Vanilla

To reach optimal conditions for growth and production, vanilla cultivation requires the following agro-ecological parameters.

Climate

V. planifolia thrives in hot-humid tropical climates.

Temperature

V. planifolia grows best in temperatures ranging from 20°C to 30°C (Childers and Cibes, 1948; Ranadive, 2005), and may tolerate high temperature of 32°C (Purseglove et al., 1981; Anandaraj et al., 2005). Temperatures reaching below 20°C inhibit plant growth and flowering intensity (Ranadive, 2005); temperatures exceeding 32°C cause yellowing of vegetative parts and premature fruit drop (Anandaraj et al., 2005; Hernández Hernández, 2007b).

Precipitation

V. planifolia requires an annual average precipitation from 2000 to 3000 mm (Sasikumar et al., 1992; Soto Arenas, 2003), it is well distributed throughout the year except during flowering/pollination. Since heavy rains may diminish successful pollination and fruit set, it is best to irrigate the plants at their bases during flowering. V. planifolia needs 2–3 relatively dry months to stimulate flowering. In areas where average annual precipitation exceeds 3000 mm, plants are more prone to fungal attack (e.g., Fusarium sp.). At the other extreme, that is, where precipitation is less than 2000 mm, and where a system of irrigation is not in place, the lack of water greatly compromises the ability of the plant to perform basic physiological functions.

Altitude

The best altitudes for cultivating V. planifolia are between the sea level and 600 m (Childers et al., 1959), although cultivation systems do occur as high as 1100 masl in Mexico (Soto, 2003). In India, V. planifolia is reported to be cultivated up to 1500 masl (Anandaraj et al., 2005; John, 2005), and in Uganda, cultivation is successfully practiced between 800 and 1200 masl.

Light/Shade

V. planifolia demonstrates most vigorous growth under 50% shade. In dry periods with intense sunlight, it is preferable to use 50–70% shade (Hernández Medina, 1943; Ranadive, 2005) for better conservation of soil and air humidity. In rainy periods, the amount of shade should be reduced to 30–50% to avoid creating favorable conditions for growth of pathogens.

Excess shade causes weak growth and poor flower production, while excess sunlight leads to burning of the leaves and stems, as well as early fruit drop. Plants that suffer from either too much sunlight or shade are the ones most likely to develop diseases.

Site Selection

The first step in designing a successful vanilla cultivation system is to choose a site to plant.

The land destined to be used for the cultivation of vanilla (vainillal) should have an excellent drainage, rich humus content, and a pH between 6 and 7 (Childers et al., 1959; Soto, 2003; Ranadive, 2005); the land should receive morning sunshine and not intense rays during afternoon, which can lead to the sunburn damages on the leaves (Sánchez Morales, 1993; Curti, 1995; Soto, 2003). Limestone soils on slight slopes are the most appropriate substrates for cultivating vanilla because they tend to be less acidic and are well-drained (Childers et al., 1948; Bouriquet, 1954; Dequaire, 1980; Ranadive, 2005); acidic soils are the least recommended because they favor the establishment of fungal pathogens (Soto, 2003).

The areas where vanilla cultivation has been practiced in the recent past are less desirable because they are likely to contain remnant populations of Fusarium oxysporum.

Land Preparation

The most prevalent form of vanilla cultivation in Mexico is the vainillal: a secondary forest regrowth the managed to cultivate vanilla. The first step in its preparation is to thin the forest to increase luminosity and space, leaving only those trees or shrubs that would serve as supports. Where deforestation has occurred, weedy herbs are cut and left to compost on-site in lieu of being burned. On level ground, where the soil may become waterlogged, drainages are dug to eliminate excess water. On steep terrains, terracing is done to control erosion, mitigate runoff of organic material, and to conserve soil humidity.

Support Trees

The primary function of the support tree is to provide an appropriate framework and microenvironment for the growth and management of the clambering vanilla plant. In addition to providing physical aid, the support trees give shade and, in deciduous or semideciduous species, apportion organic material. The choice of the support tree should be the species that is most adapted to the area, and feature the following characteristics: (1) maintain a canopy throughout the year, (2) be easy to propagate and able to reestablish readily, (3) respond positively to periodic pruning for form/shade management, (4) lack spines and leaves near its base, (5) be strong enough to support the weight of the vanilla plant, and strong winds, (6) possess bark that does not shed,(7) be fairly resistant to pests/diseases, and (8) have a deep root system that does not compete in the shallow soil layers for nutrients and water with the vanilla plants.

