seemed to be both competitive and additive. The competition was, however, less pronounced than found for goethite and gibbsite in an earlier study. The soil's pH seemed to be the only important factor in determining the amount of glyphosate and phosphate that could be adsorbed by the soils; consequently, glyphosate and phosphate adsorption by the soils was well predicted by pH, though predictions were somewhat improved by incorporation of oxalate-extractable iron. Other soil factors such as organic carbon, the clay content and the mineralogy of the clay fraction had no effect on glyphosate and phosphate adsorption. The effect of pH on the adsorption of glyphosate and phosphate in one of the soils was further investigated by batch experiments with pH adjusted to 6, 7 and 8. These experiments showed that pH strongly influenced the adsorption of glyphosate. A decrease in pH resulted in increasing glyphosate adsorption, while pH had only a small effect on phosphate adsorption."