Growth regulation is the management of plant form. The finished product reflects the skills and diligence of its producer. With greenhouse crops, many cultural and chemical tools are available to regulate finished plant form and flowering. This section discusses various aspects of crop growth regulation, such as promotion of growth and flowering, management of plant height, branching, defoliation, and longevity.

10.2 Chemical Growth Regulators

A chemical plant growth regulator is a natural or synthetic chemical substance that in very small quantities regulates or controls some aspects of plant growth, such as stem length, rooting, flowering, leaf abscission, fruiting, and winter hardiness. Regulators either promote or retard plant growth and development, depending on the chemical chosen and the concentration used.

In commercial floriculture, regulators are used primarily to enhance rooting of cuttings, control plant form (height and branching), and induce flowering. Growth retardants reduce stem elongation, strengthen stems, and darken foliar color.

Growth regulators are not substitutes for good cultural practices. When intelligently used, however, they cut labor and overall production costs and create a better crop than could be achieved otherwise. Categorization of chemicals as stimulants or retardants of plant growth and development is not absolute. Ethylene occurs naturally in plants. Low levels may promote rooting of cuttings when used in combination with auxins. Elevated concentrations reduce postharvest life of floral crops, distort foliage of growing crops, retard elongation of some bulb crops, induce flowering of bromeliads, promote branching and retard flowering of stock plants of geraniums, and cause leaf abscission.

In addition to ethylene, floral crop producers use several synthetic growth regulators to stimulate or inhibit plant growth. They are applied either as a spray on the foliage, as a drench on the growing mix, or as a dip for bulbs. Table 11.1 summarizes label-approved uses of growth-regulating chemicals. Because of the potentially detrimental effects of growth regulators on the crop, the environment, and the grower, label instructions should be read and followed carefully. Purchase growth regulators only in required quantities to ensure fresh stocks of chemicals. Consult the label for storage instructions. Once mixed with water, chemicals should be used immediately because solutions deteriorate if stored. CONSULT THE CHEMICAL LABEL FOR SPECIFIC INSTRUCTIONS ON APPLICATION.

10.3 Growth Promotion

Growth is an increase in plant size or weight. This increase can be accomplished in several ways, including elongation, thickening of leaves, and addition of branches. For greenhouse crops, growth promotion usually refers to promotion of elongation. Rarely are extraordinary steps taken to promote growth. Providing an optimal growing environment usually is sufficient.

Several factors promote stem elongation:

· An optimal growing environment increases the number of nodes on a stem.

· Shady conditions promote internode elongation.

· Long production schedules provide more time for crop growth.

· Long day lengths. For day-neutral crops (regarding promotion of flowering), long photoperiods at high light levels promote increases in both node number and node elongation, and long photoperiods at low light levels increase node elongation. For photoperiodic crops (regarding promotion of flowering), long photoperiods increase the number of nodes for short-day plants and minimize the number of nodes for long-day plants.

· Cool night temperatures and warm day temperatures promote internode elongation.

· Cold treatment. For a few crops, a vegetative dormancy is broken; growth for crops such as bleeding heart is promoted.

· Gibberellin application. Internodes are elongated; high doses may also distort growth.

10.4 Chemical Growth Promoters

Auxins, some of which are found naturally in plants, are a group of growth regulators that promote growth in length. Plants may react to these chemicals in other ways, as seen by early rooting, root initials on cuttings, fruit development without pollination, and fruit thinning. Indoleacetic acid, indolebutyric acid, naphthaleneacetic acid, and phenoxyacetic acids (e.g., the herbicide 2,4-D) are auxins. Several powdered products are available at different chemical strengths. When using auxins, transfer enough powder for one use to a container for dipping cutting bases. Discard leftover product to avoid the spread of disease through reuse.

Hormodin and Dip ‘N Grow, hormone products that promote root formation, are registered for use on several crops in New York State. Consult product labels for specific instructions. See Table 11.1. Gibberellins are also found in plants. They promote growth in length but inhibit root formation on stems and leaves. They also affect.

flowering. Their use in floral crop production is limited because they may create undesirable, elongated plants. Research has shown that gibberellins break flower bud dormancy (e.g., azalea), stimulate flower bud development (e.g., cyclamen), increase plant and flower size (e.g., geranium, hydrangea), promote stem elongation (e.g., geranium trees), and suppress senescence. See Table 11.1

10.5 Management of Plant Height

Market requirements for final plant height are met through the integration of several aspects of crop management. Light intensity, photoperiod (length of day), temperature, ethylene generation from mechanical flexing of stems or stroking of leaves, and the relationship of night temperature to day temperature are major influences on plant elongation. Mechanical treatment of the crop is not a current practice. Desirable plant height and shape are primary market requirements. Customers prefer plant heights (including the pot) that range from 2.67 to 3.5 times the pot diameter (e.g., 16 to 21 in. for a 6-in. pot). For many production situations with contemporary cultivars, crops do not require additional height management if they are grown at full light, are adequately spaced, and are forced at normal temperatures. Occasionally the following measures to manage height must be taken.