In the low-intensity vainillales of Mexico, where other crops and species of economic importance are managed, the species chosen for supports are the shrubs and trees that grow in that area. Fruit trees are also chosen: orange (Citrus sinensis), grape-fruit (Citrus grandis), and mandarin (Citrus reticulata). In intensively managed monoculture of vanilla (Figure 6.1), support trees must first be planted. The following are the most commonly planted vanilla support trees in the world:

Erythrina sp. (Mexico, Costa Rica, India, and Indonesia)

Gliricidia sepium (syn. G. maculata ) (Madagascar, Tonga, India, Indonesia, Reunion Island, and French Polynesia)

Leucaena leucocephala (Tonga and Indonesia)Casuarina equisetifolia (Madagascar, Tonga, Reunion Island, and India) Jatropha curcas (Madagascar, Uganda, Tonga, Reunion Island, and French Polynesia)

Plumeria alba (India)

FIGURE 6.1 Intensive cultivation of vanilla on living supports (Erythrina sp.).

In the most recently established system of vanilla cultivation, artificial or “dead” supports are used (Figure 6.2). In this case, the function of the support is purely physical, while the shade is provided by 50% shading cloth (green, red, or black), which is stretched above all the plants at ca. 3–5 m high, on the four sides of the planted area. These systems are referred to as “shade houses.” In size, they are usually on the order of 25 m × 40 m (1000 m²), and some are up to 1 ha (Hernández Hernández 2007a).

FIGURE 6.2 Vanilla intensively cultivated on concrete support under shade houses.

Establishing Living Support Trees

Supports such as Erythrina sp. and Gliricidia sp. are propagated mainly through stem cuttings, although some species such as L. leucocephala are propagated by seed.

In Mexico, transplanting and establishing the supports can be done at any time of the year, but is most successful when done at the onset of the rainy season. Cuttings are normally 1.5–2 m long and at least 5 cm in diameter. Cuttings are planted at about 30–40 cm depth, and six months to one year in advance of establishing the vanilla cuttings.

The most common planting distances between supports are: 1.2 × 1.50 m, 1.5 × 2.5 m, 2.0 × 2.5 m, 1.8 × 2.5 m, and 2.0 × 3.0 m, or a density of approximately 1600–5000 supports/ha (Childers et al., 1948; Dequaire, 1980; Tiollier, 1980; Sasikumar et al., 1992; Soto, 2003; Anandaraj et al., 2005; Ranadive, 2005). Smaller planting areas are problematic due to insufficient air circulation, which lead to diseases (Childers et al., 1948; Soto, 2003; Ranadive, 2005). For each support, 1–2 vanilla cuttings are planted.

Planting distance between fruit tree supports varies with the species. For orange, the planting distances are normally 4 × 4 m, 5 × 5 m, 6 × 6 m, and 7 × 7 m (roughly equivalent to 204–625 supports/ha). For each orange tree, 3–6 vanilla cuttings are planted, producing an estimated 1224–1875 vanilla plants/ha.

In polycrop systems that include vanilla, usually associated with coffee, banana, and coconut palm cultivation, it is common to use Erythrina sp. or Gliricidia sp. as vanilla supports, planting them at distances of 1.5 × 2 m. These are smaller densities than are encountered in monocrop systems, but have the advantage of allowing for at least two products to be exploited.

When used, the native tree species are planted at variable distances, depending on the standing distribution of those that are selected, and can be 500–1500 supports/ ha. In shade houses, supports are spaced 1–1.5 m between each tutor and 2–2.5 m apart between each row. In 1 ha, 4000–5000 supports are set up, with 1–2 vanilla plants/support. Vanilla generally thrives in shade house cultivation. However, at the reproductively mature adult stage, their management becomes difficult and tedious, air circulation is reduced, and the spread of pathogens can be very rapid and devastating (Ranadive, 2005).

Vanilla is propagated almost entirely by stem cutting. The cuttings are procured from another grower or from a government agricultural entity. Cuttings are made from highly productive and vigorous individuals, which are selected and marked before the harvest. The cutting itself should not be a flowering shoot and should have at least three nodes with viable axillary buds for producing new shoots from which the plant grows. Cuttings should be free of damage or of symptoms of pests/diseases so as to avoid future proliferation of the disease.

Cuttings are normally 6–8 nodes (80–120 cm long, 1 cm in diameter) in length. Longer or thicker cuttings form new vegetative and reproductive shoots more rapidly (Ranadive, 2005), but are more difficult to deal with during planting and are also more expensive. Cuttings that are less than 60 cm long are best managed as nursery plants before they are transplanted permanently (Anandaraj et al., 2005; Ranadive, 2005).

