Seeing this indoor question made me look for an outdoor version of the same but found nothing, so here goes:
We've got a balcony with a very large planter on the outside in which we have planted some small flowering plants for decoration. The planter faces south so it gets a LOT of sun during the day, and my wife waters it by hand nearly every day.
We're worried that the plants won't survive four days in direct sunlight without watering, but we haven't found a good solution for automatic watering. I don't want to spend big money on ready-made systems.
We have tried the inverted soda bottle with a pinhole in the cap but the bottle was drained within a few hours. This solution won't work over several days.
What alternatives can I try?