Plant Answers

Photoperiodism

Soon we will all experience the "delight" of moving the time on our clocks back one hour to adjust for daylight savings. Everyone enjoys the extra hour of sleep but have you ever wondered if daylight hours affect the life of a plant? A plant is drastically affected by its night life, especially how long or short the night life is. The same is obviously true for some people, too.

The effect of the length of day and night on plant growth and development is referred to as photoperiodism. Photoperiodism can be defined as the developmental responses of plants to the relative lengths of the light and dark periods. Here it should be emphasized that photoperiodic effects relate directly to the timing of both the light and dark periods.

The responses of plants to photoperiodic stimuli are numerous and very diverse. Some of the more important responses related to photoperiodism include: flowering, tuber and bulb formation, and bud dormancy.

The effect of photoperiod on reproductive development is widespread among plant species. Not all plants respond the same. There are three distinct groups: short?day, long?day and day?neutral plants.

1. Short?day plants will be induced to flower only when the day- length is below a certain critical length. If the critical day-length is exceeded, the plant remains in a vegetative state. The critical day-length varies among species and cultivars. Examples are chrysanthemums, poinsettias and Michalmas daisy.

2. Long?day plants will be induced to flower only when the critical day-length is exceeded. Until the day-length exceeds the critical length, vegetative growth continues. As with the short?day plants, the critical day-length varies both among and within species. Examples are spinach, beet, and radish.

3. Day?neutral plants will flower under any day-length and are exemplified by plants such as tomato, dandelion, rose and African violet.

Unfortunately, there is considerable confusion about photoperiodism, and much of it is the result of inaccurate terminology. The short?day plants are not named because they require a day-length that is necessarily shorter than a long?day plant. This is because that for a short?day plant to flower, the photoperiod must be shorter than the critical day-length. The long?day plant must have a photoperiod longer than the critical day-length.

Photoperiodic responses were taken into account this fall when a spectacular new bedding plant named “Mari-Mum” was introduced to the gardening public. Texas Cooperative Extension horticulturists discovered that fall-planted marigolds can give a sustained flower show from September until the first frost, or until late October or early November when it is time to "fall plant for spring bloom". Transplants that were just forming buds and flowers were sold. The formation of those buds and flowers were stimulated by manipulating the night life to insure immediate bloom. Another interesting discovery was that large-flowered marigolds actually bloom more profusely when planted in the fall. Long April and May day-lengths delay (but don't prevent) the flowering of American type marigolds. Day-lengths exceeding 12 hours both delay flowering and increase marigold height so, if you want a more compact, massive blooming marigold, plant it in the fall—not in the spring when most folks plant marigolds. This is why the Mari-Mum should provide the most spectacular fall bloom display imaginable. Day-length response was also considered when Extension horticulturists recommended that flowering tobacco (Nicotiana) be planted in April. Flowering tobacco responds and flowers more profusely in the spring day-lengths than it does in fall day-lengths.

The induction of tuber and bulb formation has several similarities to the onset of flowering. The photoperiodic stimulus is received by the leaves and is apparently transmitted to the part that actually enlarges to develop into the storage organ, such as tubers (in the Irish potato and Jerusalem artichoke) and bulbs (in the onion).

Gardeners always become impatient in the spring waiting for their onions to bulb. If you plant onion seed in the fall (October) or transplants in the spring (January?March) of recommended varieties for this area, such as Texas A&M 1015Y (Supersweet), Granex or Grano, the onions HAVE to bulb in the spring when the bulb?triggering day-length hours occur. However, if you plant bulblets or seed of the wrong varieties, the onions may never bulb. Bulblets should NEVER be planted because the proper day-length varieties are not available in bulblet form. The onion, depending upon the variety, bulbs during either long-day (13 to more than 14 hours), intermediate (11?1/2 to 12?1/2 hours), or short-day conditions (10 to 11 hours). Long-day varieties will not bulb in Central and South Texas because the days aren't long enough. In this area, long-day types are grown as green onions or scallions. Long-day varieties from seed or bulblets will produce huge plants, large necks, big blooms and no bulbs.

The onset of bud dormancy in many species is, to at least a large degree, a response to the shortening days of late summer and fall. In response to the lengthening days of spring, dormancy is broken and a new season's growth begins. The initiation of growth in the spring is also associated with the advent of rising temperatures. Since buds will not break dormancy in the short days of winter, the plant is provided with protection against premature growth, such as might occur in response to a midwinter warm spell. Fall fertilization (October) can also occur without the worry of rejuvenating plants and causing winter damage. In many deciduous species the initiation of growth in the late fall and early winter is prevented by the requirement for a minimum amount of hours of low temperatures to satisfy the chilling requirement. In the spring, however, no leaves are present, and the buds perceive the lengthening days.

When the photoperiodic phenomenon was initially discovered, the assumption was made that the critical aspect was length of the day. Subsequent research soon indicated, however, that the length of the dark period was the determining factor in whether or not a plant flowered. An interruption of the night with a few minutes of light will prevent a short?day (long?night) plant from flowering and promote flowering of long?day ones. An interruption of the day with a few minutes of darkness has no effect at all.

So as you can see, a plant's night life IS important. Now that horticulturists better understand this phenomenon, we can predict which plants to plant at which times of the year to insure optimum production of flowers and produce.

Mari-Mum Nomenclature and Bloom Explanation

The Mari-Mum is a "new" flower, which is actually a marigold rather than a chrysanthemum, that derives its name from being a marigold with a chrysanthemum flower form which is showier and longer blooming than most chrysanthemums (mums). As an added bonus, MARI?MUMS will be cheaper than most mums and perform several hundred percent better!

Well, now I've incensed the lovers and purveyors of mums. Sorry, but you mum folks have got to learn to deal with truth. Young mum plants transplanted in the fall never perform as well as the mums planted in the spring, pinched back several times during the summer so that plants are large and full?foliaged with a massive bloom bud surface. Mums planted as small transplants now never grow large enough before blooming begins to make a showy display. The bloom cycle is stimulated by decreasing daylight hours.

For more information about Mari-Mums, see:

http://aggie-horticulture.tamu.edu/cemap/marimum/marimum.html