In Vitro Plants

Another option for sourcing and reproducing commercial vanilla stock is in vitro propagation. In vitro propagules are accustomed in having their nourishment provided, and so before they are transplanted directly into a cultivated area, they must first be placed in climate-controlled greenhouses, where they can adapt in providing their own food via photosynthesis. These propagules are transplanted as soon as they reach a height of about 30 cm.

In Vitro Propagation

Vegetative growth proceeds slowly following establishment, usually taking at least one year before the plants reach the flowering/fruiting stage. Plants established from in vitro propagules are more phenotypically uniform and healthier, and consequently yield higher than vegetative cuttings. The cost and care of in vitro propagation is prohibitively expensive for many vanilla farmers.

Preparation and Disinfection of Cuttings

Cuttings are prepared prior to planting. The three most basal leaves are removed by hand by twisting at the petiole and taking care not to tear into the stem where open wounds can facilitate the spread of pathogens.

In order to prevent stem rot, caused primarily by F. oxysporum, stem cuttings are disinfected prior to planting. The basal portion of the cutting is submerged for 2–5 min in a fungicidal solution. The solution may consist either of carbendazim (2 g/L) or Bordeaux mixture (1 kg lime + 1 kg copper sulfate in 100 L of water), the latter being less effective but authorized for the production of organic crops. Fungicidal solutions are handled with rubber gloves to avoid harmful exposure to the human body.

After disinfection, cuttings are hung separately on a structure 1–1.5 m tall, in a shaded and well-ventilated area for a period of 7–15 days. The cuttings slightly dehydrate allowing for more flexible material for planting. Calluses form over areas of the cuttings that were damaged during leaf removal.

Establishing Cuttings—Timing

Cuttings are planted when support trees have developed sufficient foliage to prevent the young vanilla plants from sunburns. With shade cloth, cuttings are planted immediately after the establishment of support trees.

Cuttings can be planted practically at any point during the year given a sufficient availability of water. In the winter, new vegetative shoots are slower to develop and may be “burned” by low temperatures, which slow the overall development of the plant. In the rainy season, excess humidity can cause up to 50% of cuttings to suffer some degree of rotting.

The best conditions for planting cuttings are in humid substrates during warm, dry months preceding the onset of the rainy season (Ranadive, 2005). This timing favors a high percentage (>90%) of successful establishment of cuttings since high temperatures are conducive to the emergence of new shoots and roots.

Establishing Cuttings—Planting

Cuttings are planted in the following manner: adjacent to the support, a shallow ditch is dug 5–10 cm deep, into which the cutting is placed horizontally (but only the part that has had the leaves removed). The cutting is then buried with 3–5 cm of organic material and/or fertile soil or leaves, which will serve as mulch and as a source of nutrients. The extreme basal end of the cutting (2–3 cm) is left uncovered to prevent rot (Lepierre, 1988; Wong et al., 2003; Ranadive, 2005), especially when the substrate is humid. Some cuttings are established without making ditches, and are placed on top of a humid substrate.

Once planted, the rest of the cutting (with leaves, ca. 4–5 nodes) is positioned vertically on the support and fastened with biodegradable material such as banana leaves, tree bark, or henequen fiber.

Under optimal conditions of humidity and temperature, and with vigorous, healthy cuttings, the first roots begin to emerge the first week after planting and during the first shoots in about one month. This relatively early rooting negates the need to purchase rooting hormones or products.

After cuttings are successfully established, several activities and practices are necessary to ensure the development and optimal production of the vanilla crop.

Irrigation

Water is the main factor in the growth and development of vanilla. Sufficient availability of water is most critical during flowering/pollination. Irrigation has three benefits: (1) encouraging growth and development of the plant, (2) increasing fruit yield and quality, and (3) prolonging the productive life of the plant.

The most frequent form of irrigation in Mexico is the use of microemitters to moisten the mulch layer of the vainillal.

One criterion for irrigation is to maintain at all times a moist layer of mulch without reaching saturation levels. The frequency and amounts of irrigation therefore depend on the type of mulch, phenological stage of the vanilla plants at the time of watering, and the prevailing climatic conditions such as rain and solar radiation as well as the amount of shade covering the vainillal. Generally, in the dry season, watering is performed once to twice per week.

