Keep in mind when planting that Nairobi Purple is thought of as half hardy, so although it can survive a small mild cold snap, it is wise to ensure that this plant is protected from frost damage.
Sow
0.04 inches (0.1 cm) deep with a guideline distance of
11.7 inches (30.0 cm). Soil temperature should be kept higher than
18°C / 64°F to ensure good germination.
By our calculations, you should look at sowing Nairobi Purple about 56 days before your last frost date.
As Nairobi Purple is half hardy, ensure temperatures are mild enough to plant out - wait until after your last frost date to be on the safe side.