Since Frank made a great comment regarding the mini bearded iris - now seems like a perfect time to talk on dividing bearded iris. The best time to divide your bearded iris is in August, that will give them time enough to get establish and you will get plenty of blooms next spring.
How do you know when it is time to divide them? When your rhizomes become crowded and are growing on top of each other is a sure indication that they need to be separated. Another indication is when you notice you are getting less and less blooms each year (you may have a ton of leaves but few blooms), this indicates that the plants are fighting for nutrients in their crowded environment and only produce foliage growth.
How to plant? The most common mistake made with bearded iris is planting too deep. First make a 3" deep depression about 6" in diameter. In the center make a small, fist-sized mound of soil. The rhizome is placed on top of the mound so that 1/3 of the rhizome will be above the soil. Spread the roots out to support the plant, replace the soil, firmly pack around the roots to remove air pockets and water liberally. DO NOT plant bearded iris in a low area; they will not tolerate standing water. I would suggest 12 -14" as a good distance between rhizomes. This allows the new growth to fill the area between plants in about three to four years. And then it is time to separate again.
Fertilize or not? If your soil is rich in nutrients (a good mix of compose can accomplish this) there should be no need to fertilize. If you soil has not been amended I would suggest a fertilizer low in nitrogen such as 6-10-10. Fertilizers high in nitrogen tend to cause bacterial rot and lush, but weak, foliage growth. When selecting fertilizers for irises, be sure that the 2nd and 3rd numbers are bigger than the 1st. These numbers stand for the amounts of phosphorus and potassium in the mixture. Phosphorus and potassium are the key nutrients in root and bloom production.
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