Nutrition

The primary source of nutrition for vanilla in cultivation is organic material (humus) that results from the natural decomposition of vegetable/animal residues (mulch), composting (via microorganisms), or vermiculture (worm-mediated breakdown of organic material).

In addition to apportioning nutrients, mulch has the following benefits: (1) it helps maintain soil humidity, (2) serves as a porous substrate, aiding in soil aeration and permitting the unrestrained development of roots, (3) maintains an adequate temperature, and (4) decreases the incidence of weeds.

Mulches increase the biological activity of microorganisms and promote the development of mycorrhizal associations (Porras Afaro and Bayman, 2007). For example, the fungus Rhizoctonia sp. is known to establish a mycorrhizal symbiosis with vanilla roots, from which the vanilla derives a better absorption of nutrients and water while the fungus gains carbohydrates produced by the plant (Wong et al., 2003). Other fungal species belonging to the genera Ceratobasidium, Thanatephorus, and Tulasnella also positively effect seed germination and plant growth (Porras-Alfaro and Bayman, 2007).

The most common mulch for vanilla is from decaying leaf litter deriving from leaf fall from trees, pruning, and from herbaceous plants in the vainillal. Decomposed and rotting tree bark is also used in the form of sawdust.

When there is not sufficient organic plant material found in the vainillal, mulch can be made from stubble of kudzu (Pueraria phaseolides), elephant grass (Pennisetum sp.), or Guinea grass (Panicum maximum) (Bouriquet, 1954; Ranadive, 2005).

Coconut fiber is also a popular mulch because it is porous, lightweight, and has an excellent capacity for retention of humidity and for conserving an appropriate micro-climate for the promotion of root growth.

Sawdust in a fresh state may contain toxic substances to plants such as phenols, resins, terpenes, and tannins. Fresh manure or manure that is not well decomposed can also burn or cause root rot and mortality. It is important that these materials are composted before they are applied to the vanilla plants.

The mulch should be 10–20 cm deep and approximately 50–100 cm wide, depending on the extent of root growth. In areas of high humidity and deficient drainage, the depth is less, in order to mitigate the development of fungal pathogens, which can favor root rot.

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.

The inflorescence of vanilla is a raceme, meaning individual flowers are borne on stalks arranged around a central axis. An individual plant normally produces 8–15 racemes or more (Anonymous, 1998, 2004; Ranadive, 2005), depending on the age, the quantity of flowering and rooting shoots, and the prevailing environmental conditions. From the initial formation of the inflorescence to the time the first flower opens is a period of 45–60 days.

Each raceme develops 10–20 floral buds, which open sequentially, starting from the base of the raceme. Normally, only one flower per raceme opens per day. Sometimes two flowers open simultaneously and sometimes no flowers open when there is rain or when temperatures are low. The flowers are fully open in the early morning and last 6–8 h before closing in the heat of the afternoon.

Vanilla-producing countries in the northern hemisphere (e.g., Mexico, India) experience flowering in March–May, with a peak in April. Southern hemisphere countries (e.g., Madagascar, Indonesia) have a flowering season from September–December.

Mexico is one of the few countries where it is possible to obtain vanilla beans through natural pollination, although it happens rarely, accounting for only about 1% of all fruits. The identity of the natural pollinator(s) of vanilla is unclear, and for a long time it has been said that bees (Melipona beechii), hummingbirds (Cynniris sp.), and bats pollinate vanilla. The preponderance of evidence favors the hypothesis that the most common pollinator is the shiny green orchid bee Euglossa viridissima (Soto, 1999a, 2003; Lubinsky et al., 2006). These bees have been documented visiting vanilla flowers but their visits are irregular and their potential for effecting pollination even smaller, perhaps only just 1 fruit per 100 or 1000 flowers (Soto, 1999a, 1999b; Hagsater et al., 2005).

With natural pollination, only one fruit is observed per raceme. Sometimes up to four are observed, all of which are usually larger and heavier than vanilla beans obtained through hand pollination because more resources are allocated to their development.

Other orchid bees, namely, individuals of Eulaema sp. ( jicotes) frequently visit the flowers of Vanilla pompona (Figure 6.3) in northern Veracruz, Mexico. On rare occasions, they also effect pollination of the flowers (5%) while looking for nectar inside and at the base of the labellum.

FIGURE 6.3 Eulaema sp. (jicote) bees, probable natural pollinator of V. pompona.

The mechanism by which the aforementioned bees actually pollinate flowers of vanilla is yet to be documented.

