Is this a good time to divide irises?
Irises can be divided any time after they've stopped flowering up until early autumn. Just keep in mind that although irises are fairly rugged, they are not indestructible. In order to give them the best chance of making it through the winter, divide them early enough in autumn for them to begin setting roots before the cold weather sets in. The failure to properly set roots is one of the biggest reasons why perennials transplanted in autumn fail to survive the winter.
Dividing is an excellent way to give rhizomes the space they need to produce masses of flowers and once divided, your irises will bloom beautifully for several years until their rhizomes once again multiply to the point where they begin to compete for water and nutrients and blooms becoming increasingly sparse.
You divide irises by digging up the entire clump either with a spade or a garden fork and shaking or brushing off the dirt. Break the clump into individual rhizomes and inspect each for disease, discarding any that are suspect or rotten. Most iris clumps are easy to break apart with your hands, but stubborn clumps can be pried apart with a hand fork or a trowel.
During inspection, you'll discover a wild mix of dark (old) and light (young) rhizomes. Break off the older rhizomes and discard them along with any that are undersized. Any irises that remain should have their leaves trimmed to about six inches in length and their long roots cut back by a quarter.
Traditionally, iris leaves are trimmed in an inverted V pattern which supposedly helps to reduce any rocking caused by the wind and gives the newly transplanted iris a better chance of establishing healthy roots. I don't know if this is true or if it’s just a latter-day justification for a quirky horticultural habit, but I've seen the leaves trimmed into an inverted V and I've seen them trimmed straight across and both ways seem to work just fine.
Replant the irises in a bright, sunny location equipped with well drained soil. Each iris should be spaced roughly five inches apart and the rhizomes should be covered about halfway up the sides. Ideally, you still want to see the top surface of the rhizomes once you have finished planting them.
If you have any questions or comments, please send them to me at vanessa@gardenmuse.ca.
Originally published in the Creston Valley Advance on September 11, 2008.
