If you live in a mild area, it should be safe to put out containers of halfhardy annuals and tender plants such as nemesia, gazania, bacopa, petunia, pelargonium, verbena and argyranthemum. In colder regions such as the midlands, it is traditional to wait until the end of the month.
When buying plants, choose sturdy, healthy-looking specimens with lots of unopened buds. Don't be tempted by plants in full flower as these will soon tire out. Likewise, reject plants that are in dried-out compost (a feature in some supermarkets) as they may not have been properly looked after.
When mingling colours in a container, be just as careful as when planting a corner of the garden. Unless you want a jolly carnival effect, stick to one or two complementary flower-colours, or graduated tones of one colour. A line of pots filled with identical plants is an impressive sight and will distract the viewer from less-appealing parts of the garden.
Put a drainage layer of pot shards, stones or broken pieces of polystyrene in the bottom of the pot. Mix some pelleted chicken manure or other slow release fertiliser with the compost. Check the pots daily to see if they need watering (even if it has been raining) and pick off the dead flowers regularly.