1. Introduction
Flower forcing is an operation or treatment to the plant, after it reaches the ripeness-to-flower stage, in order to stimulate it to flower at a specific date (e.g. on New Year’s day), or during off-season period. The flowering date/period may be earlier or later than the normal date/period of flowering.
The goals of flower forcing are off-season production and specific-date production.
Cut flowers which are available during the normal season are plentiful, thus fetching a low price. Sometimes the farmers have to sell their produce even at a loss. In some cases, flowers which could not be sold are either left on the plants or are spoiled after being harvested. Thus, it would be beneficial for farmers to produce cut flowers during the off-season period to obtain higher price, although the inputs may be higher.
Similarly, the demand for cut flowers is generally very high during certain occasions such as New Year, Christmas, Mother's Day, Memorial Day, Valentine Day, Graduation Exercise Day, etc. Thus, it will be to the farmers' advantage if they can produce cut flowers to be available on these specific dates.
The objectives of forcing plants to flower during off-season or at certain specific dates are:
2. Physiology of Flowering
The process of flowering follows the following sequence:
Proper stimuli (temperature & photoperiod).3. Forcing OperationRipeness-to-flower stage · Vernalin · Florigin · Flower.
Factors affecting flowering are photoperiod, temperature and humidity.
Flowering behaviour of plants is controlled by seasonal changes. There are two types of flowers with respect to the seasonal effect on flowering:
a) Little influence of seasonal changes: e.g. roses, marigold, chrysanthemum, heliconia, etc.Seasonal factors can be of various types viz.b) Great seasonal influence: e.g. jasmine, dendrobium orchids, etc.
i) Photoperiodic influence: This includes short day plants, which are temperature and humidity influenced; also long day plants, which are also temperature and humidity influenced.ii) Temperature influence: This includes low temperature requiring plants, which are photoperiodic and humidity influenced; as also, high temperature requiring plants which are also photoperiodic and humidity influenced.
iii) Humidity influence: Including low and high humidity requiring plants.
Flower forcing can be achieved by adjusting the factors effecting flowering behaviour, viz. photoperiod, temperature and humidity.
Chemical flower forcing can be done through the application of fertilizers, used both for retarding and stimulating flowering; and plant hormones, including gibberellin, growth retardants and growth inhibitors.
Mechanical flower forcing is achieved by operations such as, pruning, leaf trimming, ringing, budding or grafting, smoking, low-temperature storage and breaking dormancy.
4. Forcing of Some Cut Flowers in Thailand
Dendrobium Orchids
Flowers are available all the year round, but more profuse in rainy season and less profuse in winter (which happens to be the time when the demand is high).
Pinching-off flower buds in August/September (to save nutrients for later blooming) and application of special fertilizer (high in P & K) during October/November, 3 to 4 times, induces blooming from November onwards for December/January harvest.
Siam Tulip[12]
Seasonal flowers are produced during rainy season (June to August), requiring longday condition. No flower develops after September when shortday condition commences. Above ground parts wither and die down, and rhizome enters dormancy period until next rainy season.
Providing additional light breaks dormancy. Most effective is 3 hours of light in the middle of the night. Should be started soon after daylength is shortened (21 September). In this way, plants will continue to produce flowers all the way up to the New Year day, provided enough humidity and nutrients are given.
Marigold
Flowers are available round the year. It is a day neutral plant which takes 60 - 70 days from seeding to harvest.
Timing of flowering can be maintained by fixing the date of seeding 60 to 70 days prior of harvest date. The recommended period is 65 days. For example, if the flowers are to be harvested for New Year, seeding should be done on 27 October, transplant on 6 November and pinching on 22 - 24 November. Flower buds emerge by 5 December, and blooming starts from 25 December to 5 January.
Lotus
Flowers are available all the year round, but the plant needs standing water. Low temperature during winter reduces the amount of blooming. Thus, it is recommended that the level of water be reduced to 50 cm in order to raise water temperature. In this way, blooming will be the same as in summer, both in terms of amount and size of flower.
High temperature during the summer speeds up the growth of lotus plant and the blooming time, but flower size remains small. Thus, water level should be raised up to 75 cm (from original 50 cm level in the winter). This way, water temperature will be reduced and the amount of bloom and size of flower will be maintained.