Inside the labellum of the vanilla flower, the part which attaches to and wraps around the column, is a tissue that flaps down from the column called the rostellum. The rostellum hangs exactly in between the stigma (female organ) and the anther sac (male organ), and is considered to be a product of evolution selected to prevent self-fertilization. In hand pollination, pollen is manually moved from the anther sac to the stigma, bypassing the rostellum.

Hand pollination is performed with a small, thin stick roughly the size and shape of a toothpick, but can be made from bamboo, bone, spines, or other materials. The relevant parts and reproductive organs of the vanilla flower are shown in Figure 6.4.

FIGURE 6.4 Floral structures of V. planifolia. (a) Complete flower, (b) column (side view), (c) column (front view), and (d) rostellum and anther sac lifted revealing the stigma.

Hand pollination was discovered by Charles Morren in 1836 and the first to put into practice on the island of Reunion was by Edmond Albius in 1841 (Lecomte, 1901; Childers and Cibes, 1948). This method of hand pollination is the same one that is in use today (Figure 6.5) and consists of the following steps:

1. Use a toothpick or similar tool to make a longitudinal slit in the labellum on the side opposite of the column to reveal the reproductive structures.

2. With the same end of the toothpick, lift underneath the rostellum and flip vertical so that the anther sac can hang down unimpeded over the stigma lobes.

3. Gently press the anther to the stigma until the two stick together and remove the toothpick.

FIGURE 6.5 Method for hand pollination of V. planifolia.

Pollen of V. planifolia has been found to be viable for a period that begins 23 h before anthesis and ending 16 h after the flower closes. Likewise, the stigma is receptive 41 h before flower opening and remains so until 17 h after the flower has closed (Shadakshari et al., 2003). For practical reasons, hand pollination is performed from 7 a.m.—noon, or a little bit later when it is overcast, but never when the flowers have already closed or withered.

Hand pollination should be conducted by able and experienced people. Women are more commonly involved in the task. An experienced person pollinates 1000–1500 flowers per 5–7 h period (ca. 4 flowers/min), assuming that the plants are more or less in the same area. The first flowers in the raceme that are pollinated yield longer and straighter fruits while the last flowers to open characteristically produce smaller and curved fruits that have less value.

Hand pollination is a daily task for a period of three months. Per hectare, 300– 600 days of work is required to carry out pollination, depending on the abundance of flowers, their location, efficacy of the pollinator, and distance between plants.

Fertilized/pollinated flowers do not separate and fall from their pedicel (in the case of vanilla, the pedicel is also the inferior ovary). On occasion, the column may abscise from the fertilized ovary, but the ideal is for the column and petals to stay attached to the developing fruit as they serve to keep the fruit hydrated and diminish the colonization of pests or fungal disease.

Flowers that are not pollinated, pollinated incorrectly, or have been damaged by rain or high temperatures abscise or abort 2–3 days following pollination. On rainy days, pollinated flowers abort up to 50% of the time because the pollen humidifies, loses its adhesive quality, and falls away from the stigma.

In general, 6–8 flowers per raceme are pollinated to ensure a minimum yield of 4–5 fruits per raceme of acceptable quality. Obtaining 100–120 fruits per plant requires 8–15 flowers per raceme to be pollinated (Anonymous, 1998). These approximations are rough since much depends on environmental conditions, the position and vigor of the plants, as well as the biological characteristics of the clone or cultivar. Vanilla growers determine the amount of flowers to be pollinated by considering pricing as well. Overpollination leads to an abundance of many smaller fruits of lesser value that increase the cost of pollination and exert a heavy cost on the plants. Overpollination is also associated with major fluctuations in production volume from year to year.

Immediately following hand pollination, pollen tubes begin their germination and growth and eventual fertilization of the ovules. The ovary quickly begins to enlarge and assume a strong, dark green aspect as it orients itself downward. The maximum length and diameter of the fruit is achieved 45 days after hand pollination (Figure 6.6). Thereafter, growth ceases, and the fruit enters into a period of maturation lasting roughly 7–8 months.

FIGURE 6.6 Time-series data on the development of a fruit from V. planifolia cv. mansa (length and diameter) observed at the Ixtacuaco experimental station (INIFAP, Mexico).

Harvest

Vanilla is harvested when it reaches its commercial maturity, when the distal point of the fruit changes from green to yellow (Figure 6.7), generally 8–9 months after pollination.

FIGURE 6.7 Commercial maturity of green beans (8–9 months after pollination).

