ANR-1125, New July 1998. J.R. Kessler, Jr., Extension Horticulturist, Assistant Professor, Horticulture, Auburn University

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Greenhouse Production of Marigolds

Marigolds have long been popular with American gardeners as one of the premiere summer annuals. They perform well in dry, hot, sunny locations and produce dark-green, fine-textured foliage and bright-colored flowers throughout the summer and into the fall. There may be more sizes, flower types, and growth habits of marigolds than of any other bedding plant.

Marigolds range in size from 6 inches to over 3 feet tall. Dwarf cultivars are appropriate to use in containers, as bedding plants, or as flower bed borders, while taller cultivars can be used as bed fillers, at the rear of the border, or as cut flowers. In the past, marigolds had a strong fragrance that some people found objectionable. Newer cultivars are somewhat less pungent, but they still have the characteristic marigold smell.

Cultivars

Two different species of marigold are commonly grown: African marigolds (Tagetes erecta) and French marigolds (Tagetes patula). Interestingly, neither African nor French marigolds come from Africa or France--both species are native to New Mexico and Arizona and southward to Argentina. Crosses between African and French marigolds have resulted in triploid cultivars; therefore, the three major horticultural types are the African type, the French type, and the triploid type.

Breeders have worked extensively to create cultivars in a wide range of colors, plant sizes, and flower forms. Marigold flowers are available in single, semidouble, or double petal arrangements with white, yellow to orange, gold, or red flowers. Red and crimson flower colors are found in the triploid and French types but not in the African type. Flower sizes range from 1 inch in the French type to 5 inches in the African type.

African Type

Cultivars of Tagetes erecta, sometimes referred to as American marigolds, are larger plants than the French type, often with fewer, larger double flowers. In the double-flowered cultivars, there are crested doubles, whose flowers are mounded and full, and anemone doubles whose flowers are flat and wide with a recessed center.

French Type

Cultivars of Tagetes patula are usually smaller than African type plants. Most are 6 to 8 inches, but some cultivars reach 12 inches in height. Though double flower petal arrangements are available, single and semidouble are more common. The single-flowering cultivars stand up to rain and humidity better in the South than double-flowered cultivars do.

Triploid Type

Triploid cultivars are breeding crosses between Tagetes erecta and Tagetes patula. One advantage of these cultivars is that they provide the longest overall display of color in the landscape, often lasting through the hot summer weather into August and September. However, seed germination during production may be considerably slower than for the African and French types. These plants are sterile, so they will not produce viable seed.

Best-Performing Cultivars

The Alabama Agricultural Experiment Station has established a trial garden at the E.V. Smith Research Center near Shorter, Alabama, to evaluate annual garden plants. Fourteen African cultivars and 25 French cultivars were visually rated bimonthly on a scale of 0 (dead) to 5 (superior plants in flower) for garden performance during the summer of 1997. The 10 best-performing African and French cultivars are listed in Tables 1 and 2. Average seasonal performance rating for the 10 best African and French cultivars ranged from 2.9 to 2.3 and 3.2 to 2.6, respectively. Cultivars in the 'Antigua' and 'Inca' series were highly rated among the African type, and those in the 'Bonanza,' 'Bounty,' and 'Hero' series were highly rated among the French type.

Plug Production

Growers pay a premium price for high-quality marigold seeds. Therefore, care and planning are necessary to ensure that the maximum number of transplantable seedlings will be produced from an ounce of seed. For the best results in starting marigold seed, purchase F1 hybrid seeds fresh each season from a reputable supplier. In planning the number of seed to order for production, consider that there are 9,000 to 13,000 seeds per ounce, depending on the species and cultivar.

If seeds must be kept from one season to the next, store them in a dark, cool, dry environment protected from insects and rodents. As a general rule, store seeds under conditions where the sum of the Fahrenheit temperature and percent relative humidity does not exceed 100. For example, where seeds are stored at 45°F, the humidity should not exceed 55 percent. Refrigerators dedicated to seed storage are often used, with the seeds sealed in containers containing a desiccant material.

