How to Grow Zinnias

Choose the right zinnia for your garden

Most zinnias are forms of Zinnia elegans, but the hybrids and related species behave so differently that the choice decides how much disease you fight. The main split is between tall cut-flower types, which give the biggest flowers but are the most prone to leaf disease, and compact bedding types, many of them bred for disease resistance.

Tall cut-flower zinnias. For Mississippi growers, Mississippi State Extension recommends the Benary’s Giant series for farmer-florists: 40- to 50-inch stems carrying 4- to 6-inch heads, with excellent vase life and good tolerance of summer heat and rain. It also lists the Giant series, the Queen series (lime and blush shades) and the Oklahoma series, whose small 1 1/2- to 2 1/2-inch flowers come on 30- to 40-inch stems with low susceptibility to powdery mildew. For South Carolina gardens, Clemson adds State Fair Giant (30 to 48 inches, bred for higher tolerance of Alternaria leaf spot and powdery mildew) and the Giant Cactus series with spiky, quilled petals. In Florida cut-flower production, UF/IFAS notes that small-flowered varieties such as Oklahoma produce more stems per square foot, about 4 to 5 per week compared with 2 for Benary’s Giant, but take more stems to fill a bouquet.

Compact and bedding zinnias. Clemson’s list for South Carolina includes Dreamland (10 to 12 inches, more resistant to bacterial and fungal leaf spot), Magellan (9 to 12 inches with 4- to 5-inch double flowers), Lilliput, Swizzle and the 6-inch Thumbelina. NC State’s plant database for North Carolina notes good leaf spot resistance in ‘Magellan Orange’.

Disease-resistant types for humid summers. These are the safest choice across the Southeast and Gulf Coast:

• Profusion (Zinnia x hybrida), a cross of Z. elegans and narrowleaf zinnia, makes mounds about 18 inches tall. Clemson credits it with higher resistance to powdery mildew and leaf spot, and NC State describes superior disease resistance, tolerance of heat and humidity, and no need for deadheading.
• Zahara (Zinnia marylandica) is compact (8 to 12 inches), heat-tolerant and disease-resistant, according to Clemson.
• Narrowleaf or creeping zinnia (Zinnia angustifolia) has small star-shaped flowers, excellent leaf spot resistance and no need for deadheading.

Texas A&M AgriLife named the marylandica types, including Profusion and Zahara, Texas Superstars for fall planting because they resist powdery mildew and hold color until frost.

When to plant

Zinnias come from Mexico and the Southwest, and they need real warmth. Clemson sows once the soil and air are around 70°F, after the last frost. UF/IFAS gives 74 to 80°F as the temperature where seed germinates in 5 to 7 days, and notes that growth slows, and can stop, when daytime temperatures stay below 60°F. That is why both Clemson and NC State wait until after the last frost, and why the frost date alone is not enough: the soil also has to warm up. The dates for your zone above show when that window opens.

For flowers sooner, sow in plug trays 3 to 4 weeks before the last frost, as UF/IFAS does, and transplant the seedlings after frost danger has passed. UF/IFAS stresses that tray-grown seedlings need high light to develop into good transplants.

Zinnias are short-day plants. They grow leaves in the long days of early summer and set buds faster as days shorten, which is why a later sowing can reach bloom faster than an early-summer one. In UF/IFAS trials, a June sowing took 100 days to bloom and a mid-August sowing 83 days. Mississippi State notes that flowering peaks once daylight drops below about 14 hours, while longer days and warmth give longer stems and bigger flowers.

Sowing and spacing

Sow 1/4 inch deep and keep the surface moist until seedlings are up. Thin when they reach about 3 inches tall (Clemson).

Spacing affects disease as much as plant size, because air moving through the planting dries the leaves. Clemson spaces dwarf varieties 8 to 9 inches apart and taller 2- to 3-foot varieties 12 inches apart, with rows 12 to 18 inches apart. Cut-flower growers do something different: UF/IFAS and Mississippi State plant 6 inches apart in rows 2 feet apart for repeated harvests, and closer still for a single harvest, because tight spacing reduces side branching and pushes up long, straight stems. In a home cutting bed, 6 inches between plants works if the rows are wide enough for air and access. In a mixed border, use Clemson’s wider spacing.

Watering

Give about 1 inch of water a week, applied at the base of the plants whenever the top 1 to 2 inches of soil is dry (Clemson). Mississippi State recommends drip irrigation. Overhead watering spots the petals, splashes soil onto the leaves and spreads leaf spot and mildew. Mississippi State also warns against watering at night, which encourages gray mold (Botrytis) in cool, wet weather.

Once established, zinnias are fairly drought-tolerant (UF/IFAS).

Soil and feeding

Zinnias want full sun, at least 6 hours, and fertile, well-drained soil with organic matter. UF/IFAS gives a pH range of 5.5 to 7.5, and NC State says zinnias are not fussy about pH. Adding well-cured compost paid off in UF/IFAS research, giving faster crops and better soil. Clemson recommends a soil test before fertilizing. For cut-flower rows, UF/IFAS side-dresses with a balanced fertilizer and advises against fertilizers high in ammonium nitrogen.

