Radish is a wonderfully useful little crop to grow in spring, as it will finish harvest by the first week of June, meaning it can be sown and harvested prior to sowing/planting out maincrop beetroot, maincrop carrot, leeks, dwarf beans etc.
Here in NW London, I have found that sowing radish direct before the end of March gives poor yields, so the earliest realistic harvest date for directly sown radish is around May 5-8th.
As I also find that the warmer temperatures see pests eating young radishes, the latest I tend to sow radish direct in spring is around April 20th, giving harvests up to around 8th June.
So, simply sowing outside limits the harvest to about four weeks.
So over the past two years, I have experimented with sowing radish in modules indoors with 160 radish seeds per 40 module tray. With rapid germination, this year I sowed modules on 25th February and transplanted the speedy half on 6th March and the slower growers on 13th March.
I sowed a second batch on 23rd March and transplanted them all on 31st March.
By doing this, I had six rows of 12 modules in a 1*1.5m area.
The aim is to harvest 12 roots a day for 20 days from April 20th, freeing the first five rows by May 7th to allow sowing of four rows of carrot maincrop.
The remainder of the radish season will then come from directly sown rows.
To harvest 12 roots a day (four each for three people) for fifty days means 600 harvested roots. With a sowing density of 200 roots per square metre and 1000 seeds sown, that implies 5 square metres of soil to cover the crop. Whilst this is 10% of my garden, I plant 12 square metres after the end of May with beans and leeks, whilst maincrop beetroot and carrot can follow earlier sowings and parsnip and carrot sown in April can be interplanted with radish too.
A nice way to add healthy bites to the salad bowl in spring.