[Robster]

Yesterday I sowed some Florence fennel.  The packet said May.  But I sowed anyway and in a sunny spot under a cloche.  My reasoning was, of course, that the cloche will keep them warm enough and I can keep it on until the frosts are over.  But some plants are more sensitive to daylight hours for germination than others.

What are the plants that are most sensitive in this way so that they cannot be started earlier in a cloche?

Is fennel amongst them?  I guess this is a question the Polytunnel Boys have all the time.

[JayG]

Florence Fennel is a crop I've tried several times to grow, but with mixed results - not very good, and not at all good (bolting.)

Germination isn't the problem, although it needs a certain amount of warmth and moisture of course - like the vast majority of seeds it doesn't need light to germinate, but it is sensitive to daylight length as well as temperature.

Interesting and hopefully useful article HERE

[Mr Dog]

Have always found that sown before midsummer's day it invariably bolts, sown after, the 'bulb' never gets big enough to be worth it.

[Goosegirl]

I have never heard of this before, so a new one for me. If your seeds are covered with compost, how does the extra daylight hour make a difference?

[DD.]

If by "extra daylight hour" you're referring to the clock change GG, we don't have an extra hour, the daylight starts at a different time, that's all.

Later in the year we have nearly four hours extra daylight and this can make a difference both to germination and growing.

A bit on it from T & M here:

http://www.thompson-morgan.com/effect-of-light

[gremlin]

I read somewhere that the light needed can be specific wavelengths of the infra-red light found in sunlight, which does penetrate deeper into the soil then visible light.
Now to go and try and find the reference.....

Love the bit in the article that tobacco seeds only need 1/100 of a second of light to wake up.