How And When To Divide Irises

A couple of decades ago, a wunderkind with some trade organization came up with the idea of a promotional campaign pushing the theme that fall is for planting. And, over the years, that theme has come and gone.It was primarily a marketing ploy to boost the nursery and garden center industry during a somewhat slower business period. There is, nonetheless, some good science behind the effort.

The science is there because, as the soil begins to cool later in the summer and through the fall, root growth in perennials, trees and shrubs goes into expansion mode from the summer survival mode. So it seems to follow that if you’re going to disturb the roots of a plant, or if you want to take advantage of a period of root growth and stimulation, then, yes, fall is for planting.

But there have been a few problems The first is that after a spring and summer of planting, tending and harvesting, many if not most of us are just plain tired and not in the mood for three more months of work in the garden. Well, hard work anyway. But in recent years our falls have been dry, leaving me reluctant to tell you to go out and buy and plant a $5,000 tree, or even a $500 tree. And in a year like 2016 with a severe summer drought proceeded by three years of dry weather, it’s been very difficult for me to say, yes, this is a fall for planting.

But 2017 has been very different. We had a wet spring, a damp and somewhat cool summer, and, with a promise for at least a normal meteorological fall, this fall could very well be the one for planting. And the time to start is now, with dividing some perennials and thinking about new trees and shrubs, and those that may need to be moved and transplanted.

Let’s hold off on the trees and shrubs for a week or so, because it’s a bit early to be working on them. However, it’s not too early to examine your landscape and see if there are spots where you want to do some new plantings, or if there’s a tree on the property that needs to be moved. Now’s the time to plan for this kind of work.

So, keep that in mind—and if you’re thinking about new plantings, you may want to visit the garden centers and nurseries from Amagansett to Moriches that have nursery stock in their yards and in the ground that you can look at for ideas and candidates.

Now, however, is the time to look at at least two perennials on your property that may need some attention in the form of division and transplanting. This is the traditional time of the year to divide irises, and for two reasons.

The first is that you may have some irises in the garden whose spring flowers seem to be on the wane. Where once you might have had 20 flowers a season, the past couple of years it’s been more like 10, or fewer. And while Iiis can grow in the same spot for a hundred years, they will flower less and less, and eventually all you have left is foliage and no flowers. This is reversible—and the process pays huge dividends in the form of many more plants that you can install elsewhere or give to friends.

As a brief aside, a short iris tale.

I steward a property upstate that was a subsistence farm in the 1800s and then, in the 1940s, became a summer getaway for a New York dentist who had a penchant for gardening. The property was donated to the New York State Department of Environmental Conservation about 15 years ago, but it’s been ages since the farmhouse gardens were tended.

Three years ago, while surveying for invasive plant species, I found two areas of iris among some stone rubble. It looked like they hadn’t flowered in years and, being a consummate plant poacher, I waited until late August and then dug a couple of samples.

One of these has not only flowered in a magnificent combination of purple, yellow and a few other hues, but it’s now ready to be divided and moved out of the trial garden and into the landscape. It could be an iris that hasn’t been otherwise cultivated in 100 years or more, and it’s certainly a stunner. The second one is being a bit more difficult and challenging—so I’m sure it’s even more beautiful.

So, if you’ve got some iris that need rejuvenation, it’s really simple, with only one hard rule to follow: Always plant the iris divisions just slightly below the ground or at the same depth as the ones being dug. Plant them deeper and they will never flower.

How deep? Well, the rhizomes should be planted horizontally, never vertically, into the soil, and the top of the rhizome always should be visible and never buried.

The digging tool of choice for this work is a garden fork, sometimes referred to as a pitchfork. The tines of the fork are inserted into the ground so as not to spear the rhizomes, and portions of the old plants that are as large as practical to coax out of the ground are lifted.

Once out of the ground, as much soil as possible is shaken off the rhizomes and roots. Then, with a sharp knife, you can cut out any rotted or diseases portions. What you will be left with are the bare rhizomes, with some roots and the “fans” of foliage attached.

At this point, you can use a sharp pruner like a Felco and cut the foliage down to 3 to 6 inches in length. This step can also be done prior to digging, but when done after digging the long foliage makes it easier to handle the divisions.

Now you will have lots of sections of short “fans” attached to rhizomes. This is when the final divisions are made. Depending on the condition of the pieces that remain, each division can contain from one to three fans. A three-fan division may flower next spring, but most of the divisions will take two years to re-flower and will reach their peak in two to three years.

Keep in mind, though, that the replanting is critical. Never cover the rhizomes with soil so they are totally buried. Always allow the top of the horizontally planted rhizome to peek above the soil, and it will be fine.

Once transplanted, use a watering can to gently water the rhizomes in, and if they sink a bit give them a gentle lift up so they are at the right level. No fertilizer should be added until next year, but you can add a biostimulant to the watering can.

It’s common to look at these transplanted rhizomes and think they can’t possibly survive a winter, especially a cold one, but they are quite amazing, and as long as you do your dividing in the next few weeks, the rhizomes will set roots that will anchor them well, and your winter losses will be few, if any.

So, dividing your irises is your project for the next week or so. Get them done so we can move on to your peonies, because they need some work every few years as well, and next week we’ll see how.

And if you want just one more thing to do: See if you can find any signs of the foliage from your poppies. These can be divided, too, in the near future. A little tricky, though, so mark or flag where these plants are, and I’ll tell you more next week.

Keep growing!