There is an easier way. If you have already sprayed, you might want to ignore this post. LOL

I have been doing lasagne beds for years. I called them instant gardens, since I make them when I want a new bed right-here-right-now! LOL I only recently found out that what I was doing is known as lasagne gardening.
First I mow the lawn in the area as short as I can. A weed-whacker helps to get it really short!

I lay down newspapers - 6-8 layers thick, watering as I go so that they don't blow away in the wind. Cover with 2-3 inches of good soil. I work in small manageable sections.

Once you have your lasagne bed all done, go ahead and water it. Yes, even though there are no plants in it! Ignore the neighbours who watched you lay down newspaper (and/or cardboard), soak them, cover them lovingly with soil and who are now wondering what the heck you think you are growing!

Regular watering will help to keep the newspaper/cardboard damp and it will already start to break down (newspapers more, cardboard takes longer) by the time you want to plant. And about those earthworms and lasagne beds - if you build it, they will come.

I only use newspapers, and so when I want to plant, I just stab that trowel into the bed straight down to get through the newspapers, pour in a bit of water if necessary, to soften the paper, and plant away. You will be pleasantly surprised next year at how nice the soil further down has become.

Oh, and any stray or homeless earthworms you find should go straight into that new bed, so they can get to work right away LOL.

I use pretty much the same technique when I want to enlarge an existing bed. I just make sure that I place the newspapers around any existing plants.

I never till, since I find that even a good digging will often bring scads of weed seeds to the surface, and I spend the next season just yanking them out. GRRRRRRRRR!!

Try it, you will wonder why you never thought of this before. It's so easy!

3 days

I'd wait longer than 3 days, give it a good 2 weeks, you might have to give it a second application.

I have also been using Roundup, but not for extending a bed: So far I have done the latter by laboriously turning the sod over, covering with mulch, and then just used the Roundup on the few bits of grass that manage to poke up here & there. There hasn't been much of that (thankfully!), so I have adopted a technique whereby instead of spraying, I put some in a jar and apply it directly to the grass leaves using a brush and a small sheet of rigid plastic.
For next spring, I'm thinking of extending the bed without turning the sod, because I don't plan to plant anything in that area -- its just to give some ground cover (Junipers) room to expand.
But I think it might be worth trying a different approach than spraying: maybe applying the Roundup with a painter's roller might be more effective ?
Any thoughts ? (thnx)

For me, the easiest technique is to cut out the extension and simply remove the sod. You will have your extension in one day.

If you wish to do this the non-laborious way, use the method of solarization which farmers use to kill weeds. It's large black plastic that stops sunlight from reaching the grass and cooking it at the same time. Normally takes 2-3 weeks to be 100% effective. I suppose if you combine the technique with roundup the effect is far more faster.

Ianna

That "solarization" technique sounds interesting - thnx.
The problem with removing the sod completely, is that it creates a big hole, which requires lots of filling (with mulch) -- particularly in cases where the the sod has been there for decades, so it doesn't come off in a nice thin sheet....
However, I do that anyway, around the periphery of the new bed, in order to prevent the neighboring grass from migrating back: I dig out an ~8" wide trench, which I then back-fill part-way with screening rock (piled up against the grass side of the "moat"), and the rest with hemlock mulch, as the rest of the bed area (regardless of whether the bed is turned-over sod, or just treated with Roundup).
Seems like a fairly bullet-proof approach -- or "belt & suspenders" if you wish :-)

Don't you have an edger, that half moon shaped digging tool? Since it's only the periphery, you could use the edger to cut out the extension, then use a pitchfork to break the soil under the sod starting from the edge closes to the bed.. With this technique, you'll find it easier to remove the sod and leave the ground ready for your bed. I wouldn't worry about the hole since you'd probably start getting that filled out quickly when you add more plants and possibly new soil to amend the exising soil. I suspect you'd have to amend it anyway before adding new plants.

Ianna