During the rainy season, due to the addition of rain water the level of water may increase, thus it is recommended that the level of water be maintained at 50 cm in order to accommodate additional rain water. The blooming will be maintained as normal.
Chrysanthemum
It is a short day plant, with critical value of 14.4 hours. Thus, it will bloom all the year round under Thai conditions, having maximum daylength of 13.3 hours in June.
Daylength can be extended by giving artifical light after sunset for about 3 hours during the early stage of growth to keep seedlings in vegetative stage until one month prior of the planned harvest date. For example, if the planned harvest date is New Year, cutting should be made in September and transplanted to the growing plot for rooting to occur. Seedling is kept under light regime of more than 14.5 hrs by providing artifical light (100 w incandescent bulb) until 1 December (seedlings should be at least 30 cm high). These will bloom on 1 January.
However, as chrysanthemum blooms profusely during the period of low temperature which commences in December, it fetches a low price in the market even during the time of Christmas or New Year. Thus, some farmers avoid producing flowers during such period but shift it to the summer. The problem is that the temperature during summer is quite high and not optimum for chrysanthemum growth. The most suitable place of production of chrysanthemum flowers is on the highlands of northern Thailand where the temperature during the summer is around 16-20°C. The same principle of flower forcing is applied, but in this case the daylength in northern Thailand may be lower than the critical value for certain varieties. As practiced in Doi Phu Kha of Nan Province, black cloth is used to completely cover the plant house from 16.00 to 08.00 hrs. for 30 days after the cuttings have been exposed to long day conditions (supplemented with artificial light).
Jasmine
Flowers are available thoughout the year, but very profuse during the rainy season and scarce during the winter. To produce jasmine for winter-season harvest, the following operations are recommended:
(i) One month before the planned date of harvest, stop watering for 2 - 3 days until the plants show sign of wilting.Amaryllis(ii) Prune the plant to a round shape so that blooming will emerge from mature branches when induced in stage (iii). In this way the blooms will be of large size and healthy as they receive full sunlight.
(iii) Apply balanced fertilizer (e.g. 15-15-15) at the rate of 30 g/plant and water heavily. Keep watering normally everyday. Flower buds will emerge within 10 days and blooming will occur within 25-30 days after pruning.
Flowers are available all the year round, although depends on the availability of bulbs which are available from the Netherlands in the summer (June onwards). It takes about one month after the bulbs are planted to have full bloom ready for the harvest of cut flowers.
It is recommended to keep the bulbs in a refrigerator (4°C) for at least two months. The bulbs must be planted one month prior to the required date of flowering.
Gladiolus
Flowers can be available all the year round, provided the weather is optimum (requires cool climate). Blooms 90 - 100 days after planting.
In cool climate preheat corms before planting for 2 weeks at 27-32°C. This will force such corms to flower early. In warm climate soak in GA3 solution (10 - 25 ppm) before planting. This will accelerate flower by hastening differentiation of floral primordia.
Growth retardants (e.g. CCC) promote initiation of floral primordia (by reducing endogenous GA3 level, or counterbalancing its inhibitory effect on floral initiation.
Roses
Flowers are available round the year but more profuse in the cool season.
For blooming during Christmas - New Year, and Valentine Day, the following procedure is suggested:
(i) Cut the branches in November. This will stimulate flowering during Christmas - New Year. Flowering will take place 43 days after this pruning. Cutting the flowers on 23 December will further stimulate flower bud initiation to bloom on 10 February. Flowering will take place 49 days after this cutting.The following operations should be followed:(ii) Pinch (i.e. cut only the tip off) instead of pruning as in (i) above, on 10 November for 23 December harvest. This operation automatically stimulates flower initiation for 10 February harvest.
- Do not pinch healthy flower buds after 10 November as blooming will be due in 30 days. Allow them to bloom for 10 December harvest.- Do not pinch flower buds which are still very small.
- Pinch all other branches, including those flower buds which are not healthy, deformed, etc. Do not pinch too far down to healthy leaves, unless they are water sprouts which should be cut off from the base.
