thank shankar & ramesh for comments.
@ ramesh,
flower bed had direct sunlight while the rest was in shadow, moreover there was huge contrast between two; hence it was simple to bring this out in b&w. and yes, about 20% canvas added on top to make it square.
for reference, you may see another compo @
http://www.flickr.com/photos/nevilzaveri/5388852783/" onclick="window.open(this.href);return false;
thanks once again.