Early harvested fruits weigh less, are more susceptible to fungal attack and, when cured, yield smaller quantities of vanillin (Tiollier, 1983). Fruits that are harvested past nine months begin to dehisce, and change from green to yellow to dark brown or black. These fruits are called “splits” and have less value.

Northern hemisphere countries have harvest seasons from December to February while Southern hemisphere countries harvest from June to September. Most vanilla growers harvest at only one time, and collect all fruits, that is, those that are mature and those that are still developing. The still developing fruits dehydrate faster than mature fruits during curing. Some growers, for example, in Uganda and French Polynesia, harvest only when fruits are mature, generally once a week, prolonging the harvest to 2–3 months.

In Mexico, the whole bundle or raceme of fruits is harvested with the central stalk, or rachis, still attached. A more ideal practice is to harvest each mature fruit individually. Harvested fruits are placed in baskets or plastic crates to prevent mechanical damage, which can lead to pathogen infection. The fruits are also kept in well ventilated and shady areas.

After harvesting, it is customary to prune shoots that have already flowered. These shoots will not produce again (or as much) unless they retain buds. The pruning is performed with a knife or blade that is disinfected prior to use in a solution of 1 part bleach to 6 parts water.

The removal of these “spent” shoots serves to eliminate unproductive parts of the plant that occupy space and take away the energy resources from the plant. Their removal facilitates the maintenance of adequate ventilation and light conditions for the plant. Some of these spent shoots may serve as cuttings to start new plants if they retain meristematic tissue.

Yields from vainillales are extremely variable, and depend on the ages of the plants, density of the plantings, method of cultivation (“traditional” or modern), source of water (rain-fed or irrigated), the soil and climatic conditions of each site, and so on.

Worldwide, average yields of green vanilla are less than 500 kg/ha, since most growers employ few technologies. Those who use technologies for irrigation or fertilization or use shade houses may obtain 3–4 tons of green vanilla/ha, or approximately 500–800 kg cured vanilla/ha (Ranadive, 2005).

The average weight of fruits per plant is 1–2 kilos and up to 5.5 kilos for plants that have been grown on orange tree supports (Lopez Méndez and Mata García, 2006).

In whatever system of vanilla cultivation, the maximum yields occur in the fourth or fifth year following planting (second or third harvest). After this time, production volume can be lower or higher, but after nine years yields steadily decline until productivity ceases almost completely by the twelfth year.

Experience has demonstrated that vainillales that are densely planted can produce high yields, but generally only once. Later, yields decline drastically and abruptly terminate usually six years after planting because of major problems with disease. In vainillales that are less densely planted, a productive life span of 12 years is expected because adequate parameters exist for ventilation and disease/pest prevention. The best model for this is vanilla cultivated on orange supports.

Labor Requirements

Vanilla is a labor-demanding crop, requiring 172–575 workdays/year ha depending on the system of production, and stage of the plants (i.e., vegetative growth versus productive). The fourth and fifth years are normally the most demanding since production is greatest then. In Mexico, in the semi-intensive system using Erythrina sp. as a support tree, growers have needed a maximum of 575 workdays over the year (on an average, it is 316.2 workdays).

The majority of workdays, more than 50%, are needed for hand pollination, followed by shoot management (looping and rooting, 10.4%), pruning (7%), and weed control, accommodation of organic materials/compost, and phytosanitary maintenance (7%) (Table 6.1).

Labor requirements are similar in other systems of vanilla cultivation. One family can cultivate a maximum of 1 ha of vanilla, but needs to contract wage labor for help with pollination in a larger area.

Perspectives on Crop Improvement

The fungus F. oxysporum is the major cause of economic damage to vanilla cultivation worldwide. Management practices and chemical control have so far not proved effective at controlling outbreaks of Fusarium. Strategies should be developed to identify a genetic basis for resistance through selection and breeding. Genomic/ genetic technologies such as marker-assisted selection and gene chips can help in this regard.

Knowledge of the nutritional requirements of vanilla is still imperfect. The quantity and types of nutrients needed by vanilla at each phenological stage should be studied in more detail. Such information can be used to generate mulches and fertilizer mixtures specific to vanilla. This in turn would have a beneficial effect on yields. Similarly, water is an indispensable element in the cultivation of vanilla, but the precise water needs of vanilla—how much, and at what intervals—are yet to be studied.

Vanilla pests are readily managed with organic products, when such chemicals are used efficiently and opportunistically. In addition to learning more about the effects of specific management practices, vanilla cultivation stands to improve from insights into how flowering can be induced using chemical or hormonal applications.