Keep in mind when planting that Astra Violet is thought of as half hardy, so it will need cloching or fleece protection when the temperature dips.
Look to ensure a distance
11.7 inches (30.0 cm) between seeds when sowing - bury at a depth of at least
0.04 inches (0.1 cm) deep. Soil temperature should be kept higher than
18°C / 64°F to ensure good germination.
By our calculations, you should look at sowing Astra Violet about 56 days before your last frost date.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Astra Violet is a half hardy plant.