A high dose of urea will definitely kill the grass. It will leach out relatively quickly and new plants will be able to grow there.

Having said that, I do not consider that a good idea. The reason it won't harm plants in the area isn't that it breaks down into less harmful substances, but that it leaches out of the ground (and into the groundwater). That kind of pollution is pretty rough on the groundwater.

If you feel you need to kill the grass and you're not opposed to using chemicals to kill the grass, glysophate (RoundUp is the best known brand) will kill the grass and dissipate quickly so it would be a better choice than an overdose of urea.

I don't think you need to do anything other than the layers of newspaper or cardboard, though.

"Having said that, I do not consider that a good idea. The reason it won't harm plants in the area isn't that it breaks down into less harmful substances, but that it leaches out of the ground (and into the groundwater). That kind of pollution is pretty rough on the groundwater."

Would it be any saving grace if my lawn was only 100 sq ft and the beds will only be about 20? Then again, I guess bad is bad.

"If you feel you need to kill the grass and you're not opposed to using chemicals to kill the grass, glysophate (RoundUp is the best known brand) will kill the grass and dissipate quickly so it would be a better choice than an overdose of urea."

I've never had experience with chemicals before. Would it be possible to purchase a really small amount, just for this purpose? I suppose I would need to buy an applicator and protective gear as well?

"I don't think you need to do anything other than the layers of newspaper or cardboard, though."

You're probably right. First-time jitters, I guess :)
But I've also heard that zoysia can be pretty hard to eradicate.

I wouldn't use the urea overdose on a small area either.

I don't know how small thee containers are, or how readily available they would be for you. If I remember correctly, some things that would be pretty inexpensive in the US end up pretty expensive for you with shipping, currency conversion etc.

I don't have experience with zoysia, so I may be offbase here, but I think I'd try skipping the chemicals and just using the cardboard.

The use of Urea in excess quantities to kill off the grass is unnecessary and not something you want to do for the reasons bpgreen has already stated, nitrate poisoning of groundwater, the water you drink. What the paper over the grass will do is cut off the light that grass, and any "weeds" growing there, so the plants will die and when they die the soil bacteria will digest those plants adding organic matter to that soil. A bit of mulch is needed to hold the newspaper in place and hide it and it is unnecessary to use more mulch than 3 or 4 inches, although you could. Spraying with a weed killer will kill what is growing there, but also will inhibit the activity of the soil bacteria although eventually even that dead material, minus the nutrients they had when alive, will become part of the organic matter in the soil. Since it is unnecessary to spray anything, because the newspaper will do that job, why spend the money to do something you do not need to do?

I guess you're probably right. I'm just worried about not laying newspaper in the correct thickness - too few, and the grass might grow through once it decomposes (depending on its invasiveness), too many and it might take a long time before my new plantings can root properly through the layers. But I think I'll give it a shot :)

While we're on the subject of urea, there is one other thing I would like to ask without having to start an entirely new thread. I have collected so many fallen leaves over the months, that the only practical thing I can do with them is make leaf mould. However, I would prefer not to leave them there sitting for ages, so I was thinking of adding some liquid urea or bloodmeal to speed up the process a little. What is an appropriate dosage (say, per cubic foot of shredded leaves)?

Leaves well wetted will compost on their own and produce leaf mould. Occasionally I collect a lot of leaves and put them into a compost bin wetting and turning them as I fill the bin. A full bin about 1 cu. metre in size easily goes to 50C ( 120F ) withing a week. This is October when the ambient temperature ranges from 0C to 15C ( 32F to 60F ). So certainly the addition of blood meal or urea will increase the temperature and hasten the decomposition.

In your climate, there should be no need to add anything to the leaves. If you decide to add bloodmeal or urea, I would just sprinkle in a little bit as I add and wet the leaves.

