Keep in mind when planting that Castle Scarlet is thought of as tender, so remember to wait until your soil is warm and the night time temperature is well above freezing before moving outside. Castle Scarlet requires a soil ph of 6.1 - 7.5 meaning it does best in weakly acidic soil - weakly alkaline soil.
By our calculations, you should look at sowing Castle Scarlet about 49 days before your last frost date.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Castle Scarlet is a tender plant.