Poinsettias
Why do both dandelions and some violets flower in early spring and late fall? Why do other plants flower only in the spring, others only in the summer and still others only in the fall? Here in the Temperate Zone, we know that plants must survive and mature seed within the growing season. We also know that in the spring and fall, day lengths are short, and that June 21 is the longest day. Surely temperature and the varying length of light and dark periods provide the triggering mechanism for flowering.
Mary and I and our four children spent 15 months in Cali, Colombia. I had a sabbatical from Heidelberg College and was sponsored as a special field staff member of the Rockefeller Foundation. My assignment was to help establish a young biology department at the Universidad del Valle in Cali, to work with staff members, and to do tropical research.
Calli is only a few degrees north of the equator and there is a small part of Colombia south of the equator. At the equator the day and night lengths and the temperature remain constant throughout the year.
Yet, it was observed that certain species of trees and herbaceous plants flowered at the same time every year. It appeared that the amount of light and temperature did not influence flowering time. There were distinct wet and dry seasons that might provide a clue. It was also noticed that there were some species of trees, especially the oaks that had originated in the North Temperate Zone. They had migrated down the Mountains of Central America to the Andes and still maintained a regular rhythm of fall coloration and leaf fall.
After a search for long-term environmental records from government and private sources, and, after pursuing this question for only one year, I had to conclude in a paper that was published at the University, that I could not shed any light on the triggering mechanism that resulted in the production of flowering hormones.
Poinsettia with its large, mostly red bracts (modified leaves) is one of the most favored Christmas flowers. It is a tropical American species of Euphorbia. In its native habitat, it is suitable for garden culture and is considered to be a shrub that may grow up to 10 to 20 feet tall.
If you would like to care for a poinsettia and keep it for a second season, there are certain conditions that need to be followed to have the plant flower for next Christmas. First, the plant needs to be protected from drafts, placed where it receives the most light and with a daytime temperature of around 70 degrees and a nighttime temperature of no lower than 60 degrees. Water every day from above and never let the soil dry out, nor allow the soil to become soggy. Excess water will shorten its attractive display.
Under the most perfect conditions, the plant will drop its leaves late in the winter until only a few of the bracts remain at the top. The plant is in a natural resting period and water should be gradually withheld.
When only naked stems remain, store the pot in a warm dry place and stop watering. In May remove the old dry soil, repot with good loam or compost. The stems may be cut down to two or three eyes (nodes) from the base, although if you want more of a shrub appearance, the central stems are left longer than the outer stems. Then water the plants; sink them into the ground outdoors in the sunniest location. In a very short time the plant will resume growth. The plants require a lot of light and moisture during the summer. Occasionally add a water-soluble fertilizer.
In late September or before frost, bring the plant in and provide a sunny location and a temperature of 65 degrees or above. About the first of October make sure that the plant receives 12 or more hours of uninterrupted darkness. The continuous long night hours are crucial for promoting the flowering response. Poinsettias are thus classed as a short-day type. Technically it should be called a long night type. Once the flowering response is observed, the plant is placed in the best light conditions.
Some plants, for example tomatoes do not require any set hours of light or dark in order to flower. They are called day-neutral. They will flower any time once a certain amount of vegetative growth occurs and the temperature is correct. This is the basic reason why tomatoes can be grown in greenhouses without having to pay attention to day-night lengths.
In February 1998, I planted six tomato seeds in our solar greenhouse. There is no supplemental heat besides the solar energy. About 400 gallons of water, in barrels, and a black-painted concrete block wall in the back of the greenhouse provide a heat sink.
The six tomato plants were placed in a bed of soil about 12 inches deep. Three were staked and three were allowed to drape over the sides. They have continuously produced tomatoes since June of 1998.
Foliar applications of fertilizer were irregularly applied. One time during the summer of 1999, these plants received no water for almost two weeks. With little care four of the six plants are still alive and are producing all the fruit that Mary and I can use in salads as well as BLT’s. At this time and after about one month of little sunshine and low night temperatures, the tomatoes have grown very little and there are no new flowers.
Apparently we have a type of tomato that has indeterminate growth. That means that the stems and branches may continue to grow and continuously produce new flowers and fruit. There is another type of tomato that has determinate growth and most of the flowers and fruit mature at the same time. These tomatoes can be machine-picked.
Many plants have a determinate type of growth and once the flowering response is initiated, whether it is a short-day or long-day plant, it will have no further vegetative growth. All of the plant’s energy is directed to the production of fruits and seeds. They can sometimes be fooled into continued flowering by picking the old flowers.
– Percy