Laundry detergent bottles, big or small, don't need to end up in the recycling or rubbish bin when empty. Frugal living blog Wisebread suggests turning them into watering cans instead.
[Y]ou can [...] use your old detergent bottles as watering cans. The small ones are great for the younger kids, and the big ones work almost as well as the pricey ones from the hardware stores. Just wash your bottle thoroughly, then poke many small holes (a metal skewer is good for this) in the lid. Fill with water, tighten the lid, and you have a great watering can for nothing.
I think it's worth emphasising this again: clean your detergent bottles thoroughly before doing this. You don't want dead, soapy plants. That said, while watering cans aren't incredibly expensive, putting an otherwise wasted bottle to good use is better than tossing it.
15 Ways to Reuse Detergent Bottles [Wisebread]