Hello Tamara, and welcome to the iris forum! Like Tom stated above, gardening, even with irises, is a learn as you go or hands on learning experience. When you speak of planting irises 15 inches apart do you mean each rhizome or each cultivar?
Whether you are starting with one rhizome or a group of the same irises be sure and make a map of the bed with each iris listed. Sometimes the markers, even with our best efforts, seem to disappear, break or fade over time and with a map you will always be able to reference where each cultivar is located.
Like Tom said check the bloom time for each rhizome - some bloom early-early in the spring bloom season, middle-the middle of the spring bloom season or late-late in the spring bloom season. Rebloomers, often marked RE, will often bloom in the spring and then once or even many more times throughout the summer/fall. Also check for height. I hate planting an iris bed only to find them in full bloom with a 'shortie' over in the very back. TB's do vary in height and I like the taller ones in the back of the bed, obviously.
You will be getting a lot of great advice/comments on this site and yes, ask any growing question that pops into your head. We love to talk iris and the accumulated iris growing knowledge from the other participants on this site is just amazing. Our growing areas range from all growing zones. What zone are you planning to grow irises in?
I think you will be very pleased with your iris growing efforts. They are very adaptable and hardy. They love to grow and bloom.
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