Daylight Hours for Marigolds to Bloom

Photoperiod

• Marigold seeds do not need light to germinate. However, flowers planted in containers or beds need bright sunshine or high-intensity artificial light to survive and bloom. Photoperiod requirements vary, notes Auburn University’s College of Agriculture, depending on marigold species, cultivar and when the seeds are sown. Photoperiod is the amount of light necessary for a plant to grow during a particular time period. In a 24-hour span, for example, some plants may require 12 hours of light and 12 hours of darkness. Others may need eight hours of sunlight and 16 hours of darkness. African -- also known as American -- marigold (Tagetes erecta) seeds planted in February require between 12 1/2 and 13 hours of daily sunlight within a few weeks after seedlings sprout. Artificial light from high-intensity discharge lamps can help indoor plants grow.

Light

• French (Tagetes patula) and signet (Tagetes tenuifolia) marigolds growing outdoors in direct sun do not typically need photoperiod control in the spring; six hours a day is adequate, notes Iowa State University Extension. After the danger of frost has passed -- usually in mid-May -- and before you place tender young marigolds into the ground, plants must adapt to outdoor climate and temperature. Harden young marigolds in partially shaded locations for several days and then expose them to longer periods of sunlight before planting in flower beds. Blooms appear throughout spring, summer and early fall.

Seeds and Soil

• Marigold seeds typically germinate within three to five days when soil is at 75 to 80 degrees Fahrenheit. Plant marigold seeds indoors in late March or early April; blooms appear within 45 to 50 days. Although they adapt to a variety of soils, marigolds prefer well-draining loam or peat-lite with pH of 5.8 to 6.2. You can plant seeds in peat or clay pots, flats, seedbeds and cold frames. Place growing seedlings in filtered or artificial light.

Planting and Care

• When starting marigolds indoors, about six to eight weeks before outdoor planting, crush the soil and sow the seeds 1/4 inch deep into furrows. Place about 1/4 inch of vermiculite or perlite over the seeds to help keep the soil moist. As marigold stems produce leaves, you can transplant the seedlings into individual containers and place them in partial sunlight or artificial light. For plants to thicken and spread as they establish root systems, set African marigolds about 18 inches apart and French and signet marigolds 6 to 9 inches apart. Monitor soil moisture during the first two weeks after planting and give the plants at least 1 inch of water per week -- more often in periods of drought. Pinch faded flowers to encourage new growth. Add organic matter such as pine bark or leaf mold to the soil to help keep plants healthy.

Pests and Diseases

• Marigolds emit odors that typically drive away animals and insects. Flowers do attract aphids, spider mites, leaf miners and thrips but the insects do very little damage to the plants. Marigolds are susceptible to damping off, root rots, Alternaria leaf spot, gray mold and powdery mildew. Southern bacterial wilt (Pseudomonus solanacearium) can kill marigold and other flowering annuals. Destroying infected plants helps keep fungal spores from spreading.