If planning to start the flower zinnia from seed, probably the most important consideration is whether to start the seeds indoors or to plant directly into the garden. Starting indoors requires an understanding of the germination process, and poses some risk to the plants when transplanting. Outdoor planting often produces better results, but timing is critical. Regardless of whether starting zinnia from seed indoors or in the garden, the spacing of the seeds is one of the most important factors.
Before deciding on direct sowing or indoor containers for zinnia seeds, gardeners should determine the number of plants they are going to need. If only a few zinnias will be planted, starting the zinnias indoors will not be as difficult and will not require a great deal of space. Those who plan a large bed of the flowers should probably sow the seeds directly into the garden, thus avoiding having to tend a large number of containers through the winter months.
Starting zinnia from seed indoors will require the use of containers, and it is important to place as few seeds as possible for each container. Crowding the seeds could result in smaller, weaker plants, and since transplanting can sometimes be a shock to zinnias, it is important that young seedlings be as strong and healthy as possible. Containers should be large enough to provide ample space for the seeds as they grow, typically 2-4 inches (5-10 centimeters) in width, and at least 6 inches (15 centimeters) deep. Ideally, no more than two seeds should be placed in containers of this size. Place the seeds at a depth of about an inch (2.5 centimeters), as burying too deeply will limit the amount of sun the seeds receive.
After planting, the seeds will need to be thoroughly watered and covered in plastic to seal in moisture and then moved to a warm location where they will receive indirect sunlight. When the zinnia seeds have sprouted, the plastic covering can be removed, and a normal watering routine can then be established. The soil should be kept moist, but not entirely saturated. In many cases, potted plants will tend to learn toward the source of light, so it is a good idea to rotate the pots on a regular basis. When the potted zinnias reach a height of about 6 inches (15 centimeters), they can be moved directly to the garden, as long as all danger of frost has passed.
Sowing zinnia seeds directly into the garden bed is not that much different from starting zinnia from seed indoors. It is important to begin sowing after any likelihood of frost has passed, and just as indoors, spacing is still critical. The seeds need to be placed far enough apart so that the plant can grow to maturity without crowding, ideally leaving about 12 inches (30 centimeters) between each seed. Choose a sunny location and water only if the soil begins to dry out. Zinnias are very hardy plants and may not need fertilization, but if the plants do not seem to be thriving, a time-release fertilizer can be used.