1) If you're growing plants in pots, ideally start feeding them about six weeks after planting, when nutrients from the compost will start to diminish. Stop feeding at the end of summer.
3) Feed little and often! This will ensure plants regularly have enough nutrients, and prevent losing too many nutrients to rainfall.
4) If a plant is stressed, wait until it has started to recover before feeding.
5) Not every plant needs food - for example nasturtium only flowers in poor soil, so feeding it will produce leafs with no flowers!