Keep in mind when planting that Double Pleat Blackberry is thought of as hardy, so Double Pleat Blackberry will tend to go dormant or grow slowly over the winter months. Double Pleat Blackberry requires a soil ph of 5.1 - 6.0 meaning it does best in weakly acidic soil.
By our calculations, you should look at sowing Double Pleat Blackberry about 56 days before your last frost date.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Double Pleat Blackberry is a hardy plant.