I've read that some cats who don't cover their messes do this a sign of dominance, it's like leaving his or her scent just like in the wild. He or she is letting other cats know who is the boss. Domestic cats will tend to do this if it feels threatened by the arrival of a new cat or kitten into the family abode. To verify if this is the situation with Ziya, she would have stopped covering her messes in her litter tray too. N.B. > All other cats will remain covering theirs...
With regards to the garden, it may help to place some soil in the areas the cat commonly uses as a latrine so that the cat 'accidentally' elimates on it when the cat goes in his or her usual areas? If the cat starts this, you could then add more soil and the cat may eventually begin to dig and cover his or her mess in the usual way. In the meantime, it is best to clean up after the cat with a 'pooper scooper'.
With regards to the house, it is always useful to have as many litter trays as there are cats. This way each cat will have it's own litter tray... Place them in different areas of the house, in places where the cat isn't going to be disturbed while it is doing its 'business'. Remember to remove all clumps of litter and 'number 2s' on a daily basis and replenish with clean litter as necessary. Clean the litter tray completely at least once a week; avoid using strong smelling chemicals or cleaning products when washing the litter tray, as doing so may cause your cat to avoid using the litter tray. N.B. > Some cleaning products are toxic to cats. Check the following link for alternative cleaning solutions that you can find around the house such as Vinegar, Baking Soda and Lemons! hehe... http://housekeeping.about.com/od/environment/a/vinbaklemons.htm
If anyone else has any helpful tip please don't hesitate to leave a comment!