The mulch is laid down on either side of the support where the vanilla roots will grow. To prevent the loss of mulch from runoff from heavy rains, most prevalent in vainillales managed on slopes, borders are constructed out of trunks of wood, bamboo canes, rocks, or other materials.
New applications of mulch are made when roots are observed growing out of the surface of the mulch, generally 2–3 times/year, and mostly in the hot/dry months, when mulch is carefully managed to prevent dehydration.
The most important nutrients for vanilla are calcium, potassium, nitrogen, phosphorous, iron, and copper (Cibes et al., 1947; Childers et al., 1959; Domínguez, 2005; Ranadive, 2005). The normal or optimal levels of nutrition required by vanilla has not been studied in detail, and in practice, vanilla nutritional requirements are inferred from horticultural species of other members of the Orchidaceae (A.S. Anderson, pers. comm.).
Generally, vanilla is not fertilized beyond provisioning of mulch. In India, growers are encouraged to apply 40–60 g of nitrogen, 20–30 g of phosphorous (P2O5), and 60–100 g of potassium (K2O) per plant per year. Foliar sprays are also recommended such as 1% applications of Triple 17 fertilizer (17:17:17) once per month to stimulate growth and flowering (Anandaraj et al., 2005).
Management Activities
Weeding is done by hand. Between rows, tools such as a hoe or machete are used, but at the base of the plants themselves, weeds are carefully pulled out by hand so as not to disturb the shallow rooting structure of the vanilla plants. Once deracinated, weeds that are annual herbs can be added to the mulch or composted and later added. Perennial weeds such as Commelina diffusa and Syngonium podophyllum are removed from the vainillal because they do not readily decompose. Weeds should be dealt with whenever they impede access to the vanilla plants and/or when support trees defoliate in a disproportionate amount. In general, weeding is performed 3–4 times/year.
In vainillales with living support trees such as Erythrina sp. or Gliricidia sp., shade is controlled by periodic pruning, usually two or three times/year.
Pruning should be timed to take place in the rainy season to avoid the development of diseases in vanilla due to inadequate sunlight. Shade levels are between 30% and 50% during the rainy season. In dry and hot times of the year, which coincides with flowering/pollination and fruit development, support trees should have a denser canopy to provide 70–80% shade, which conserves humidity, prevents burning from intense sunlight, and decreases the incidence of young fruitdrop.
Pruning is accomplished by removing the thicker central branches and leaving the laterals in order to achieve a canopy in the shape of a parasol that also maximizes the equitable distribution of vanilla shoots. Branches are pruned with either saws or machetes, down to about 40 cm from where they diverge from the trunk. The thinnest of the cut branches are broken into longitudinal pieces and placed at the base of the support as an additional source of organic material. Thicker branches are removed from the vainillal entirely. Overpruning results in sunburns to the vanilla plants, and should be avoided.
In cultivation systems with artificial or “dead” supports, 50% shade is provided year-round by shade cloth. This system allows for uniformity of shade and negates the cost of having to periodically prune, but in hot months the 50% shade has been observed to be inadequate.
The most common practice involving shoot management is “looping,” that is, redirecting a growing shoot over a branch and toward the ground once it reaches the height of the first branches of the support tree. This practice maintains the height of the vanilla at roughly 2 m, facilitating hand pollination and harvesting. Another consequence of looping is hormonal induction promoting flowering and new shoot formation (usually just below the height of the fork in the tree where the shoot is bent). Shoots are managed so that they are equally distributed among the branches of the support tree such that no shoot shades out another.
Once a shoot has been looped and has reached the level of the ground, a portion of it, usually 2–3 internodes long, is buried, leaving the growing apical meristem uncovered. This practice promotes root formation at the buried nodes. The shoot apex is fastened back to the support tree to continue growth. Rooting of shoots is performed every instance a new shoot has reached ground level, which helps maintain the vigorous growth of the plant that obtains more nutrients and is more resistant to F. oxysporum. In this way, rooting helps counteract the mortality of plants due to pathogens (Hernández Hernández 2005). Alternatively, for plants that are at least three-years old and are near or at ground level, a grower may elect to trim off the apical growing tip (ca. 20 cm) to induce flowering in lieu of rooting (Anonymous, 1998, 2004).
Another technique for shoot management is to let the shoots attain a height of 1.5–2 m, at which point the shoots are trained onto horizontal supports such as bamboo canes, hoses, or plastic PVC pipes.
Keeping vanilla plants free of pests/disease requires frequent oversight and consists of removing any parts of the stem, leaves, or roots that manifest disease. Sometimes entire plants require removal to terminate the spread of disease from plant to plant. Diseased material is burned or buried outside the vainillal to eliminate sources of inoculum. Leaves attacked by pests are also removed.
Flowering and Pollination
In general, the first flowering happens three years following planting. When Citrus spp. are used as supports, or when vanilla is cultivated in shade houses, flowering initiates in the second year, even when smaller cuttings (80–100 cm long) are used, since the plants tend to grow more vigorously as a result of more consistent shade and management. Vanilla normally flowers once a year.
The physiological cue to flower is promoted by climatic or mechanical stress. The following are some examples:
a. Drought: Water stress induces reproductively mature individuals of vanilla and many other plants to flower. In Uganda, vanilla flowers twice a year, a consequence of the country having two distinct dry seasons (Anonymous, 2000, 2005).
b. Cool temperatures: The principal stress in Mexico that induces flowering are the low temperatures of autumn–winter, when cool air masses known as “nortes” blow down more or less unimpeded from the Arctic Circle, dropping temperatures to below 10°C; the lower the temperature, the greater the expectation of a good flowering year. The cool temperatures “burn” the apical tip, killing it, and break the apical dominance of the plant while stimulating lateral floral buds to develop.
c. Plant management: In India, growers are recommended to perform a series of tasks two months before flowering: (1) pruning of the apical tip, at least 10–15 cm, (2) temporary suspension of irrigation, (3) abundant use of irrigation once 10% of blooms have opened, (4) pruning of the support tree canopy up to 75% luminosity (Ranadive, 2005), but only when the intensity of sunlight is not high to cause burning (light is an important factor in activating floral buds; Hernández Apolinar, 1997), and (5) allowing the shoots to reach a height of 1.5 m and training them on a horizontal support to increase flower quantity and the number of aerial roots.