Answer: Grass and weeds in the iris bed are an ongoing battle, as you have seen. Iris should be lifted and divided about every three or four years, and as it happens this is usually about when the weeds have managed to slowly infiltrate and grab hold anyway.
The best way to rejuvenate the beds would be to lift the rhizomes and sort them, keeping the most vigorous and healthiest to replant. In most cases you will discard the original worn out old section and keep only the plump newer sections of the rhizomes. The time to do this is in midsummer.
Next, take care of the weed problem. Annual weeds can be pulled, but perennial weeds and grasses may be better treated with a nonselective hwerbicide such as glyphosate. Be careful to follow the label instructions and allow the full waiting time so that the chemical translocates to the roots where it kills the weeds. Meanwhile, the bare rooted iris can be held in one layer in a dry, airy location such as a garage while you do this part of the job. To promote air circulation, store them in cardboard boxes rather than plastic bags. They will be fine for the weeks that it takes to work on the weeds and prepare the soil.
Next, prepare the soil deeply, adding organic matter such as compost, rotted leaves, aged stable manure and bedding, or milled spagnum peat moss and any additional amendments as indicated by basic soil tests, so that you are ready to replant into a deeply dug, loosened, weed-free environment. Your goal is to provide a soil that is rich and well drained. Adding the amendments and working the soil may result ina slightly raised bed effect and that is fine because it helps ensure good drainage. This soil preparation is an important step because the iris will be in place for a number of years.
Replant the iris and water them in. Be careful not to plant them too deep or they will rot. You may mulch between but not over top of the rhizomes since mulch will hold moisture against them and invite rot. You could use any organic mulch such as shredded hardwood bark, half finished compost, or leaf mold. Some gardeners will use a pre-emergent herbicide closer to the rhizomes; in my experience corn gluten works well for crabgrass. Unfortunately, you will still need to do hand weeding from time to time.
Iris require a routine "eyeballing" to check for insect activity such as borers as well as for fungal problems and to catch any weeds that may be starting up. The foliage should be trimmed back in mid summer when it begins to deteriorate. This will encourage fresh healthy growth. Although I have seen this cutting done by lawnmower set on high, in my experience it is better done by hand so that you can check the individual rhizomes for any problems at the same time. In addition, the lawnmower wheels tend to damage the rhizomes and this in turn invites rot.
The bed should also be gone over carefully to remove any discolored or damaged foliage in the fall, to remove any faded foliage in late fall and again in the early spring to remove any deteriorated foliage, as well as at any time you notice a problem beginning. The spent flower stems should also be cut off at the base and removed promptly. Always remove and destroy and trimmings. Sanitation is in many ways the key to maintaining a healthy iris bed.
All of this sounds like a lot of work, and in this respect iris are no different from most "low maintenance" perennials. In my experience that is a bit of an oxymoron, especially when a planting is an extensive one. Depending on the condition of the rhizomes, your bed could bloom beautifully next spring or it may take a year to recover its strength before it puts on a good show. Often a neglected bed responds eagerly to the attention and renewed soil, especially since the "survivors" are certainly good sturdy varieties.
Good luck with your project -- you will be well rewarded by the display!
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