|This is a picture of a Dahlia X hybrida in a garden in British Columbia.|
Dahlias are a flowering plant from Mexico. They were first discovered by a French friar named Bernardino de Sahagún who landed in New Spain, Mexico, in 1529. The Aztec word dahlia means “plant with tube-like stems.” This name is appropriate because the petals are shaped in a tube form. In the late sixteenth century two graduates from the College of Santa Cruz, Tlatelolco by the names of Martin de la Cruz and Juannes Badianus wrote the Codex Barberini. This book noted that not only are dahlias useful for carrying water, but they were also used to treat epilepsy. In more recent years, scientists found that Atlantic starch, a diabetic sugar taken from dahlias, was used in the treatment of diabetes before the development of insulin.
Dahlias reproduce asexually through vegetative tissue called bulbs. A bulb is a round, underground storage organ that contains the shoot of a plant. But, in fact, dahlias do not produce true bulbs. Instead, they form a type of pseudobulb called a tuber. Tubers are underground organs with buds where new shoots, stems, and root-like structures develop. The functions of the bulb of a dahlia are similar to that of roots of regular plants; to store food, water, and to transport dissolved minerals and nutrients to all parts of the dahlias. When water is not available, like in winter, the bulb keeps the plant alive.
Like most flower plants, the dahlia has a stem, petals, root, and a flower. Inside the flower, there is the stamen and the carpel (the male and female reproductive organs), the ovary (a special "sack" that holds and protects the seeds), and lastly, the petals. Unlike most flowering plants, the dahlia has numerous flower sizes and colors. There are six categories of flower sizes in the dahlia family; giant dahlias grow more than 10 inches in diameter, large dahlias grow 8-10 inches, medium dahlias grow 6-8 inches, small dahlias grow 4-6 inches, miniature dahlias grow 2-4 inches, and mignon dahlias grow less than 2 inches in diameter. There are eleven bloom types: decorative, cactus, fimbriated, ball, waterlily, anemone, collarette, orchid, peony, single, and novelty blooms. The flowers also vary from cool and solid colored to warm and multicolored. Their petals can be short, tall, fat, skinny, wavy, straight, round or twisted as well.
The way the dahlia and almost every other flower reproduces is by pollination or cross-pollination. Pollination is the transfer of pollen particles in seed plants from their development site in pollen "sacs" in the male reproduction structures. Cross-pollination is the pollination between two plants. Pollen is a made by male gametophytes. When the pollen particles reach the female reproductive organs, they will produce a pollen tube, which grows inside the flower to carry the sperm cells close to the female reproductive cells. The pollen tubes transport the sperm cells in the seed plant to meet the egg held in the ovary. From here, a new plant, flower, or shoot will develop. Although pollination may occur often, it does not always result in fertilization.
During pollination, individual pollen particles remain dry and inactive. Once deposited on the female reproduction organ, the pollen will be moistened and become activated. The growth of a pollen tube from a pollen particle, or grain, requires metabolic activity and the consumption of starch. The pollen and the female reproductive organ must be mature and have chemical interactions between one another. After this process is finished, a new shoot grows out from the bulb under the soil and starts either a new plant or a new stem.
The dahlia is found in all parts of the world, but perform best in warmer climates with temperatures of 70° F and above, such as Mexico and other countries in South America. They also can survive in cooler temperatures as low as 60° F if they receive direct sunlight. Dahlias require minimum soil temperatures of 58°-60° F, and a 12 inches in diameter by 12 inch deep planting hole. When grown in gardens, the soil should contain compost, bone meal, and dolomite lime. This makes the soil rich in nutrients and will make healthy and strong blooms.
In the US they are in almost every state but are most abundant in North Carolina and Mississippi. Around the early to mid 1500's, Dr. Francisco Hernández was sent to New Spain by King Philip II for geographical purposes. Later, he wrote a book about the flowers and vegetation in New Spain. In his book, he determined that the dahlia plant was native to the state of Morelos, near the region around Cuernavaca and Tepozotlán. Modern botanists agree that the dahlia is native to Mexico and Guatemala.
Near the end of the eighteenth century, two physicians from Spain named Martin de Sessé y Lacasta and José Mariano Mociño collected plants for Mexico's Botanical Garden and recorded what they found in Plantae novae Hispaniae. Vincente Cervantes was a colleague of Sessé's and sent some dahlia bulbs to a famous Spaniard botanist, Antonio José Cavanilles. After receiving the bulbs, Cavanilles sent some of them to a Swedish botanist Andreas Dahl who gave the first variety its name, Dahlia pinnata. This began the spread of dahlias northwards across Europe. In 1803, a explorer named Alexander von Humboldt found a field of wild dahlias and also sent the bulbs back to Europe. The bulb that was sent is now know as Dahlia coccinea. By 1820, about 100 dahlia varieties were being cultivated. This number increased to more than 2,000 by 1840.
Dahlias are prone to bacterial diseases, fungal diseases, and viral diseases. Bacterial wilt is caused by Ralstonia solanacearum or Erwinia tracheiphila. Bacterial wilt is a disease of the vascular tissue and is spread by contaminated soil, soil surfaces, and infected seeds. When the bacteria E. tracheiphila multiplies within the xylem, it will eventually cause the blockage of the water transport system. As a result, individual leaves on a single stem will wilt. Eventually, this process will continue and spread down the runner and stem, and then infect the whole plant, causing it to shrivel up and die. There is, however, a diagnostic test for bacterial wilt. The presence of the E. tracheiphila causes the sap to become a milky white color and appear to have a sticky consistency. If an infected stem is cut near the crown and the ends are slowly pulled apart, the sap should form a viscous string. Unfortunately, there is no cure.
The second category of diseases of a dahlia are the fungal disease. There is a total of eight fungal diseases. They are flower blight, leaf spot, powdery mildew, southern blight, smut, stem and tuber rot, cottony stem rot, and vascular wilt. Powdery mildew is one of the most common diseases of ornamental plants such as nursery flowers, and woody plants. It can be mistaken as sack of spider eggs with its white to gray looking color on the leaves and sometimes the stem. Although most fungal diseases of any kind can kill the host, powdery mildew does little to no damage to the plant, but distorts the shape, size, and color or the leave(s) or plant. Depending on the environmental conditions and the age of plant tissues that are affected, the dahlia may or may not be affected. Temperatures above 90° F kill some powdery mildew fungi and spores and the presence of free water can reduce spore germination. Powdery mildew prefers high humidity of greater than 95%, moderate temperatures ranging from 68° to 86°, and low light to thrive. The best way to get rid of powdery mildew is to put the plant in the shade because of the cooler conditions. This disease is more active in the spring and fall when there are large temperature differences between day and night, so it would be wise to keep the dahlia(s) that are affected in a green house where there are few temperature changes. The third and last category of dahlia diseases are viral diseases. There are only three known viral diseases that affect dahlias; they are mosaic cucumber, dahlia mosaic virus, and ringspot.
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