Which part of zinnia to pinch or cut? My zinnias are growing beautiful but flower blooms are very slow. A lot of leaves.
I wouldn't pinch anything back at this point as your trying to get them blooming. After they are blooming you can take off cut flowers and dead head to promote more flowering and a bushy plant if you prefer.
Lots of lush green and few flowers generally indicates to much Nitrogen in the soil.
It's likely needing Phosphorous. You can use Bone Meal to help with this.
Here are a few links for you.
https://www.gardeningknowhow.com/ornamental/flowers/zinnia/growing-zinnia-flowers.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm
How deep to plant them and do I put a lot of space between each one?
Space small (dwarf) varieties 6-8 inches apart and space large varieties (those that grow to 2 feet or more) 12 inches apart.
I would refer to your seed packet for planting instructions, but usually zinnia seeds are planted 1/4" deep. If you are planting actual plants, dig a hole as deep as the root ball and twice as wide.
For more information on the care of zinnia plants, please visit the following link:
https://www.gardeningknowhow.com/ornamental/flowers/zinnia/growing-zinnia-flowers.htm
What creature eats ZINNIAS?
There are many different pests that will eat zinnias. Can you please describe what the damage to your plants looks like? That will help us with IDing your pest.
Does soaking seeds in water for several hours help speed up the germination?
You can soak the dried seeds 4 to 6 hours in water prior to planting.
https://www.gardeningknowhow.com/ornamental/flowers/zinnia/growing-zinnia-flowers.htm
As zinnias don't like to be transplanted, can I just place the whole potted zinnia container and all, into the ground?
No, go ahead and remove the plants from the pot and plant directly into the soil.
https://www.gardeningknowhow.com/ornamental/flowers/zinnia/growing-zinnia-flowers.htm
Is pinching the bloom a good idea when you first plant bloomed zinnias purchased from the nursery? Does it encourage growth of the plant and root development?
Dead heading the spent flowers should be the only pruning you need to do on your Zinnia plants.
Should zinnias be fertilized as soon as they start growing or wait until they are blooming?
When the seedlings are about 4 inches tall, you can fertilize with a light application of a 5-10-5 fertilizer.
You can repeat a feeding in mid-season to help them keep growing.
Do not over fertilize, as excessive nitrogen will cause lots of green leaves and little or no flowering.