As well as the advice elsewhere in the thread about reducing soil fertility to prevent grasses outcompeting wildflowers, it's worth noting that many species may

As well as the advice elsewhere in the thread about reducing soil fertility to prevent grasses outcompeting wildflowers, it's worth noting that many species may take more than one year to establish. Annuals like — in the UK — poppies and cornflowers are more likely to grow the first summer after they're planted so can be used as attractive cover whilst perenials are taking seed.