rasik wrote:

I have created a new flower bed area running the side of my garden (20
meters on each side of the garden). The area was once grass and I had
someone remove soil and then grass. I still see bits of grass around
and it seems it been a bad job by the labourer. I will be adding 50mm
of new top soil but today I had pulled hidden grass roots. I am not
sure if grass will grow through the new soil.

I questions are;
1. Will grass grow through the new 50mm soil?

Yes, definitely.

2. Is there anything I can do to the existing soil (bits of grass)
before adding new soil?

Till the entire area removing roots and rocks and then till in the new
soil to about a 6" depth.

3. Are there any chemical based products that I can use to kill grass
and when should it be applied (before or after new top soil)?

There are plenty of products that will kill grass but they will also
kill your new plantings... there is no need to use any chemical
defolients. Prevent grass/weeds with a good layer of mulch... medium
pine bark pellets work very well for flower beds. For long strip beds
roll out heavy duty weed block cloth and then cover with pine bark
mulch to about a 4" depth. Cut slits in the cloth to insert your
plants. Good quality garden cloth should last some 20 years.
This one costs a bit more but it is truly heavy duty:
http://www.leevalley.com/US/garden/p...t=2,2300,33272