Hardy annuals can be sown direct outside, where they are to flower. These do best on 'poor' soil, so do not enrich it with fertiliser. Half-hardy annuals such as dahlias and cleome, and perennials are best sown under glass at this time of year.
Here are just some of the flowers that you can start to sow in March.
Annual flower mixes can be sown in March. As with hardy annuals, prepare the soil well by removing weeds and raking to a fine tilth, but do not enrich the soil.