Bedding plants are often on sale early in the year from March onwards and the temptation is to plant them out. Because bedding plants are tender, (which means they are not frost hardy,) they should not be planted out until the risk of frost has passed. Most areas of the UK are frost free by the end of May which means, although it is tempting to plant out earlier, it is best left until May.
It is not just northern areas which can suffer cold in May. Remember May 2017 when there was a sudden and unexpected frost and temperatures dropped to -6. This frost was widespread across the country damages gardens and crops to the extent that the grape harvest in the south was badly damaged.
You can plant out earlier provided you are ready to cover with fleece or cloche in the event of a frost. Knowing if a plant is fully frost hardy or not will also help when selecting plants for the garden.
If you have a greenhouse or lean to and can shelter plants under glass, it is a good idea to plant up a hanging basket or tub and grow on in the greenhouse. This will give the plants time to settle and establish roots in the container. Bedding plants grown under glass will need to be "hardened" off before planting out. This means getting the plant accustomed to the outside conditions and not moving it straight out from the calm, still and warm greenhouse conditions to the less perfect outside weather. Move the trays of bedding plants outside, starting on milder days and leaving out for longer as the weather improves until they are out all the time and fully accustomed to our weather.
Bedding plants have to grow a lot and produce lots of flowers quickly. It follows they do best in good compost and will always enjoy a feed during the growing season.
Dead heading is also essential to keep bedding plants flowering. In summer when in full bloom, dead heading may need to be done several times a week depending on the plant.
Bedding plants are also attractive to slugs, some more than others. For example, Marigolds are slug magnets, but Calendula are not. Protect with slug protection such as pellets or slug traps.
Some bedding plants are easier to grow and easy to grow than others. By way of example, Mesembryanthemum common name ice plant is a fabulous showy bedding plant, 4th image bottom left, but look at the dead heads in the image next to it removed in just one day. If a plant is not dead headed, it will cease to flower, but taking off 50+ spent flowers is very time consuming. Compare that with Pelargonium, image 3 top right, which needs much less dead heading and Nasturtiums, 1st image top left, hardly need any attention.
Check out Best Bedding plants for ideas on which bedding plants are slug resistant and which are low maintenance.