Zinnias grow best when you plant the seeds right in your garden soil, just about 1/4-inch deep. Zinnia seeds are large, which make them a good choice for children to plant.
Thinning the seedlings increases airflow between plants and helps prevent powdery mildew later in the summer.
Zinnias are considered annuals, meaning they will not return the next spring. But if you live in USDA hardiness zone 9 to 11, you can enjoy zinnias year round. If not, try growing smaller varieties in containers and bringing them indoors before the first frost.
Different varieties of zinnias can grow from 6 inches to 3 feet tall. When planning your garden, be sure the zinnias do not block sunlight for other plants.
Season and Location
Zinnias are from Central America. They like direct sunlight and cannot survive frost.
Zinnias grow best when the daytime temperature stays above 60°F. They may then produce flowers throughout the entire summer.
There are many different sizes and colors of zinnias. The most attractive flowers will be visited by many pollinators such as butterflies, hummingbirds and bees.
Oregon State University scientists have noticed that male bumblebees out late in the day will sometimes spend the night nestled between zinnia petals.
Key Pests and Diseases
Diseases include powdery mildew: water early in the day, and allow enough space between plants for airflow.
Insect pests include aphids and whiteflies. They can be washed off with water.
When to Plant and Harvest Zinnias in Oregon
Central and Eastern: Plant seeds indoors in June to harvest July through September.
Willamette Valley: Plant seeds indoors late April through May to harvest May through September.
Coast: Plant seeds indoors April through May to harvest June through November.