When I was a kid in the 1950’s I ordered this variety from the long gone Murvon Seed Company based in Norwalk, Connecticut along with Yard Long Beans and Japanese Climbing Cucumbers and it started a life long curiosity about those vegetables we never see in the market.  Sakurajima is the mammoth round white radish that can reach 100 pounds.  Mine grew only softball sized from a midsummer sowing but I remember them to be just about the best radishes I ever grew.  Listed here, only for the nostalgia.  
70-90 days   

200 seeds  

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Garden Radish
A packet sows 10 ft row (250 seeds)

You can sow radish seed around the average last frost date;  mid April in zone 7 and continue to sow seed every 2-3 weeks through early summer and again in late summer for production into early winter.   You will have plenty of radishes over a long harvest period by succession planting.  

Sow the seeds in drill where they are to grow thinly at a rate of 20 seeds per foot and cover the seed with 1/2 of fine soil.   The best radishes grow rapidly in potassium rich soils rich in trace elements.  Drought and other stress conditions delay growth adversely and affect the crop.  Hot temperatures seem to affect even mild radishes making them spicier.

A late crop of oriental type radishes heavily mulched will allow you to harvest the roots into the winter.  They can also be stored as a “root cellar” crop where winters are too cold. Small, early radishes so not keep well though the winter.

Most radishes will mature for harvest and if left in the ground past maturity will bolt into seed if they are started from seed early enough in the spring.  The abundant pods should be swollen and start to turn gray.  We generally will pull the plants and then allow them to further ripen in a dry area.  The pods have to be thoroughly dry to easily thrash the pods (on a tarp) and release the seed.