@cowgirl

1 cu. metre within a week? That's some pretty active pile you've got there! Sort of embarrasses me when I think of how I always neglect to turn mine. Then again, I get a little squeamish when I consider the possibility of skewering one of the many fat staghorn beetle larvae that that made my pile a home :)

Here is what I have done over a lawn. Put newspaper down about 6-10 layers thick. Bury that with your lasagna fixin's, let it sit all winter and plant away next spring. Now you being in the tropics probably means waiting 6-10 weeks for the grass to die- depending on the type of grass.

tj

Several years ago, I thought I'd try to get rid of some sort of course, horizontally growing rough-stemmed, matting grass that I have in my lawn. Here they call it "orchard grass" but I have no idea what the latin name is. My theory was to pour about 1/4 cup of high nitrogen fertilizer granules on the unwanted grass clumps, and, the theory went, that would zap the unwanted grass, and then the fertilizer would be ready to go for the surrounding good grass and it would rapidly grow in.

Well, it worked, but this is nothing I would recommend. It killed the bad grass just fine, and then the surrounding grass went on a growing spree, but vertically, not horizontally, as hoped. it needed mowing every 2 days. The yard had all these dark green spots with a brown spot in the middle for the entire summer. By next spring, it was all ok.

How well the newspaper will work will depend on the type of grass you have, if it is an invasive type the newspaper will kill the top growth easily but later the the roots will send up more grass. I find that with Quack Grass, aka a lot of other names, as well as Johnston Grass. I most often will layer 4 to maybe 6 pages, usually 1 section of the local paper, down. I seldom find a need for more than that.

We had an area where we tried to smother the grass with thick layers of mulch, landscape fabric, and weeding. What we found is that essentiallly all the weeds were killed by the effort, but that the really invasive grasses were not. Even worse, since everything else had been killed off, the invasive grasses started covering everything. As noted above, the roots continued to send up more shoots. Some of the areas became so bad that we gave up trying to kill the grass, and have left the area alone. I highly recommend killing the underlying weeds with some kind of herbicide if you have grasses with persistant rooting systems. Where we did not have these invasive grasses, the mulching worked fine, and produced beautiful beds.
Renais

What I have found is that after the underlying plant material is killed off and the soil bacteria have digested it when the incasive species of grass does start to grow back the soil is sift enough that the roots, rhizomes, pull out so easily that control is fairly simple. If an herbicide is used first that will suppress the soil bacteria enough that the soil will not be soft and workable and the dead plant tissue will be there for a long time. Spraying an herbicide is unnecessary.

There have been many discussions about invasive grasses and lasagna gardening. I have found in my sub-tropical beds, some grass finds holes and gets itself up into the middle of the bed where I wasn't careful about overlapping layers well, these are hard to pull up as the ground under isn't well improved yet. Along the edges the spredding grasses like zoysia climb over/through the edging and will root into the mulch. You need to really stay on top of pulling these back out of your beds or you will find it covered in grass quickly and mowing over a lasagna bed can be tricky. My understanding of zoysia is that it has been bread to spread and smother out other grasses and weeds. Here in FL you can't buy it as sod, you buy plugs that you space around the lawn so that it can take over in a few years.

Most people who have battled stuff like bermuda grass in lasagna beds will tell you that you must dig out the grass roots and rhizomes before building the bed to be successful. And keep on top of protecting the edges of your beds.

Where I have used logs and sticks as edging, I find weeds tend to love those locations and pulling them is difficult and the edging make hoeing impossible.

If you can make your lasagna bed much deeper you will have better luck (depending on the plants you want to put there.)
If the bed is deeper you can make your paper/cardboard layer at the bottom thicker without worry that your plants will run out of root space. You don't necessarily need edging up to the height of your bed but without it your bed will be more of a mound.
This is how I planted my squash, cucumbers and pumpkins this season. Make a line of cardboard and mounded mushroom compost 2-2 1/2 feet high in a ridge along the center of the cardboard and covered with mulch (wood chips I had on hand.)
I did something similar with corn but the bed was too thin (less than a foot deep) and the corn is stunted with not enough space above the cardboard for the roots so some of the stalks are falling over. This might have worked if I waited for the cardboard to decompose before planting the corn but then the weeds might be back.

I'm still learning as this is my first year doing no till organic gardening here.