How to Grow Zinnias
If you're looking for a prolific annual flower for gardens and bouquets in a wide range of colors, try zinnias. The plants range from low growing, to tall and statuesque. Flowers come in all the colors of the rainbow depending on the variety. Group them in a flower or vegetable garden for color and to attract bees and butterflies.
When & Where to Plant Zinnias
Light: Zinnias grow and flower best in full sun. They can flower in part shade, especially in warmer climates with afternoon shade, but they may be more susceptible to disease and have fewer flowers.
Soil: Zinnias grow best on fertile, well-drained soils high in organic matter. Well-drained soil is important because zinnia seedlings can be prone to rotting in cool, wet soils.
Spacing: Plant zinnia seeds a few inches apart in rows or clumps. Thin to 8 to 18 inches apart, depending upon the variety, once the plant has grown four leaves.
Planting: Plant zinnias in spring after all danger of frost has passed, around the same time you'd plant tomatoes. Zinnias are easy to grow directly-seeded into the garden. For sooner blooms, start seeds indoors 4 to 6 weeks before your last frost date.
How to Grow Zinnias Throughout the Season
Growth Habit: Zinnias come in a wide variety of plant sizes. Some are low-growing. 1-foot tall plants with a mounding habit. Others are 3-foot tall plants with big flowers. Plant the right variety for your usage and that your space allows. Mounding varieties have smaller flowers and produce more ornamental looking plants. Taller varieties are often used for cutting and are best grown in rows or clumps for a better visual effect.
Staking: These flowers have sturdy stems, so do not need staking.
Watering: Zinnias need moist soil to grow their best. This is especially true of young plants. Water deeply a few times a week so the soil stays moist 6 to 8 inches deep. Don't overwater because zinnias can also succumb to rot diseases especially on wet, clay soils. Once established, zinnias a relatively drought tolerant.
Fertilizing: Amend the soil before planting zinnias with compost. When flowers start to form, side dress with an organic 5-5-5 fertilizer to get more and bigger blossoms.
Mulching: Mulch zinnias once they are established with a 2 inch layer of straw or bark mulch. This will help preserve soil moisture and prevent weed growth.
Trimming & Pruning: Deadhead (trim) spent flowers regularly to promote more flowering and to keep the plant tidy. To create bushier plants on tall varieties, pinch the growth tip of the plant when young. This will stimulate more side branching, a shorter plant, and more flowers. However, it may delay flowering, especially in cool summer areas.
Zinnias: End of Season Care
Dividing & Transplanting: Zinnias are annual flowers so don't need dividing. However, they can be finicky about being transplanted. If growing them indoors for transplanting, use larger peat or plastic pots so you don't have to transplant more than once. Try not to disturb the roots when moving the plants.
Pests & Disease: The biggest pests of zinnias are powdery mildew disease and Japanese beetles. Powdery mildew disease is especially prevalent in humid areas of the country in late summer. A white film develops on the leaves. Eventually the leaves turn yellow and drop. The whole plant can quickly get infected making it look messy and reducing its overall flowering. To prevent powdery mildew, space plants further apart to increase air flow, grow disease-resistant varieties, and apply preventative sprays early in the season. Neem oil and Serenade (Bacillus subtilis) are two organic sprays that work well to prevent the spread of powdery mildew. You can make your own organic spray by adding 1 tablespoon baking soda to 1 tablespoon of horticultural oil in 1 gallon of water. Aphids also can attack zinnia leaves causing them to curl and yellow. Knock aphids off zinnia leaves with a strong spray of water from a hose.
Japanese beetles love zinnias and are often found feeding and shredding the leaves. To control Japanese beetles, apply beneficial nematodes or milky spore powder in the early summer and fall to kill the c-shaped, white grubs in the soil. These are the larvae of the adult beetles. Use beneficial nematodes in colder areas. To reduce the adult beetle population in summer, set out Japanese beetle traps at least 200 feet away from your garden in a perimeter around your property. In this way, the flying beetles will find the traps before they find your plants. You can also handpick the adults in the morning while they are sluggish and drop them in a pail of soapy water.
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