, Newburyport, MA


August 28, 2013

Tim's Tips: Time to divide iris plants

August is rapidly winding down, and there are things you need to be doing in your gardens.

The end of August is the best time to divide your bearded iris plants. These irises tend to grow into large clumps that eventually produce few flowers unless you divide the plants. If you look at the iris leaves, they go into a swollen root called a rhizome. It looks like a small sweet potato.

If you look closely, you will notice that half the rhizome is above the ground and half is below ground. The rhizomes form a mass on top of soil that needs to be broken up into individual rhizomes. You can use a shovel or a spading fork to lift the mass of rhizomes out of the ground. Once you have that mass of rhizomes out of the ground, you want to divide them up. The larger rhizomes that are in the center of the mass are the ones that most people would naturally want to plant. However, there is an insect called the iris borer that will many times infect those large rhizomes. The newer rhizomes that are around the edges of the clump are the ones that you want to replant.

Once you have selected two or three rhizomes, you want to cut the leaves back to about 6 inches. This allows the rhizomes to get re-rooted without having to struggle to keep those long leaves healthy. You want to add some fertilizer to the soil and mix it into the top 4 inches of soil. When you plant the rhizome, remember that only half of it is planted below the ground. Firm the soil around the rhizome, and give the plant some water.

The rhizomes should be planted about 6 inches apart. In the end, you will have some rhizomes left over that you can plant in other sunny spots in your yard. The older rhizomes should be discarded to eliminate any iris borers.

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