Idea : Self-Watering by Water Reservoir

Keep your container garden happy and thriving on long hot days by installing a hidden water reservoir. All you need is an empty plastic bottle around the same height as your planter. It will be easier to start with a new planting than an established container.

Cut. Using a craft knife, cut the bottom off a plastic bottle. The cut bottle should be no taller than the distance from the bottom of the pot to the top of the soil line. Remove the cap.
Place. Fill your container up partially with soil, and then place the empty bottle, upside down, in the center. Add more soil around the sides so that it stands up.
Plant. Add your new plants in the donut shape around the bottle.
Water. On hot days, just fill up the bottle with water, and it will slowly trickle down to the deep roots of your plants. You may still need to top-water at first while your little plants are getting established.

Source : dabbletree