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Dig and divide now for more irises next spring

Bearded irises need late-summer rejuvenation to keep blooming

Golden iris
These are Tennessee Gentleman irises. Divide and
replant irises every three to five years for best
production. (Photo: Debbie Arrington)

Have your bearded irises stopped blooming? When was the last time you dug them up?

Every three to five years, bearded irises need to be dug up and started over. That’s the key to their durability and consistent bloom.

Each thumb-like growth on the rhizome – the plant’s fleshy underground stem – only flowers once. The rhizome then grows and produces another thumb, which will bloom the next year. But if the rhizomes run out of room to grow, they stop blooming.

Timing is everything with this chore; irises need to be divided and replanted in late August or September to get ready for spring bloom. So, now is the right time to dig up the iris bed.

Why grow irises? They’re among the best water-wise perennials for the greater Sacramento area. They’re low maintenance, drought tolerant and reliable. Deer won’t eat them and most pests leave them alone. They pretty much take care of themselves most of the year, dying back in fall before re-emerging for a massive spring flower show.

What you see above ground is tied directly to what’s below ground. Over time, iris rhizomes lose their vitality. They rot or wither away. Side shoots grow from the central rhizome to form new baby rhizomes. It’s those baby rhizomes that will produce the future blooms.

Fans of leaves sprout from the tips of those thumbs. Since those thumbs can bloom only once, discard any segments attached to flower stalks when dividing iris.

How do you know which to keep, which to cut? Dig up the rhizomes and look.

Shallow-rooted, they come up very easily, pried from the ground with a pitch fork or spade.

Once they're unearthed, wash the rhizomes off with water and start working them apart, using your fingers and a trowel or sharp knife.  Keep the young healthy rhizomes with one or two fans of leaves and healthy roots attached. Break off the aged, bloomed-out, rotted or dried-out pieces and discard. Also look for insect damage; cut that out, too. Of the fans you keep, trim the leaves down to 4 to 6 inches.

Now you’re ready to replant. Rejuvenate the planting area with some well-aged compost and a little bone meal, worked into the top 6 inches of soil. (That’s going to be all the fertilizer the irises need for the next three to five years.)

Plant the rhizomes about 12 inches apart so the fan of leaves sits right on the soil surface, with the rhizome just under the soil. The rhizome will grow in the direction the fan is facing (with the curve of leaves to the outside); position plants accordingly so they have room to spread and sprout new flower stalks.

Unlike most perennials, don’t cover with mulch. (That can promote rhizome rot.) Water deeply once, then let rest.

Want to learn more about irises? Check out the Sacramento Iris Society:


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Dig In: Garden Checklist

For week of Sept. 24:

This week our weather will be just right for fall gardening. What are you waiting for?

* Now is the time to plant for fall. The warm soil will get these veggies off to a fast start.

* Keep harvesting tomatoes, peppers, squash, melons and eggplant. Tomatoes may ripen faster off the vine and sitting on the kitchen counter.

* Compost annuals and vegetable crops that have finished producing.

* Cultivate and add compost to the soil to replenish its nutrients for fall and winter vegetables and flowers.

* Fertilize deciduous fruit trees.

* Plant onions, lettuce, peas, radishes, turnips, beets, carrots, bok choy, spinach and potatoes directly into the vegetable beds.

* Transplant cabbage, broccoli, kale, Brussels sprouts and cauliflower as well as lettuce seedlings.

* Sow seeds of California poppies, clarkia and African daisies.

* Transplant cool-weather annuals such as pansies, violas, fairy primroses, calendulas, stocks and snapdragons.

* Divide and replant bulbs, rhizomes and perennials. That includes bearded iris; if they haven’t bloomed in three years, it’s time to dig them up and divide their rhizomes.

* Dig up and divide daylilies as they complete their bloom cycle.

* Divide and transplant peonies that have become overcrowded. Replant with “eyes” about an inch below the soil surface.

* Late September is ideal for sowing a new lawn or re-seeding bare spots.

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