Commercial Greenhouse Production
Scientific Name: Tagetes erecta (African forms),
Tagetes patula (French forms)
Common Name: African and French Marigold
Dr. J. Raymond Kessler, Jr.
One of the premiere of garden annuals, marigolds perform well in dry, hot conditions. Excellent in a border, pots, or as a cut flower. Marigold flowers are either single, semi-double, or double in colors ranging from white or yellow to orange, gold, and red. Red and crimson are found in the triploids and French forms but not the African. Flower sizes range from 1" in the French to 5" in the African forms. Plant sizes range for 6" to 3'. Marigolds has a strong scent which some people find offensive.
Neither African or French marigolds come from Africa or France, respectively, but both Tagetes erecta and Tagetes patula are native to Mexico. Breeders have worked overtime to create cultivars in a wide range of colors, plant sizes, and flower forms. Crosses between Tagetes erecta and Tagetes patula have resulted in triploid cultivars. Available forms:
African: Hybrids of Tagetes erecta are larger plants than the French forms, often with fewer, larger double flowers. In the double flowering forms, there are crested doubles where the flowers appear mounded and full, and anemone doubles where the flowers appear flat and wide with the center recessed.
French: Hybrids of Tagetes patula are usually smaller than the African forms, 6" to 8" and up to 12" tall. Thought doubles are available, singles or semi-doubles are more common. The single flowering forms stand-up to rain and humidity better in the south than double forms.
Triploids (Signet hybrids): Crosses between Tagetes erecta and Tagetes patula provide the longest overall color in the landscape, often lasting through the hot weather in August and September. These plants are sterile.
Seed may be purchase as raw seed, or to facilitate sowing in automatic seeders as detailed, or coated seed. Seed germinate in 3-5 days at 75-80F (stage 1). Marigold seed do not require light, therefore, cover the seed lightly with course vermiculite to retain moisture around the seed. Keep the germinating medium moist but not saturated.. Germination medium pH should be 6.0-6.2 with an EC <0.75 mmhos/cm. Reduce the moisture level once the radicle emerges and reduce the temperature to 68-70F in stage 2. Begin fertilization with 50-75 ppm N from calcium/potassium nitrate once the cotyledons unfold. In stages 3 and 4 temperatures should be 60-65F
If seed must be kept from one season to the next, store in a cool, dry environment away form insects and rodents. As a rule of thumb, store seed where the sum of the temperature and relative humidity in percent does not exceed 100, e.g. at 55F, the humidity should not exceed 45%. Refrigerators dedicated for seed storage are often used.
Temperature: Night temperatures 60-65F, day temperatures 65-72F for high quality plants. Use 65F night temperatures for a week or so after transplanting, then drop to 60F if desired. DIF (-5F) may be used for height control.
Photoperiod: All marigolds are quantitative short-day plants, thought Tagetes erecta displays a response more than Tagetes patula. The critical photoperiod for Tagetes erecta is between 12.5 and 13 hours. Plants of Tagetes erecta should be grown under artificial short days starting the first 2-3 weeks after germination during late February and later. Photoperiod control is rarely needed nor practiced for the French and Signet marigolds during normal spring production.
Light: Marigolds require high light intensities and supplemental lighting is often practiced under low light conditions to prevent stretch.
Growing medium: Light, well-drained, peat-lite medium with a pH of 5.8-6.2 with an EC less then 1.0 mmhos/cm. Lower pH should be avoided to prevent iron and manganese speckling, necrotic margins, necrotic spots on the older leaves, and death of the growing tip. Avoid excess application of micronutrients.
Fertilization: Fertilize at 100-150 ppm N using a complete fertilizer. Soluble salts should be around 1.0 mmhos/cm. Use a fertilizer balanced in nitrogen and potassium (15-15-15). Do not apply fertilizers containing ammonium if the media temperature is below 65F.
Growth Retardant: Usually not needed on the French and Signet forms. Plants respond to two applications daminozide at 2500 ppm (B-Nine) one week apart or 2-3 applications of chloromequat chloride at 1500 ppm (Cycocel) 10 days apart.
|Marigold normal foliar analysis ranges|
Supplemental Light and Carbon Dioxide: Supplemental light from HID lamps benefits growth at 450-700 ft.ca. during the plug stage (begin stage 2). Supplemental carbon dioxide also improves growth at 800-1000 ppm. A 2-3F increase in day temperature should be used with supplemental CO2.
Pests: Spider mites, aphids, thrips, and leaf miners. Slugs and snails can be a problem under damp conditions.
Diseases: Apply a protective application of a fungicide for Alternaria leaf spot, especially the African forms. Tomato spotted wilt virus (TSWV) can be a major problem and is carried by thrips. Damping-off (Pythium and Rhizoctonia) and grey mold (Botrytis) of flowers can be problems. 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.
Seedlings require 4-5 weeks in plug flats and 4-6 weeks in finishing flats. In the southeast, African marigold flats require a total of 10-11 weeks (11-12 weeks in 4" pots), and French marigold flats a total of 7-9 weeks (10-11 weeks in 4" pots).