How To Grow And Care For Zinnias
Zinnias are popular, flowering annuals that thrive in warm weather and are often cut for floral arrangements.
Zinnias are warm season, flowering annuals that are easy to grow from seed. Grown in full sun, these herbaceous plants will bloom from summer to frost. Intense breeding efforts have produced such a wide variety of plant sizes, flower colors, and flower shapes making them very versatile in the garden. Depending on the size, zinnias can be grown in containers or window boxes, as small border plants, in the garden, and for cut flowers. Seeds can be sown directly in the soil or small bedding plants can be purchased from garden centers. The flowers support beneficial insects, bees, butterflies, and hummingbirds.
Plant Attributes
Common Name: Zinnia
Botanical Name: Zinnia spp.
Family: Asteraceae
Plant Type: Herbaceous, Annual
Mature Size: 6-48 in. tall, 12-24 in. wide
Sun Exposure: Full Sun
Soil Type: Moist but Well-Drained
Soil pH: Neutral Soil pH
Bloom Time: Summer, Fall Bloom Time
Flower Color: All colors except blue
Hardiness Zones: 6-11 (USDA)
Native Area: North America, South America, Mexico
Zinnia Care
Zinnias are very popular summer-blooming annuals. They are easy start from seed and grow, and they make excellent cutting flowers. There are about 20 species, both annual and perennial, from the Southern United States, South America, and Mexico. Typically, gardeners grow the summer blooming annuals that last until frost. There is a wide variety of cultivars producing flowers in every color but blue and every imaginable shape. Plants can be as small as 6 inches to as tall as 48 inches.
Zinnia seeds can be sown directly into the garden bed or container. They are quite commonly sold as bedding plants at garden centers in small six packs at the beginning of the season. They should be planted, or seed should be sown, in the summer, after the last spring frost, when temperatures are a consistent 50 to 55 °F.
Zinnia flowers support beneficial insects, bees, butterflies, and hummingbirds. Depending on the variety, flowers should be deadheaded (removed) but some varieties do not need this (see Types of Zinnias).
Light
Zinnias need to be grown in full sun, at least 6 hours a day.
Soil
Part of what is “easy” about zinnias is that they can be grown in average soil, but they perform better with fertile soil or soil that has been amended with compost. Although established plants may be resistant to a dry spell, they do best with evenly moist, well-drained soil.
Water
Immediately after planting or sowing, water well and water often during the germination process. As plants grow, water at the base of the plant, not overhead. Water in the morning, not in the evening. Because some varieties are prone to fungal diseases, it is best to keep the foliage as dry as possible. They do not need an irrigation system like drip irrigation. Once established, usually rainwater is enough but if they seem to be wilting, water at the base in the morning.
Temperature And Humidity
Zinnias flourish in the heat, but they dislike humidity. A hot humid environment encourages fungal diseases.
Fertilizer
For small, low growing types of zinnias, a fertilizer may not be needed especially if the soil is high in organic matter. However, if one is cutting flowers often and pushing the plant to constantly bloom, an application of 10-10-10 will help keep the plant nourished and healthy. Zinnias grown in containers will benefit from a water-soluble, balanced fertilizer as nutrients will leach out of a container.
Types of Zinnias
Zinnia acerosa
Dwarf, mound shape, about 4 to 10 inches tall with many branches and gray-green, needle-like leaves
A native perennial type, hardy to Zone 7-9 USDA but found in very dry areas
Summer bloomer, flowers are simple, daisy shape, about 1 to 2 inches wide
White flower and a yellow flower form
Zinnia angustifolia
Compact, mound shape, about 12 to 16 inches tall
Single, daisy-like flowers in orange, red, yellow, pink, white, and bicolors
Narrow leaves
High drought tolerance, deer resistant, powdery mildew resistant, and deadheading not required
Annual that blooms from summer to fall
Zinnia elegans
Annual that blooms summer to fall.
