Generally, people only look after their tree in the beginning when the tree is first planted.
But you need to look after them as the tree matures and grows.
The general health of your trees is reflected on the amount of care you put in.
It is important to water your trees when it is initially planted, watering throughout the first year it was planted and also during dry spells.
It is very beneficial to deep soak your trees by letting your hose run slowly at the base of the tree.
Depending on several factors like soil type, you will need to investigate if water is reaching the root structure of the trees.
The only real way of finding out is by digging a foot down to check.
Quite often you will find that you will need to water your trees a lot more.
Young trees would need soaking twice a week during their first year and older trees can go longer.
Naturally the soil and weather are influential factors so the exact amount of time varies.
Generally though, you can water older trees around once or twice a month.
You can use water bags to water new trees as they provide a release of water at a slow rate keeping the soil around the roots saturated.
Depending on the quality of your soil, you can use fertiliser if you wish.
If you feel that the soil is not good enough to support a new tree, then fertilise monthly or more during the initial period.
You can get a good tree fertiliser from any good garden center or nursery.