Keep in mind when planting that Celtic Copper is thought of as very hardy, so this plant will survive though longer winters with little or no damage. Celtic Copper requires a loamy soil with a ph of 6.0 - 7.0 - it grows best in weakly acidic soil to neutral soil.
By our calculations, you should look at sowing Celtic Copper about 56 days before your last frost date.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Celtic Copper is a very hardy plant.