Pinching, support and deadheading

Sources differ on pinching because they grow for different results. Clemson suggests pinching back leggy plants for fuller growth, and Texas A&M AgriLife says pinching young standard zinnias makes them bushier (the marylandica types branch without it). UF/IFAS, growing for long cut stems, does not pinch before the first flower and removes side shoots to get larger blooms. For more, shorter stems and a bushier plant, pinch. For fewer, bigger flowers on long stems, don’t.

Tall cut-flower varieties can fall over in wind or heavy rain. Mississippi State and UF/IFAS stretch wire or mesh netting horizontally over the bed so stems grow up through it.

Cutting or deadheading keeps new buds coming. Clemson notes that cutting just above a leaf node also prevents brown stubs on the stems. Expect stems and flowers to get shorter and smaller as the season goes on and you harvest repeatedly (UF/IFAS).

Regional differences

Cold north. The season is the main limit. Wait for warm soil, consider plug-tray transplants for a head start, and expect zinnias to slow in cool late-summer nights below 60°F. NC State notes that zinnias bloom continuously from summer to fall in cool-summer climates.

Humid South and Gulf Coast. Disease is the main limit. NC State advises growing disease-resistant varieties in hot, humid climates and warns that fungal problems are common in shaded or moist sites in the South. It also notes that zinnias may slow at the peak of summer. Plant Profusion, Zahara or narrowleaf zinnias in beds, and give tall cut-flower types full sun, wide rows and drip irrigation. Where summers are long, a mid- to late-summer sowing gives fresh plants for fall color, as Texas A&M AgriLife recommends.

Arid West and low desert. Zinnias handle dry heat well. The University of Arizona’s low-desert guide lists planting from March through June, with blooms from April to November. Watch for spider mites, which thrive in dry conditions.

Texas. Texas A&M AgriLife in Bexar County (San Antonio) recommends sowing zinnias in July for late-summer and early-fall color. Fall zinnias in 4-inch pots usually reach Texas garden centers in September and bloom until the first frost.

Florida and warm-winter areas (zones 9 to 11). The UF/IFAS planting calendar lists zinnias from March through September in North and Central Florida. South Florida can also plant in February, November and December.

Cutting zinnias for the vase

Zinnias are cut fully open, not in bud. UF/IFAS says to cut when the flower is fully developed and pollen is starting to show. Harvest in the cool part of the day, early morning after the dew dries or in the evening, and leave two or three pairs of leaves on the stem so the plant can regrow (Mississippi State). Strip the lower leaves and put stems straight into water with flower food. UF/IFAS research showed that floral preservatives greatly extend vase life. Clemson suggests changing the water every 2 to 3 days.

Mississippi State gives a vase life of 7 to 10 days; Clemson says a week or more. Zinnias are not sensitive to ethylene, so Mississippi State notes they can be stored alongside other crops. Mississippi State stores cut stems for up to 7 days at 38 to 42°F, while UF/IFAS uses 42 to 45°F.

Problems and fixes

• White, powdery coating on leaves: powdery mildew, worst in humid weather. Space plants farther apart, water at the base and grow resistant types.
• Reddish-brown spots with pale centers: Alternaria leaf spot, worse in warm, wet weather. Keep foliage dry, remove infected parts and stay out of the bed when plants are wet.
• Small translucent spots with yellow edges that turn reddish: bacterial leaf spot, which Mississippi State says favors cooler weather. Remove infected plants and avoid overhead watering.
• Gray mold on flowers and buds: Botrytis in cool, wet weather. Remove affected parts and improve air flow.
• Stunted plants with green, deformed flowers: aster yellows, spread by leafhoppers. Remove infected plants and nearby weeds that host the disease.
• Aphids, spider mites and Japanese beetles: wash aphids and mites off with a strong spray of water or use insecticidal soap. Lady beetles and parasitic wasps help control aphids.
• Chewed flowers: budworms and grasshoppers, according to UF/IFAS.

Deer usually leave zinnias alone (NC State), and Clemson says deer and rabbits avoid narrowleaf zinnia, which makes it a good choice for exposed beds. Zinnias are also good pollinator plants and attract butterflies, bees and hummingbirds.

Common mistakes

• Sowing on the frost date in cold soil. Wait until soil and air are around 70°F (Clemson); seed germinates best at 74 to 80°F (UF/IFAS).
• Crowding a border. Air moving through the planting dries leaves and helps prevent mildew. Use 8 to 12 inches between bedding plants.
• Watering from overhead in the evening. Wet leaves overnight spread leaf spot, mildew and gray mold.
• Planting only tall elegans types in a humid climate. Mix in Profusion, Zahara or narrowleaf zinnias for beds that stay clean all summer.
• Cutting zinnias in bud. UF/IFAS and Mississippi State both harvest when flowers are fully developed.
• Letting plants go to seed. Unpicked, spent flowers slow new bloom on standard types. Keep cutting.