Re: What determines when the flowers of plants appear on the plants?

Ultimately, when a plant flowers is determined by its genetics. However, its 
environment, particularly light and temperature, often can promote or delay 
flowering. Plant age also often has a major effect on the ability of a plant to 
flower. Applications of certain plant hormones promote flowering in a few 
species. Other factors that can promote flowering are low-nitrogen fertilizer 
and small pot size, termed a potbound or rootbound condition, of potted plants 
such as African violets.

Many trees will not flower until they are 5 or 20 years old. Beech (Fagus 
sylvatica) may need to reach 40 years old before it will flower. The period 
during which a plant cannot flower is termed its juvenile phase. When it can 
flower, it is in its mature or adult phase. Some familiar houseplants, such as 
common philodendron and pothos, are in the juvenile phase and rarely enter 
their mature phase under home conditions. The century plant (Agave americana) 
doesn't actually take a century until it blooms. However, it takes at least 10 
to 15 years, sometimes much longer, and each shoot blooms just once, then dies. 

The effect of daylength, actually nightlength, on flowering is termed 
photoperiodism. Plants are classified into three main photoperiod groups. Day 
neutral plants are not affected by daylength. Short day plants flower only when 
the daylength is less than a critical daylength for that variety or species. 
Long day plants flower only when the daylength is greater than a critical 
daylength value for that variety or species. Sunflower, pea and corn are day 
neutral. Chrysanthemums and poinsettia are short day plants. Spinach and radish 
are long day plants. Different varieties of the same species may be in 
different photoperiod groups.

The plant pigment that senses daylength is phytochrome. The photoperiod 
category of a plant can change depending on the temperature. For example, 
poinsettia is a short day plant under warm conditions but long day plant under 
cold conditions. There are also several other photoperiod groups. For example, 
there are intermediate day plants, such as coleus, that flower when days are 
neither too short nor too long. Plants use the daylength as a calendar so they 
flower in the appropriate season.

Plant flowering is often given a time classification for cultivation purposes. 
Seed catalogs and vegetable seed packages have a cropping time in days which 
can be an indication of how soon they flower, if the crop produces a fruit, 
such as tomato or corn. Even chrysanthemums and poinsettia varieties are 
classified by the number of weeks it takes them to flower once they are exposed 
to the required photoperiods. There are 10, 11 and 12 week varieties of 
chrysanthemums for example. 

Biennial plants grow from seed the first season, overwinter as a rosette of 
leaves, then flower in the spring of the second season. Carrot, some beets, 
cabbage, celery, hollyhock, foxglove, and money plant or honesty are biennials. 
They require a cold treatment before they will flower. Many also require long 
days after the cold treatment in order to flower. A cold treatment that 
promotes flowering is termed vernalization. Winter wheat is planted in fall or 
winter so it can be vernalized by the naturally cold temperatures allowing it 
to flower the next summer.

Botanists long ago speculated that there was a flowering hormone that 
stimulated a plant to flower. They even gave it the name florigen. However, no 
chemical has yet been identified as florigen. Many books and websites 
incorrectly state that florigen actually exists but we are not really sure. 
There are grafting experiments that show that something that promotes flowering 
is transmitted across a graft union from a plant induced to flower to a 
vegetative plant. The transmitted chemical or chemicals stimulates the 
vegetative plant to flower. Therefore, there is evidence that a chemical or 
combination of chemicals acts as a flowering hormone. The frustrating part is 
that despite the evidence no specific chemical(s) has been identified as 
florigen.

Some plant hormones do promote flowering in a few species. For example, the 
hormone ethylene promotes flowering in pineapple and other bromeliads. Home 
gardeners who grow pineapple tops often enclose the plant and apple in a 
plastic bag. The apple gives off ethylene. Treating a biennial rosette, such as 
carrot, with the plant hormone gibberellic acid stimulates it to flower. The 
gibberellic acid substitutes for vernalization. Application of gibberellic acid 
prevents flowering in other species, such as strawberry and fuchsia.


References


Photoperiodism and Phytochrome


Agave americana


Florigen


How to grow a pineapple at home


EFFECT OF GIBBERELLIC ACID (GA3) ON SEEDSTALK DEVELOPMENT AND FLOWERING IN 
CARROT (DAUCUS CAROTA L. VAR. SATIVA DC)

Taiz, L. and Zeiger, E. 1991. Plant Physiology. New York: Benjamin/Cummings.

Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth.