By our calculations, you should look at sowing Japanese Imperial Long about 14 days before your last frost date.
Sow in deep, well-worked, stone-free soil after danger of heavy frost from spring to midsummer. Do not transplant, as crooked roots may result. Cultivate periodically or mulch to control weeds. Keep soil evenly moist.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Japanese Imperial Long is a very hardy plant.By our calculations, you should look at planting out Japanese Imperial Long about 14 days before your last frost date.