The species from which we have most of our modern cultivars
Size ranges from 8 to 48 inches tall
Flowers in every color except blue
Flower shapes vary tremendously
Zinnia grandiflora
Native perennial in CS and TS, Zones 9-11 USDA, but an annual in other areas
Spreads by rhizomes so acts like a groundcover, mounding shape, 6 to 8 inches tall
Flowers from summer to fall with bright yellow, simple, daisy-like flowers comparatively large to the rest of the plant, about 1 to 2 inches wide
Multi-branched, tiny needle-like leaves
Zinnia haageana
Annual, bushy, leafy plant about 12 to 24 inches tall with narrow leaves
Flowers about 1 inch wide in various bicolored shades of yellow, red, orange, and brown
Floral shapes are single, semidouble, and double
Blooms from summer to fall
Heat and drought tolerant
Deer and powdery mildew resistant
Zinnia marylandica (Zahara)
Annual often called the Zahara series
A cross between Zinnia angustifolia and Zinnia elegans (syn. violacea) to combine Zinnia angustifolia’s powdery mildew and leaf spot disease resistance and Zinnia elegans’ large flowers in a variety of colors
About 12 inches tall with flowers about 2 inches wide
Like Profusion, bred for disease resistance but flowers are larger and have more color variation
No deadheading needed
Zinnia peruviana
Annual, about 18 to 24 inches tall with single blooms, about 1 ½ inch diameter
Flower colors are yellow, orange, or red in solid matte or flat colors, not bright
Profusion Series
Annual that is a cross between Zinnia angustifolia and Zinnia elegans to combine Z. angustifolia’s disease resistance with Z. elegans’ large flowers in a variety of colors
Resistant to powdery mildew and bacterial leaf spot
Many flower colors including bicolor and double flower form
Plants are 16 to 18 inches tall and 20 to 24 inches wide
Flowers are about 2 inches wide
Drought tolerant
No deadheading needed
Zinnia pumila
Often called “cut and come again”
Bright, solid flower colors in pink, orange, yellow, and red
Plants are 24 to 36 inches tall, with 2 to 3-inch blooms in single, semi-double, and double forms
Pruning
When the plant is about a foot tall, pinch or cut above the node. The node is where the leaves attach to the stem. This is done with the tall types to encourage branching, thus more flowers. This is not necessary for the small, mounding types. When cutting flowers for vases, cut above the node and this will naturally prune the plant to encourage branching (as well as more flowers).
How to Grow Zinnias from Seed
Zinnias are easy to grow from seed. Seeds can be sowed directly in the garden bed or container. This is the recommended method for several reasons:
They do not always do well if they are transplanted
Most people do not have the space to start many zinnia seeds indoors
One does not gain time or get a jump start by starting indoors, because zinnias are quick growers.
However, if you only need a few for a container, you can start the seeds indoors.
How To Start Seeds Outdoors
When the last spring frost has passed and night temperatures are 50 to 55 °F consistently, sow seeds in the garden bed (or container) that has been prepared in advance.
The garden bed should have no weeds and compost should have been added if needed.
Sow seeds ½ inch deep and space depending on the ultimate size of the plant (refer to the seed packet). The larger the plant the more space is required among them.
Cover with soil lightly, water, keep moist until germinates. They should germinate in about 7 to 10 days.
Once they have germinated, you may have to thin seedlings to maintain spacing.
How To Start Seeds Indoors
In clean seed starting trays (with drainage holes), add moistened seed starting mix. Insert zinnia seeds about ½ inch deep and cover with the mix.
Place under grow lights or fluorescent tubes, leaving lights on for 14 to 16 hours per day.
The lights have to be adjustable. They should be only a few inches away from the plant.
Mist with water frequently so seeds do not dry out. It is important that they do not dry out when they begin the germination process because germination will stop if allowed to dry and the seeds cannot be “revived.”
As the seed germinates and grow, may have to adjust lights to continue to be only a few inches away from the plant.
As the seedling grows, can water or mist less often because the roots have formed and are able to obtain water from a lower depth in the mix.
Thin the seedlings, which is to reduce the number in order to create space for the rest. Cut the weakest seedlings with nail or manicure scissors at the base. This will make room for the strongest. Best to cut, do not pull seedlings out as this will disrupt the rest of the seedlings.
When true leaves have developed, the last spring frost has passed, and night temperatures are 50 to 55 °F consistently, can transplant outside into a slightly larger container.
Zinnias should be hardened off before moving to the garden bed in this small container and this can take 2 weeks.
Harden off by putting the small containers in the shade first, protected from heavy rains and winds. Gradually move the containers to full sun, rain, and the rest of the elements so it is “hardened” to the elements. More roots should have formed.
Transfer the plant from the small container to the ground (just like you would if you had purchased the plant from a garden center).
Water to establish the plants.
Overwintering
Because zinnias grow, flower, and set seed in one growing season, they are not “overwintered” or saved for the next season. In late fall, when frost arrives, the plants are cut at the ground level and removed. If they had been suffering from a disease, especially a fungal disease, they should be bagged and thrown away, not in the compost pile. One can easily collect the seed and sow them next year keeping in mind that if the plant was a hybrid or a cultivar, one may not obtain the desired characteristics that were in the parent plant.
Common Pests & Plant Diseases
Fungal Diseases
The most common disease associated with zinnias is powdery mildew, a white fungal growth on the upper surface of the leaves. This usually occurs during hot and humid weather and is unsightly. If only a few leaves are infected, they can be removed and destroyed. Although a fungicide could be applied, the fungicide will adversely affect beneficial insects and pollinators for which zinnias are a magnet. It is best to plant resistant varieties, provide adequate space among plants for increased air circulation, water in the morning, and water at the base of the plants.
Another fungal disease is Alternaria leaf spot which causes red-brown or purple spots on the foliage that may eventually create holes or kill the leaves. Again, although a fungicide could be applied, the fungicide will adversely affect beneficial insects and pollinators. It is best to keep the foliage as dry as possible by providing adequate space among plants for increased air circulation, water in the morning, and water at the base of the plants.
