Zinnias are often recommended for beginner gardeners because they are seen as an easy plant to grow. And with one caveat, this is true.
The one thing, the single most important thing, a gardener should know about this plant is that it is a heat – seeking plant. It is not a plant for cold soil. So while many gardeners try to beat the season, planting early, this is generally a recipe for failure with Zinnia.
So if I were to tell you only one thing about this plant, it would be to wait until the soil is warm (touch the soil with your wrist as if it were a baby’s bottle – and if your wrist feels cold, delay planting.)
This is a plant for the full sunshine garden. It is not a plant for shade of any kind.
When the flowers begin to fade, cut them off. Regular deadheading – the term we use to denote removing spent flowers – will encourage the plant to continue producing new flowers.
In general, they do benefit from rich soil, but if you find the taller varieties flopping over you may be over feeding. And, if the plants are flopping, then they can be staked and tied up so you can continue enjoying the blooms or using them as a cut flower.
I feed my zinnias once a month with a liquid fish emulsion. If you feed too much, they will flop and be ugly.
Note that the Zinnia is not particularly fragrant, but that it makes an excellent cut flower.
Direct sowing: this seed will do much better if you wait until the soil warms up, which in a zone 4 would be in early June.
I’d suggest you gently pat the soil, lay the seed on the soil, and cover so you only lose sight of the seed. Do not bury it. After you have covered it, pat the soil down again and then water it thoroughly. Note you should use a water breaker on the end of the hose so you do not disturb the seeds.
If you find you have planted them too closely together, you can transplant them when they have produced four true leaves. (A true leaf is one that looks like the adult plant and is not one of the first seedling leaves – there are two of those.)
Sowing indoors: I have never been a fan of sowing Zinnia seeds indoors. They seem to grow better for me if I direct sow them as above. But, having said that, if you keep them very warm – soil temperature in the high 70°F – they should germinate in a week. If you so them indoors, always use lukewarm water – never cold water – to keep them growing. Transplant seedlings as soon they have four true leaves.