What do you use to ensure good drainage in window boxes and containers? I've seen lots of recommendations for using packing nuts or broken pots but unfortunately I have access to neither. I read last night that aluminium cans can be used... I have a constant supply of those little metal cases that tea light candles are in... Can I scrunch those a bit and use as a bottom layer? Don't want so poison my plants!!! Also, should I mix some grit with the compost for all containers or just those housing plants that need good drainage? Thanks!