10.5.1 Plant Height Graphical Tracking

Graphing height of several representative plants during crop growth is valuable because attention is directed to height control while that is still possible.

To track height on a graph, place dates of observation along the horizontal axis of a graph and plant height along the vertical axis. The curve rises slowly as plants establish, rises steeply as plants grow, and usually plateaus as plants flower and become ready to market. At any point along the curve, a realistic expectation of maximum and minimum tolerable height can be plotted, consistent with this basic pattern of plant growth. When plants are taller or shorter than expected on any observation date, the grower can implement remedial action. If plants are too tall, retardation is needed. Removal of shading or overhead plants to provide more light, adjustments of temperature (DIF), an application of a growth retardant or other actions are required. If plants are too short, stimulation of stem elongation is needed. Adjustments of temperature (DIF), shading, providing long day lengths, or other actions are required. (See Figure 10.5.1)

10.5.2 Height Management with Light Adjustment

Many greenhouse crops require high light levels. Many also respond to the length of day (photoperiod). Year-to-year variation in crop height is partly caused by differences in light each year. Stray light from other greenhouse cropping areas and from streetlights also affects crops.

Figure 10.5.1. An example of height graphical tracking for chrysanthemum. Target finish height was 15 to 18 inches. A growth regulator application (B-Nine) was applied on March 31^st when plant height was above the desired height curve.

Plants remain compact when grown in full light. Even insulated greenhouse coverings that attenuate light slightly promote stem elongation. To provide full light, coverings must be clean and highly transmissive and the pots must be placed in the brightest greenhouse locations. Plants typically “stretch” when placed on benches near walls, under overhead storage of blackout material, or under hanging baskets or shelves. Close spacing causes plants to stretch. Respacing the crop twice during production so the leaves between the plants barely touch usually is both an economical practice and an effective way to regulate plant height.

If plants are too tall or too short, height sometimes can be adjusted by controlling the photoperiod. If a crop is estimated to be too short, it sometimes can be stretched by long photo-periods. Providing incandescent light (2 to 3 umol/sec./sq. m., or 10 to 20 foot-candles) nightly from 10 p.m. to 2 a.m. is effective. For example, new growth of Easter lilies estimated to be too tall can be kept compact by creating longer nights with a daily cover of blackout shading from 5 p.m. to 8 a.m. This is especially useful during long natural photoperiods in late spring, summer, and early fall.

10.5.3 Height Management with Temperature Adjustment

Past crop growers and scientists noticed that warm temperatures stretch crops, and a system was developed to manage crop height based on the relationship between day and night temperatures. This system is based on three principles:

Table 10.5.3. Examples of day/night temperature regimes that provide a similar average daily temperature
Day			Night			Total Heat Units (Degree-Hours)

Degrees F^1,2	Hours	Degree-Hours	Degrees F	Hours	Degree-Hours		Differential³ (Degrees F)
70	9	135	60	15	75	210	10
64	9	81	64	15	135	216	0
54	9	0	69	15	210	210	-15
¹The base temperature for calculating heat unit accumulation is 55˚ F.
²Temperatures are rounded to the nearest degree.
³Differential is between day and night temperatures.

1. The forcing time of many crops is primarily related to air temperature when temperature rather than light is the factor limiting growth.

2. The crop schedule, or days from planting to flower, depends on the average daily temperature and the number of heat units accrued. For example, assume that forcing temperatures of 60° F at night and 70° F at day are required for a crop and that the crop grows slowly between 30° and 55° F. The accumulation of heat units can be calculated using 55° F as the base temperature. Assume that the 70° F day temperature is set from 8 a.m. until 5 p.m. (9 hours) and the 60° F night temperature is set from 5 p.m. until 8 a.m. (15 hours). The number of heat units accrued in 24 hours are 135 degree-hours during the day [(70 - 55) ° x 9 hours] plus 75 degree-hours during the night [(60 - 55) ° x 15 hours], or a total of 210 degree-hours. Similar totals can be achieved by other day/night temperature configurations.

3. Crop height is controlled by the difference between day and night temperatures. Day temperatures higher than night temperatures (a positive differential) encourage elongated growth, and day temperatures lower than night temperatures (a negative differential) encourage compact growth. Temperature differential is carefully controlled to achieve a predetermined finished crop height.

Crops growing in night temperatures higher than day temperatures may discolor and leaves may downturn. When treatment is prolonged, these characteristics may become irreversible.

Lowering day temperature for only two hours early in the day may be nearly as effective for limiting height as lowering the temperature for the entire day. (See Table 10.5.3.)

