Regarding Dandelions,
I have seen over many years that this plant when in flower exhibits 
periodic opening and closing of the yellow heads.

To such an extent that at mid morning there is little else on my lawn 
but dandelions - then by early afternoon, back to a normal green. Rain 
will cause premature closing of the flower heads.

See link below:
http://www.timeproject.org/Content/Utils/ContrDetail.asp?itemid=415

For those who choose not to visit the site noted, the dandelion opening 
and closing times average is listed as :

Dandelion (Taraxacum officionale) 5-6 a.m. -- 2-3 p.m.

Regards,
Peter

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