Here are a few more steps to assist keeping your trees hydrated:
Add a handful of water crystals to help the plant survive in drought and periods when you neglect to water them. The clear jelly-like crystals absorb and store water.
Plastic aqua spikes or water cones are pushed into the root zone of a seedling or plant to give it a regular watering. A plastic water bottle is added on the other end. You can also add kelp and fish fertilizer to the water to fertilise at the same time.
The local council uses pipes with their road side plantings. These pipes are usually 30 cm long with perforated holes. They need to be inserted into the ground before the seedling or tree is planted in place, and allow water to be directed at the root zones of the plant. This targeted watering prevents water loss from evaporation and is more efficient then watering at ground level.
Tree guards are triangle plastic sleeves that held in place by a three wooden stakes. Other versions are made from pink coreflute plastic. They help increase the success rate of seedling plantings by reducing cold, wind and animal damage. They are used by Landcare in Australia for bush regeneration projects and highway plantings. A smarter version includes a water pouch to provide self-watering to the root zone.
Homegrown Evolution has a great list of self-watering planters.