Why does Black Radish bolt?

The most common failure with Round Black Spanish Radish in Edmonton, Alberta is bolting. When you get bolting you don’t have even a chance to complain about root size and shape. Why is it bolting? To understand plant behaviour I start to look for crop origin. A few literature sources are pointing down to its Mediterranean history. The word Spanish in the name of this radish indicates that it came to America with Spaniards. I would not be surprised if I found that it was widely cultivated in Spain at this time. Having said that I think most likely black radish outstanding storability  was the main factor in getting  ticket to the conquistadors ships.
Mediterranean is around 35 latitude with the longest day roughly about 14 hours. The longest day in Edmonton is 17 hours. Experiments with day length documented that bolting gave a clear response to day length. At day lengths of 19 and 24 hours the plants bolted very quickly, at 14 hours more slowly, and at 8, 10 and 12 hours very slowly. That explains why late seeding is recommended in my region. By seeding after summer solstice we are shifting plant development phases to the period with shorter days that are similar to the  day length in Mediterranean. Another option to bit the day length is to plant early. It is not very visible in Edmonton due to cold weather but it should work well in south Ontario and USA. Right now I am looking forward to spring 2012  as it is  promising to be early  and I am going to try early seeding. Visit my blog in June to see results.

The following picture shows the day length for 35, 50 and 55 latitude.

Noteworthy that this rule does not work straight forward for selected material. Selection may be result of an intensive professional breeding or result of natural elimination by years of cultivation in the same area. For example, with  intensive breeding in red radish the nature was broken and we have some varieties bred for long days and some for short days. It means seed source is also a factor but not so much for heirloom as for new varieties.

It would be very simple if the day length was only a factor of bolting. Stress is very important also. It could be lack of water, extremely low temperatures or extremely high  temperatures, soil salinity or acidity,  Not all plants react the same way to similar stress but all of them has some mechanism of surviving under the pressure. For many plants accelerated switch from vegetative growth to generative growth is one the most common response. This mechanism get incorporated in plant genome by thousands years of natural selection. You might be interested to know that greenhouse tomato growers use this response in commercial production?  To stop vegetative growth and initiate formation of fruit they cheating plants by cooling greenhouse at night. Plants react on it in the same way as they response to cold fall nights. As night getting cold tomato plants start to use all resources for fruit formation instead of vegetative development.

So, stress is important and if you don’t like your radish to bolt do everything to prevent stress condition. In Edmonton is very important irrigation  as our air here is very dry and soil dry fast. In my view the most popular top  surface irrigation, such as sprinkle one, is not very good for any crop with deep root system. Watering the top layer of soil you reduce root development going down and when hot summer days come your plants will be under stress for sure as top layer dry fast.