What To Know About Growing Marigolds

This week’s column is the second and last part in a series on marigolds. As you may recall, the scientific name for the marigold is Tagetes. It’s a plant native to the Americas, but it had to be reintroduced from foreign shores before it could be appreciated. To complicate matters there are some who consider this plant a bit pedestrian, but the fact is that marigolds can be quite spectacular and an anchor in the summer garden.

T. erecta, the species marigolds, are day-length sensitive. This means that their flowering season is related to the amount of daylight we get. Each cultivar varies in the response to day length. If a home gardener is growing T. erecta from seed sown after March 1 and wants earlier-flowering plants, a short-day treatment can be used. Just cover small seedlings with a light-proof cover at 4 p.m. and remove at 8 a.m. This treatment can be applied for two weeks.

Triploids, or 3-N hybrids, are a “wide” cross between the African (American) and French species. The cross between species results in a plant that is sterile, unable to reproduce, and not pollen or nectar producing. In the plant world these are often called “mules,” and one of their best assets is that they need no deadheading. Since the triploid is not capable of setting seed, the plant produces more flowers.

This characteristic is significant when compared to T. patula. Most T. patula plants will decrease or even cease flowering under hot summer temperatures. It’s called heat stress, and “shy” blooms are the result. The triploid marigolds are not subject to heat stress and continue blooming prolifically regardless of the heat.

The triploid blooms are 2 to 2 1/2 inches, and mature garden height can be 10 to 18 inches. The flower form on triploids can be single, double or semi double. The color range is similar to T. patula with solid colors and bicolor designs. The triploid seed germination is less than the T. patula germination.

The first triploid marigold was introduced in 1939 and since then there have been many triploids introduced with improvements in flower size and compact plant habits reducing some early sloppiness. Triploids are capable of literally covering the plant with blooms and can make quite spectacular displays, especially in drifts.

Remember that marigold seeds are large, easy to handle and germinate reliably in warm, moist soil. The seeds are easy for kids and adults to handle. T. patula can be sown directly in garden soil after the soil has warmed to 70 degrees Fahrenheit in late May.

Soil should be worked so that it drains and has a fine, loose texture. Dig a furrow about 2 inches deep with the corner of a garden hoe. Water the furrow slowly to soak the soil. Scatter seeds in the furrow about an inch apart. Cover lightly with dry soil, sand, or vermiculite. Water again with a fine mist. Continue watering with a fine spray for 10 to 14 days when seedlings should appear.

As seedlings grow, water less frequently but apply more water to encourage deep root growth. The seedlings can be transplanted when small to other garden locations but this is best done on a cloudy or rainy day. If garden soil is fertile and rich in organic matter, supplemental feeding might not be necessary. Overfeeding or a rich organic soil can result in vegetative (green) growth and a lack of flowering. T. patula will flower in 6 to 12 weeks from sowing, depending upon variety and weather conditions.

T. erecta marigolds are best started indoors and transplanted into the garden. Sow seeds eight weeks prior to planting outdoors in warm garden soil. That means starting the seed in mid to late March out here. Cover seeds lightly and maintain uniform moisture. Transplant into larger containers at the three to four true leaf stage. Provide as much direct sunlight as possible while indoors.

Two diseases that might infect marigolds in gardens are aster yellows and botrytis, the most common problems encountered by North American gardeners, though most gardeners will not encounter any problems or diseases with marigolds.

The marigold aroma, produced by oil glands on the undersides of leaves, is thought to repel some harmful insects. Thusly, marigolds are one of the most reliable annuals any gardener can grow for summer color and durability.

Aster Yellows: As the name indicates, this disease affects asters and marigolds as well as many other garden flowers. The disease is spread by six-spotted leafhoppers, which become infected by feeding on infected weeds. The leafhoppers spread the disease as they feed. The higher the number of leafhoppers, the higher the chance of plant infection.

The symptoms include a yellowing of the foliage, pale greenish yellow abnormally shaped buds and blooms, and an overall stunting of the plant. A diseased plant stands out from among healthy ones by its lack of flowers and odd shape. By the time the buds and blooms become misshapen, there is no alternative but to remove the plant. The only control over this disease is to control the population of leafhoppers, an unlikely solution at best.

Botrytis: This fungus, Botrytis cinerea, thrives in cool, moist conditions attacking injured tissues, dying blooms or foliage. The fungus is a mold, which produces masses of gray spores that are spread by wind or water. Botrytis usually affects double marigold blooms late in the growing season when heavy dew and cool temperatures create the perfect environment for this fungus growth. It appears as brown, dying tissue at the base of the flower petal. If allowed to grow, the fungus can spread rapidly to infect healthy plants. The best control is to remove spent flower heads from the plant. This is particularly important near the end of the growing season.

Nematodes are a diverse group of microscopic worms. Some nematodes are beneficial to the soil or plants, others invade or penetrate plant roots and feed on nutrients thereby robbing the plant of them. Some of the harmful nematodes can be reduced by the presence of marigolds.

Scientific studies have shown that chemical compounds produced by most Tagetes are toxic or antagonistic to certain harmful nematodes. Evidence suggests that the chemical compounds are toxic to nematodes both upon entry into the root system or in the vicinity of the roots.

On the insect side, aphids can be a minor issue on these plants but in a hot and dry summer spider mites can do damage if they are not spotted early. Watch for stippling of the foliage (tiny yellow spots) and if we do get a hot, dry spell, spray the underside of the foliage every couple of days with water. This washes the mites off and drowns them.

Marigolds adapt well to container gardening. Remember to match the mature plant size to the container size. The T. patula, French marigolds, can be grown in smaller containers due to their smaller plant size. One French marigold will fill a 6-inch pot. French marigolds can be planted with vegetables in the same container. Combining various cultivars requires more attention to the water and fertilizer needs of the plants. French marigolds will thrive in larger containers such as wine barrels, urns, or redwood planters. One consistent rule for all container gardens is to ensure that water drains from the soil. Use containers with holes on the bottom or sides.

Both T. tenuifolia, signet, and T. erecta, African or American marigolds, can brighten any patio with bold color grown in containers. Signet marigolds require a 6-to-10-inch container while the African or American needs more space and soil. Use a 12-to-16-inch diameter container at least 18 inches deep for the mid-height range T. erecta.

America continues to lead in the breeding advancements of marigolds. The two Tagetes species receiving the most breeding and research effort continue to be the T. patula, French and T. erecta, African or American. Breeders are selecting marigolds for earlier flowering with specific improved characteristics such as increased flower size. The single marigold flower form has been given recent attention with several new varieties introduced. Crosses between species such as the T. patula x T. erecta or triploid marigolds will continue to be improved for germination and seed vigor.

Someday, breeders will be able to introduce the T. patula mahogany red genes into the T. erecta species for huge 3-inch red blooms. Or the reverse, introduce the white T. erecta genes into a dwarf French marigold.

As for a reliably white marigold, the one Burpee paid $10,000 for many years ago, don’t hold your breath. Yes, you can buy white marigolds, but the white gets “dirty” quickly, and most are more of a cream color.

Buy them as cell packs or as potted plants. You’re choices will be limited, but if you grow your own from seed, this summer or next, you can have some pretty spectacular plants. Keep growing.