Bacterial Disease
Bacterial leaf spot appears as angular, red-brown spots on the leaves, often with yellow halos. This is caused by bacterium Xanthomonas. Again, although a fungicide could be applied, the fungicide will adversely affect beneficial insects and pollinators. It is best to keep the foliage as dry as possible by providing adequate space among plants for increased air circulation, water in the morning, and water at the base of the plants.
Viruses
Zinnias can be affected by a virus called aster yellows which deforms and discolors the flowers. Aster yellows is primarily transmitted via leafhoppers, which are only a few centimeters long, with wedge-shaped, brown, yellow, or green bodies. As the name suggests, these insects quickly hop from plant to plant. When a leafhopper feeds on a plant infected with aster yellows, the pathogen enters the leafhopper’s body and stays within for as long as that leafhopper lives. As the leafhopper feeds on plants and moves around from plant to plant, it spreads the phytoplasma thus spreading the disease. Once the zinnias are infected, they must be removed. If they are not removed, they serve as host plants, as a source of phytoplasma, thus infecting other garden plants. Keep in mind that aster yellows affect other popular flowering plants so if you see it on another plant, you want to remove that plant so it does not spread to the zinnias.
Pests
In the summer, Japanese beetles may feed on the leaves. In addition to seeing the brown and green iridescent beetles, you will see holes and ragged edges in the foliage. To control, hand pick the beetles in the morning and drop in a bucket of soapy water. You can spray with neem oil (when pollinators are not around) or insecticidal soap but keep in mind that beetles are feeding for about one month while most zinnias bloom from summer to frost.
Zinnias may be attacked by spider mites, aphids, and whiteflies. To control them, spray the plants with insecticidal soap. Aphids are tiny insects that suck the plant’s nutrients from the foliage, thus weakening the plant and making it unsightly. Aphids also excrete a sticky substance called honeydew which attracts sooty mold, and they can transmit viruses. Whiteflies operate in a similar manner and actually look like tiny white flies. They congregate under the foliage but will fly around for a second if the plant is disturbed.
Spider mites are tiny spider-like mites that also suck nutrients from the foliage. It is easier to see their webs under the leaves. They weaken the plant and create specks on the leaves or cause the foliage to be a lighter yellow.
How To Get Zinnias To Bloom
Zinnias need full sun, at least 6 hours, in order to bloom. Usually, they are very reliable flowering annuals and there are no blooming issues. If they have been blooming well and there seem to be less flowers, it may be that the plant was not deadheaded when it needed to be. Be more vigilant about removing old flowers.
You can encourage more blooms per plant by pinching the plant back when it is young to encourage more side branches. This is more important with the tall varieties, less so with the small bushy, mound types.
It seems counterintuitive but the more you cut the flowers (for arrangements), the more flowers you will have. If the plant thinks it has not completed its mission by setting seed, it will continue to produce more flowers until frost. The more you cut the flowers, especially the large plants, the more you may want to amend the soil with a balanced fertilizer to make sure it has the nutrients to continue to produce blooms.
Common Problems With Zinnias
Zinnias are easy to grow and will bloom profusely despite the heat. However, they do not care for the humidity. The most common problems are fungal diseases exacerbated by humidity.
Why are my zinnia leaves coated with a white or gray powder?
If it looks like someone sprinkled corn starch on the leaves, it probably is powdery mildew. If it is toward the end of the growing season, you can simply remove the plant. If it is in the beginning of the growing season, you can apply a fungicide. However, the fungicide will adversely affect beneficial insects and pollinators for which zinnias are a magnet. It is best to plant resistant varieties, provide adequate space among plants for increased air circulation, water in the morning, and water at the base of the plants.
Z. angustifolia and Z. haageana are naturally resistant and Z. marylandica (Zahara) and Profusion were bred to be resistant to powdery mildew.
Why are there are brown spots on my zinnia leaves?
If the foliage has spots or is becoming brown, it can be the fungal disease such as Alternaria or a bacterium such as Xanthomonas. Again, although a fungicide could be applied, the fungicide will adversely affect beneficial insects and pollinators. It is best to keep the foliage as dry as possible by providing adequate space among plants for increased air circulation, water in the morning, and water at the base of the plants.
Why are my zinnia leaves drooping and look wilted?
Wilted and drooping leaves may be because of lack of water. The hotter it is in the garden, the more likely the plant will need water, especially if the plant is in a container. Water deeply and see if the plant revives. In the future, monitor the plant and the weather forecast more often. Container grown plants will dry out faster than plants in the garden bed.
Why are my zinnia flowers deformed and stunted?
This is caused by a virus so there is no treatment. The minute you see this, remove the plant, bag, and throw away. Do not put in the compost pile. Try growing zinnias in a different place in the garden.
Why is my zinnia flower color different than what I expected?
If the flower color is not what you expected or is not the same as last year, it may be that you had sown seeds saved from a hybrid or cultivar. The next generation may be reverting to one of the parent’s color. As long as the plant is healthy, there is nothing wrong with this. If you still want the other color, then purchase a new seed packet or a new plant, and don’t sow your saved seed.
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