Taking an environmentally sensitive approach to pest management
The National Garden Bureau has chosen marigold as its 2010 Flower of the Year. According, the “Year of the Marigold” is an ideal time to reacquaint ourselves with this colorful and durable flowering annual that asks very little of us as gardeners. Marigold’s ease of care and striking display of color throughout the summer makes it one of our most popular annuals.
Marigold (Tagetes sp.) are native to the Americas from Argentina north to New Mexico and Arizona. The earliest use of marigold was by the Aztec people who attributed magical, religious and medicinal properties to it. Ancient writings indicate marigold was used as a cure hiccups, being struck by lightning, or “for one who wishes to cross a river or water safely.” The latter attests to the magical properties ascribed to it by the Aztec.
Early Spanish explorers are credited with taking marigold seeds from the New World to Spain. There marigolds were cultivated and grown in monastery gardens. From Spain, marigold seeds were transported to France and northern Africa. The taller marigolds, now called African or American, became naturalized in North Africa. Later, marigold returned to America where they were improved by plant breeders to give us our modern-day garden plant.
The genus Tagetes contains 40 species but only two of them are of significance to gardeners: T. erecta (African or American marigold) and T. patula (French marigold). African marigold is characterized by tall plants with large leaves and large double or semi-double flowers. Flowers might reach four inches in diameter and are solid in color. The latter ranges from white and cream to primrose, yellow, orange and gold. Early cultivars of African marigold were quite tall, some reaching heights in excess of three feet. Recent breeding efforts have been directed toward the development of African marigold with a dwarf stature but large flower size.
In contrast, French marigold (T. patula) is characterized by dwarf, compact plants ranging in mature height of between six and fourteen inches, depending on cultivar. This species also contains the widest color range of marigolds. The flowers can be pure or solid orange, yellow, gold, mahogany red, or bi-color. The latter offers a spectrum of vivid color combinations. Petals can be edged with a contrasting color or the color can be placed at the petal base. Additionally, flower form including carnation-flowered, crested and anemone differs between cultivars. Because of its small size, French marigold is often used in the front of borders or to edge beds.
It should also be noted that inter-specific hybrids between T. erecta and T. patula are available to gardeners. These crosses between species result in a sterile, triploid plant that is unable to reproduce. Since the triploid is not capable of setting seed, the plant produces more flowers and is capable of literally covering itself with blooms. Mature of height of triploid marigolds range between 10 to 16 inches. Flower form can be single, double or semidouble, with a color range similar to that of French marigold.
Marigolds can be purchased and set out as transplants or directly seeded into the garden. The seeds are large, easy to handle and germinate reliably in warm, moist soil. In either case avoid planting marigolds in the garden until the air and soil temperature warms.
Marigolds prefer a loose, well-drained garden loam rich in organic matter. They thrive in the sun and heat and should receive at least six to eight hours of sunshine daily. As is the case with most flowering annuals, excessive fertilizer application or very rich soil leads to lush vegetative grow and poor flowering. Although drought tolerant, marigolds respond well to supplemental irrigation during periods of dry weather.
Marigolds are relatively pest free. Some attribute this to their strongly aromatic foliage which also makes them a good choice in areas where animal deprivation is a problem. Two diseases which occasionally become problematic are asters yellows and botrytis.
As the name indicates, asters yellows is characterized by yellowing of the foliage and pale greenish-yellow, abnormally shaped buds and blooms. Overall stunting of the plant also is common. The pathogen that causes this disease is related to a virus and is spread by leafhoppers, which become infected by feeding on infected weeds and spread the disease as they feed on susceptible garden plants such as marigold. Prevention is the only cure for asters yellows; this is accomplished by controlling leafhopper populations in the garden.
Botrytis is a fungal disease that attacks many garden plants. The pathogen (Botrytis cinerea) thrives in cool, moist conditions and attacks injured tissues or spent flowers. Typical symptoms include brown, dying tissue a grayish colored mold that produces masses of spores that are spread by wind or water. Botrytis is most problematic either early or late in the growing season when cool temperatures and moisture from rain or heavy dew create the perfect environment for this fungus. Although there are preventative fungicides that can be applied, satisfactory control often can be accomplished by removing spent flowers from the plant. This is particularly important near the end of the growing season.
Marigolds often have been used as companion plants in gardens to repel insect pests from neighboring plants. Again, their strongly aromatic foliage is considered to be responsible for this beneficial action. Recent research has demonstrated some of the harmful nematodes that occasionally infest garden soil can be reduced by growing marigolds in the soil. Chemical compounds produced by most Tagetes species have been demonstrated to be toxic or antagonistic to certain harmful nematodes and are helpful in their control.
Very few annuals are more versatile and easier to care for than marigolds. They adorn informal borders or formal beds with equal grace and beauty. The available range in mature plant height and flower color makes them a good choice for many different garden situations. Additionally, marigolds adapt well to container gardening. Because of their smaller size, French marigolds are favored for most containers, but don’t overlook African marigold as a colorful focal point for the center of larger containers. In both cases good drainage is necessary and can be facilitated through the use of a soilless growing medium. Additionally, always use containers with drainage holes in the bottom or sides.
Credit: National Garden Bureau
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REVISED: October 23, 2012