As with most annuals, it is usually easier to start zinnia from seeds than to try and root the plant from cuttings. To get the best results, you should begin by conditioning the soil, and be sure to mulch after the plants have gotten started. While zinnias are not finicky about the type of soil, the health of the plant depends on the soil moisture content, which will help prevent many types of disease, including powdery mildew.
Step 1: Soil Preparation
As an annual, zinnia is great for propagating through seeds. Use a potato rake and break up the soil in the area you will be planting. Condition the soil with compost, or mix it with potting soil. Sifted compost should be thinly spread over the planting area, and then well-mixed into the exiting soil.
Step 2: Planting Zinnia Seeds
To start zinnia seeds, sow directly in ground that has been loosened and conditioned. For best results, soak dried seeds in water for 4 to 6 hours before planting in slightly moist soil. If the soil forms a wet sheen when it's squeezed, don't plant zinnia seeds until the soil has dried out. Adding sand may help aerate the soil and allow it to drain better. As the seeds begin to sprout, weed out weak plants to encourage the stronger ones to grow.
Step 3: When to Mulch
After the zinnias have reached a height of 6 to 8 inches, it's time to add mulch. For the best plant protection use 2 to 3 inches of mulch, and spread it out carefully around the base of the plant with your hands. Do not broadcast the mulch, as it will damage tender shoots.
Step 4: Periodic Fertilizing
Every 4 to 6 weeks, pull back the mulch around your zinnias and add a thin layer of sifted compost. return a layer of mulch over the compost. Spreading 1/2 inch or so of compost will allow plant nutrients to absorb naturally into the soil. The mulch prevents moisture from evaporating out of the compost, which could allow it to be blown away by wind.
Step 5: Avoiding Zinnia Diseases
The single largest danger your zinnias will face is related to the amount of water they receive, and how it's delivered. Don't spray the leaves. Apply water directly to the mulch and let it seep naturally into the soil. Zinnias prefer soil that is somewhat dry, which also prevents the growth of the zinnia's most common enemies: fungal and bacterial growths.
Step 6: Zinnia Pests
Japanese beetles, mites and aphids are the most common insects that feed on zinnia leaves and flowers. These pests can be controlled with commercial pesticides, but an organic garden uses other insects instead. Ladybugs are attractive predatory insects that feed on aphids and mites, and praying mantis are relatively indiscriminate, feeding on many types of insects, including spiders, flies and wasps.