The first zinnia species introduced to the United States was Zinnia elegans from Mexico in 1793. It was a single form; double forms were introduced in 1858. Zinnia haageana, another Mexican species was introduced in 1876. A nursery catalog from 1876 sent out by Henry H. Dreer of Philadelphia describes Zinnia haageana as “A double variety of Zinnia mexicana; flowers deep orange, margined in bright yellow.” These species were crossed together and a wide color range created, as well as new flower forms. Few old seed strains exist today; the oldest hybrid seed strains I have located are the ‘Cactus’ mix from 1928 and the ‘California Giants’ mix, also from 1928.
In the photos above, the red semi-double zinnia is one I grew this season, of the variety ‘Will Rogers’ from 1940. The flowers of this strain are most often semi-double, but sometimes fully double. The flowers are a beautiful warm shade of red. The other two pictures above show the much smaller flowers of Zinnia peruviana, a species from Mexico with a range extending into South America. These come in earthy yellow and red shades.
Zinnias can be started indoors if you need to start them early and in areas with cool or short summers transplants can be set out after danger of frost has passed. The seedlings will need plenty of light in all stages of growth (inside, too) so they will not become thin and floppy. Seeds can be direct sown about the date of the last frost or later if your season permits. Zinnias enjoy heat and sun. Some of the most prolific plants I have seen were grown in the desert Southwest, but they seem to do well in most of the U.S. Why not try one or two older seed varieties of zinnia?
heirloomgardenerdotnet 
I have had some luck with zinnias in the past, but I believe where I live in the semi-arid mountainous area of Montana where the growing season at 4500 ft is quite short and the on-going draught is becoming a challenge to my once flourishing flower & herb garden which is looking rather bedazzled! What is the tip on soil conditions and fertilizing and watering.
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in a mountain climate it would be practical to start zinnias inside at 70 degrees. Once they begin to germinate, give them as much light as possible to avoid leggy growth. The plants will be ready to transplant in about 6 to 7 weeks. Set them out after the last frosts are over. Zinnias enjoy hot weather and sun, so in your climate with its cool nights, it might be advantageous to choose your warmest spot to plant them; next to a wall facing south or a south-facing slope. You could try direct planting the seeds as well, as soon as frosts are over.
I would choose several varieties to try, and select seeds to save from the best early plants. Zinnias like good garden soil and grow well in the conditions tomatoes and peppers like. Do not overwater them, especially toward fall when nights are even cooler. Good luck!
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Evy – You might also try adding a little bone meal to the soil when you transplant (about a teaspoon per plant mixed with the soil in the planting hole). Bone meal is rich in phosphorus which is very important for healthy roots and fruit set. Bone meal is organic as well.
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