The 406 or 512 plug flats are small enough for economic production but large enough to accommodate growth until transplanting. However, larger plug sizes may be used to meet special production goals. The most effective way to sow marigold seeds is using an automatic seeder. Using an automatic seeder to sow marigold seeds can be problematic because the seeds are long and thin and the "fuzzy tail" (bristlelike projections at the basal end of the seed) interferes with the mechanical seed-sowing process. Some brands of seeders come with special attachments for handling marigold seed. Seed suppliers have also developed a seed that has been "de-tailed." Marigold seeds that are coated with a water-soluble, inert material to facilitate sowing can also be purchased.

Sow seeds in plug flats containing a moist, well-drained, sterile medium such as one of the many peat-lite mixes available commercially for plug culture. Germinating media pH should be in a range of 5.8 to 6.5, with an electrical conductivity level of less than 0.75 mmhos/cm based on the 2:1 extraction method. Before sowing the seeds, thoroughly moisten the medium with room-temperature water. Light is not required for marigold seed germination; therefore, sow seeds directly on the medium surface, and cover the seeds with a thin layer of #2 coarse-grade vermiculite. Vermiculite helps maintain adequate moisture around the seed.

Seeds of the African and French types germinate in 3 to 5 days at 75 to 80 degrees F medium temperature. Seeds of the triploid type may require several days more. Bottom heat greatly enhances seed germination and early growth, especially on crops early in the spring season. Keep the germinating medium moist, but not saturated. Reduce the moisture level and reduce the night temperature to 68 to 70 degrees F once the radicle (root) emerges. Begin fertilizing seedlings once or twice per week at 50 to 75 ppm nitrogen, using a 15-0-15 fertilizer or calcium nitrate and potassium nitrate when cotyledons (seed leaves) unfold. Increase this rate to 100 to 150 ppm nitrogen when true leaves develop.

Growing On

If marigold plugs are purchased from a propagator, unpack and examine the seedlings carefully. The plants should be an appropriate size for transplanting. Overgrown seedlings are difficult to grow into high-quality plants, and seedlings that are too small are difficult to transplant and slow to establish. Remove several seedlings from the sample flats, and examine the root system. Marigold roots should be large, white, and fuzzy, covering about two-thirds of the outer surface of the medium. Look for signs of overwatering and root diseases indicated by brown or black discoloration. Examine the foliage for diseases, insects, or nutrient problems.

Be prepared to transplant plugs immediately. Generally, marigold seedlings are ready to transplant from plugs when three to four mature leaves are present. It is important to transplant when seedlings just begin to crowd. Marigold plugs are usually shipped at a size that is ready to transplant. Holding plugs in the greenhouse for too long results in stunting and premature flowering. Plant seedlings at about the same level at which they were growing in the plug trays, but no deeper. Dibble the transplant medium to the depth of the root ball, and gently firm-in the seedlings. Water newly planted seedlings thoroughly immediately after transplanting them.

Growing Medium

A light, well-drained, peat-lite medium with a 5.8 to 6.5 pH and an electrical conductivity less then 1.0 mmhos/cm based on the 2:1 extraction method is recommended for growing on. The media pH should be 5.8 or higher. A media pH lower than 5.8 may cause iron and manganese toxicity. Symptoms of toxicity appear as speckling, necrotic margins, necrotic spots on the older leaves, and, in extreme cases, death of the growing tip. Toxicity symptoms may also appear with the overapplication of micronutrients. This may occur when the grower uses a water-soluble fertilizer containing micronutrients after adding a commercial micronutrient fertilizer during the mixing of the medium. Some marigold cultivars are more sensitive to this problem than others are.

Temperature

Grow marigolds at 60 to 65 degrees F night temperatures and 65 to 72 degrees F day temperatures for high-quality plants. Use 65 degrees F night temperatures for a week to 10 days after transplanting to establish the root system, and then drop to 60 degrees F if desired.

Fertilization

Do not fertilize marigolds for 7 to 10 days after transplanting or until the roots reach the sides and bottom of the container. Thereafter, fertilize on a constant liquid fertilization basis at 100 to 150 ppm nitrogen, using a fertilizer about equal in nitrogen and potassium content. Many growers alternate between 20-10-20 and 15-0-15 or calcium nitrate and potassium nitrate applied once or twice per week. If the media temperature is below 65 degrees F, avoid fertilizers with nitrogen high in the ammonium form (greater than 40 percent), such as some 20-20-20 formulations. Growers should test medium pH and soluble salts in-house on a weekly basis and send samples for laboratory testing every 2 weeks. Recommended foliar analysis ranges for marigolds are given in Table 3. Fertilizer rate can be reduced by one-half in the last week or two to harden-off plants before they are shipped to the retail market.