10.5.4 Height Management with Growth Regulators

Chemical growth retardants are also used to manage plant height (see Table 11.1). The amount of growth retardant to apply depends on (1) chemical used, (2) height reduction desired, (3) cultivar, (4) crop size at the time of application (applying a chemical growth retardant when the plant is small may reduce leaf area and retard growth more than expected; application after stretch begins detracts from the plant’s final shape), (5) method of application, (6) growing mix (bark-containing mixes can reduce the effectiveness of a portion of a drench application), and (7) cultural conditions, as described above. The formulation of the product affects the degree of retardation; adding a surfactant and applying the product as a foam sometimes increases activity.

Time and method of application are interrelated. Early application provides early moderation of elongation. Early application is feasible either by dipping the plant material into a growth-retardant solution before planting or by drenching the growing mix after planting. Dips have been studied because some growing mix components may reduce the effectiveness of a drench application. Many growing mixes, however, are adequate for drench applications; the chemical is present and effective for an extended period.

Treatment with a growth retardant when the plant is small in size results in leaves closer to the pot rim, which improves plant form. Although early application is desirable, often the need for growth retardation is not yet dramatically apparent. Application after growth begins is popular because then the need for a retardant is obvious, but it is less successful.

To apply a growth retardant after growth begins, either a drench application to the growing mix or a spray application to the foliage after sufficient absorbing leaf area is present is made. For pinched plants, applications are made when new shoots are 3/4 to 11/2 inches long. Multiple low-dose spray applications increase labor costs associated with application, but they minimize unattractive clustering of nodes midway up the finished plant.

10.6 Growth-Retarding Effects of Some Fungicides

Some of the fungicides labeled for use on greenhouse crops also retard crop growth. These include triazoles that are related to “Bonzi,” e.g., triadimefon, and pyrimidine-methanols, e.g., triforine. If these fungicides are applied, crop height control procedures may need to be moderated.

10.7 Promotion of Branching

Branching is a desirable characteristic for many pot and bedding plants and some cut flowers. Many crops must be pinched to achieve the level of branching desired. Pinching usually delays flowering by promoting a flush of vegetative growth. Because manual pinching is laborious, chemicals are sometimes used. Branching promoters such as Atrimmec, Off-Shoot-O, and Pistill replace or supplement manual pinching.

10.8 Promotion of Flowering

For several crops, causing plants to transform from growing to flowering is an important step in production. The precise technology required depends on the species. Typical procedures include the following:

Optimizing the growing environment promotes completion of the crop life cycle in a short time.

Selecting a cultivar suitable for the season minimizes production time for crops such as snapdragon.

Using cold treatment promotes initiation of flowers (e.g., calceolaria, iris, lily, and primula) and expansion of flowers (e.g., azalea, daffodil, hyacinth, and tulip).

Using photoperiodic treatment of mature crops promotes initiation of flowers for short-day crops (e.g., chrysanthemum and poinsettia) and for long-day crops (e.g., China aster) and promotes expansion of flowers for short-day crops (e.g., China aster) and long-day crops (e.g., cineraria).

Applying growth regulators sometimes also promotes flowering. Promotion occurs in one of several ways. In some crops flowers are induced to form by chemical application, e.g., Pistill on bromeliads and aroids such as Calla lily, or spathiphyllum and one of several growth retardants on azaleas.

10.9 Management of Flower Size

A flower grows larger if immediately adjacent flowers in a spray are removed when the buds are young (about pea-sized). Disbudding to a single flower on a stem is performed on some cultivars of crops such as carnation, chrysanthemum, and rose. Disbudding is manual.

In experimental work, the carefully controlled application of methyl esters of long-chain fatty acids onto young growth kills terminal or lateral buds without damaging other buds or the foliage. Unfortunately, the optimal concentration changes with the hardiness of the plant and the cultivar. Concentrations that are too high burn plants and concentrations that are too low do not disbud. Therefore, few floral crop producers use this technology. No products are labeled for this use.

10.10 Suppression of Senescence

Leaves, flower buds, and flowers can sometimes be retained on plants and cut flowers longer with chemical treatment. Leaf yellowing after storage, flower bud drop, and deterioration of open flowers is delayed in some crops by application of an inhibitor of ethylene action (EthylBloc) or gibberellin/benzyladenine formulation (Fascination).

10.11 Crop Defoliation

Crop defoliation is sometimes desirable for prevention of disease during storage. This procedure is also used to store rose bushes and dormant hydrangea plants. Ethylene (apple) gas, ethylene-generating products such as Pistill(see Table 11.1), and several other chemicals have been successfully used for this purpose.

Pest Management Guidelines A Cornell Cooperative Extension Publication	Cornell Guide for Pest Management of Greenhouse Floral Crops
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