ZinniasZinniasHave you included zinnias in your garden this year? If so, you're in good company, since the National Garden Bureau has designated 2000 the "Year of the Zinnia." These easy, long-blooming tender annuals provide lots of color for the summer garden. The plants come in a variety of sizes, from 6-inch dwarfs to almost 4 feet high, and the flowers range from tiny button-like heads to large heads with double petals, in almost every color except true blue.
Zinnias
are American natives that originated from the Southwest U.S., Mexico and Central
America. The zinnia was named after the 18th century German botanist Dr. Johann Gottfried
Zinn, who wrote the first description of this nondescript wildflower that grew
in the Mexican deserts in the 18th century. Original wild zinnias were small,
weedy, dull purplish-red, daisy-like flowers with single petals and a protruding
cone in the middle. It would take nearly a century before European breeders
seriously began developing it as a garden plant. Early varieties were introduced
in the U.S. in 1796, with double forms appearing in the mid 1800's, but the plant didn't really take off until 1920, when Bodger Seeds
Ltd. introduced the dahlia-flowered "Giant Dahlia," and
"California Giant," a natural mutation of "Mammoth." The
large, flat-flowered heads and multiple colors started a new trend in plant habit and form.
The latter variety even
won a gold medal from the Royal Horticulture Society of England.
Hybridizers
have turned this lowly wildflower into one of the most popular bedding plants. Zinnias
now come in a wide variety of flower forms: single, semidouble, or double.
Single-flowered zinnias have just one row of petals and the center of the flower
is exposed. Double-flowered zinnias, with so many rows of petals that the center
is hidden, have several shapes. Beehive types have small blooms with stacks of
flat petals that resemble little beehives. Button-type flowers are similar to
beehive except it's flatter. The edges of each petal on cactus-shaped flowers
roll under and the petal twists and bends. The petals on dahlia-flowered zinnias
are large and flat and usually semi-double, which means that the flowers have
many rows of petals but the center can be seen.
Although there are more than a dozen species of zinnias, only a few species, all annuals in our climate, are regularly planted in gardens:
Zinnia elegans is the most common zinnia, and comes in
heights up to 3 feet with single or double flower heads from 1 to 7 inches
across. They bloom in all colors: pink, rose, red, cherry, lavender, purple, orange,
salmon, gold, yellow, white, cream and light green. Some flowers are solid colored,
while others are multicolored or zoned. Flower shape varies from round, domed or
ball-shaped to dahlia- or chrysanthemum-like flowers. Tetraploid varieties (they
have four, rather than the normal two, sets of chromosomes) were developed in
the 1950's that produced larger flowers on
stronger stems, and displayed vigorous growth and increased disease resistance.
Hybrid varieties were soon developed after that. Some common varieties of Z.
elegans include:
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Z. angustifolia (= linearis) has smaller, single golden orange
flowers with yellow stripes and narrower foliage than Z. elegans.
The
compact plants grow 8 to 12 inches high, and can spread to 2 feet. The variety
"Crystal White," with pure white flowers with yellow centers, was an
AAS winner in 1997. In 1999, the "Profusion" series, "Cherry" and "Orange," won gold medals from AAS – the first awarded to a flower in 10 years. This series is the result of crossing Z. angustifolia and Z. elegans. They are tolerant of heat and humidity, disease resistant, and are compact growers, with 2- to 3-inch single flowers.
- "Persian Carpet" was an AAS winner in 1952. It has 2 inch double, bicolored flower heads of gold, maroon, purple, chocolate, pink or cream on a 15-inch plant.
- "Old Mexico," AAS in 1962, has fully double, 2½ inch blooms of deep, rich mahogany highlighted with yellow-gold, on bushy, compact, 18-inch plants.
Z. pauciflora
(=peruviana) produces 1½ inch, single red or yellow
flowers with button-like centers on sturdy stems. It is good for cutting and
drying. The 30" tall plants are resistant to powdery mildew. This species
is not commonly offered, with only the varieties "Bonita Red" and
"Bonita Yellow" available.
Zinnias are good for edgings,
massed in
borders or beds, or the taller varieties as background plants. Many varieties
make excellent cut flowers and the smaller varieties are suitable
for container plantings. Most varieties begin to bloom when still very small and
continue until frost. Zinnia flowers are also attractive to many species of
butterflies.
Zinnias can be started from seed, either indoors 4 to 6 weeks before the last frost, or directly in the garden when the soil has warmed sufficiently. You can also purchase zinnias as bedding plants, although you may not have as many choices of cultivars. Plant zinnia seedlings outdoors only after the danger of frost is past as young plants are susceptible to chilling cold. They will not grow substantially until temperatures are above 50 degrees.
Zinnias
are one of the easier annuals to start from seeds.
Germination takes 5-7 days. I've been very successful in growing plants when
seeded individually in cells of seed-starting trays or when transplanted from a
group of seedlings in a single cell – despite warnings in many publications
that zinnias resent being disturbed and should only be seeded in place or grown
in peat pots. When transplanting, wait until the first
true leaf is emerging. Separate the roots as carefully as possible and place the
seedlings into their new cells as deep as possible, up to the first set of
leaves. Select appropriately sized tray cells depending on the predicted size of your
cultivars. Tall varieties grow quickly and may outgrow their container before
it's time to plant outside!
Zinnias do best in full sun in fertile, well-drained soil, so amend your site first if necessary, and fertilize twice monthly for optimum bloom. When planting in the garden, space the plants 4 to 24 inches apart according to variety – whatever their height will be at maturity. For cut flowers, however, I crowd my plants in their raised bed in the cutting garden to encourage longer stems. To produce bushier plants, pinch the tops out of young plants when they are 4-6 inches high (but some varieties have been bred to be compact). Remove faded blossoms to keep them producing. Plants should be kept well-watered.
Zinnias have few insect pests. Aphids can be controlled with sprays of insecticidal soap or synthetic insecticides, or by releasing predaceous green lacewing larvae. Naturally-occurring predators and parasitoids may also decimate aphid colonies. Feeding by four-lined plant bug causes small, round, brown sunken spots on the leaves. This insect generally only causes sporadic damage, and no good controls are available. Spider mites are generally only a problem in hot, dry weather. Their feeding causes the foliage to become bronzed or stippled. Insecticidal soap or miticides can be used to suppress their populations.
Powdery
mildew is the most significant disease of zinnias that are not resistant to the
fungus. It is particularly a problem late in the season when white to grayish
powdery growth appears on the leaves. The fungus can be controlled with
applications of fungicides (chlorothalonil or benomyl). Planting zinnias far
enough apart to provide good air circulation will help reduce or delay
development of disease. If you have persistent problems with powdery mildew, try
to plant resistant varieties.
Other diseases include blight, or alternaria leaf spot, which starts as reddish brown spots with graying centers. Eventually dark brown cankers form on the stems and flowers become spotted or completely blighted. Bacterial leaf spot also causes reddish brown spots, but these are angular and only on the leaves. This disease can cause plants to die out by mid-August.
– Susan Mahr, Department of Horticulture, University of Wisconsin - Madison