Photoperiod

Many marigold cultivars are facultative short-day plants, meaning that plants flower more quickly under short day lengths but will eventually flower regardless of photoperiod. African cultivars display more of a photoperiodic response than French or triploid types do. The critical photoperiod for Tagetes erecta is between 12.5 and 13 hours, meaning that plants flower more quickly at day lengths shorter than this. Cultivars of the African type can achieve faster flowering if provided with artificial short days. Cover the cultivars with black cloth from 5:00 p.m. to 8:00 a.m. daily. Start applying black cloth in the first 2 to 3 weeks after seed germination, beginning in late February. Using the black cloth will reduce the flowering time by up to 2 weeks and will produce more compact plants. Not all cultivars require black cloth, however. Recent breeding work has focused on developing African cultivars that are day-length neutral; therefore, black cloth is not beneficial. Photoperiod control is rarely needed or practiced for the French and triploid marigolds during normal spring production.

Light

Marigolds require as much light as possible, especially with early spring crops. When arranging bedding plants in the production area, place marigold crops in the brightest areas possible, preferably areas with glass or clear plastic glazing. Be sure the greenhouse glazing is clean and free of shading compound.

Growth Retardant

Generally, control seedling growth and prevent stretching by managing the environment, nutrition, and water first, and then apply chemical growth retardants if necessary. Height control is usually not needed on the French and triploid types. Marigolds respond to two applications of B-Nine at 2,500 ppm 1 week apart or one application of Cycocel (750 to1,500 ppm), Bonzi (4 to 10 ppm), or A-Rest (50 to 100 ppm).

Scheduling

Crop production times differ not only for the three horticultural types of marigolds but also among cultivars within each type. Production time will generally decrease as light intensity and temperature increase during the spring production season. Production time for African marigold cultivars can also be modified by natural photoperiod. Growers should therefore keep detailed records of crop performance and timing to improve future scheduling efforts.

African Types

Generally, the African cultivars require up to 2 weeks longer to flower than the French and triploid types do. In the Southeast, seedlings in 406 plug flats generally require 5 weeks from sowing to be ready to transplant and 5 to 6 weeks in jumbo market packs for a total production time of 10 to 11 weeks. The total production time for a single plug-grown seedling in a 4-inch pot is 11 to 12 weeks.

French Types

In the Southeast, seedlings in 406 plug flats generally require 4 to 5 weeks from sowing to being ready to transplant and 3 to 4 weeks in 48- to 32-cell market packs for a total production time of 7 to 9 weeks. The total production time for a single plug-grown seedling in a 4-inch pot is 10 to 11 weeks. Ten-inch hanging baskets with four or five seedlings per container require 10 to 13 weeks total production time.

Triploid Types

In the Southeast, seedlings in 406 plug flats generally require 5 weeks from sowing to being ready to transplant and 2 to 3 weeks in 48- to 32-celled market packs for a total production time of 7 to 8 weeks. The total production time for a single plug-grown seedling in a 4-inch pot is 10 to 11 weeks.

Common Problems

Insects and Related Pests

Spider mites, aphids, thrips, and leaf miners can be problems on marigolds. Slugs and snails can be problems under damp conditions.

Diseases

A protective application of a fungicide for Alternaria leaf spot is recommended, especially on the African type. Damping-off (Pythium and Rhizoctonia) in seedlings and grey mold (Botrytis) on flowers can be problems. Tomato spotted wilt virus (TSWV) can be a major problem and is carried by thrips. Southern bacterial wilt (Pseudomonus solanacearium) and bacterial leaf spot (Pseudomonas syringae var. tagetes) are serious diseases. Southern bacterial wilt causes stunting, wilting, and death, while bacterial leaf spot causes small black spots that turn necrotic. There are no known controls, so infected plants should be destroyed.

Specific control measures for insect and disease problems can be found in the Alabama Pest Management Handbook--Volumes 1 and 2 (ANR-500A and 500B). For information about obtaining this publication, contact your county Extension agent.

For more information, contact your county Extension office. Look in your telephone directory under your county's name to find the number.