Illinois State Florists' Association 401
Day-Length Response All marigolds will bloom rapidly under long night conditions. In fact, some of the new dwarf 7. erecta hybrids are being grown for sale as winter pot plants because they will set large blossoms while the plants are quite short. And in mild winter areas, dwarf marigolds are often planted right after Christmas for very early spring bloom. Butthis very trait makes old fashioned tall marigolds slow to bloom during mid-summer when the nights are short. Fortunately, certain breeding lines of 7. erecta have been developed that respond only moderately to changes in day length so that they will bloom early and not sulk during the dog days of summer. If seedlings of American marigolds sown after March 15 are given 2 weeks of 9-hour days immediately after germination, they will bloom much earlier and at a shorter height. You can control the length of days by covering the seedling flats with black cloth in late afternoon.
Source: • Illinois State Florists' Association Bulletin # 401
Keywords: Day-Length Response Marigold Odor Hybrids Open-Pollinated Germination Nematode