WICHE: Dividing plants improves health, bloom

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We have all asked the question: “Why hasn’t my plant bloomed?”

By Jeneen Wiche

We have all asked the question: “Why hasn’t my plant bloomed?”

Sometimes the answer is as simple as not enough sunlight, not old enough, not cold enough or hot enough, etc. In fact it could be as simple as a little digging and dividing for some renewed blooming attitude.

For most of my herbaceous perennials I wait until early fall to do a good portion of my digging and dividing. The advantages include the fact that the new divisions have a chance to develop new roots in cooler weather; and roots are established before the plant goes dormant for the winter.

By the following spring the plant’s root system is in fine shape, and the new growth will be pristine. In my experience spring digging and dividing usually results in an unkempt appearance.

For other plants mid-summer digging and dividing is better. This includes bearded iris, which benefit a great deal from dividing every three years or so. If they receive adequate sunlight (at least 6 hours a day) but do not bloom well, the rhizomes are likely getting over crowded. Summer is the best time to tackle the chore because irises go through a dormant period beginning in July.

The best way to approach the task is with a pitchfork, lifting the iris rhizomes (roots) off the top of the soil where they grow. Once you have them out sort through them, discarding the older rhizomes and saving the lighter-colored young rhizomes that are growing off their sides.

While you are sorting and separating, you may also notice that some bulbs are mushy or rotting. If holes are present in the rhizomes, then the iris borer has already made its way, burrowing through the foliage, to the rhizome.

This black-headed, pink grub is the larval stage of a brown-to-black moth. The moth lays its eggs on the iris foliage starting in the summer and continuing through until fall. Next spring the pink grubs appear and chew their way into the foliage, down through the stems and into the rhizome of the plant. The grubs feed on the rhizome until they hatch into moths and the cycle starts again.

For future control inspect the iris foliage in late spring and early summer. Here’s the process:

  • Remove any leaves that begin to die out and look for holes in the foliage.
  • Split the leaf blade open, and you will see the thin larval stage of the grub burrowing through the leaf (the goal of the larva is to feed on the leaf as it journeys to the rhizome, once to the rhizome the grub stage has the real meal).
  • This proves to be easier and more effective as a control because pesticide treatments have to be timed just right in the spring before the moths lays its eggs on the foliage.
  • Once you have sorted out the old rhizomes and finished trouble-shooting for the iris borer, it is time to replant.
  • Cut the fans back to about 2 to 3 inches in height and group them in sets of three to five fans.

When replanting bearded iris, my Dad always instructed me to plant the rhizomes like “ducks on water.” The rhizome should not be completely buried, leave half of it above the soil level.

If it is completely buried, it will surely rot (and definitely not bloom.) Be mindful of mulching over the rhizomes because this will encourage them to rot, as well.

And remember: Come next spring while you are perusing the garden take time to inspect the foliage. If it appears that something is feeding on the blades, remove them or run your fingers along the length of the blade, applying slight pressure.

If the iris borer larva is there, you have likely crushed it. Late spring is the best time to control the borer because once it has reached the rhizome, there is little you can do.