Keep in mind when planting that Indian Summer is thought of as very hardy, so this plant will tend to survive through freezing conditions. Indian Summer tends to grow best in a soil ph of between 5.6 and 7.5 meaning it does best in weakly acidic soil - weakly alkaline soil.
Soil temperature should be kept higher than
18°C / 64°F to ensure good germination.
By our calculations, you should look at sowing Indian Summer about 49 days before your last frost date.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Indian Summer is a very hardy plant.