Question: This summer you discussed dividing irises. My question is this: How can I prevent weeds and grass from overwhelming my iris beds? Since the rhizomes must be left half-exposed above the soil line, it seems that it would not do to mulch the bed, as this would completely cover the rhizomes. What do you suggest to keep the weeds and grass out?
Answer: I've faced this challenge over the years, and here's how I've dealt with it.
If your iris beds are overwhelmed with grass and weeds, begin by digging all the clumps of iris rhizomes out of the beds in summer or fall. Carefully sort through each clump of rhizomes and tug out any and all weed and grass roots you can find.
Because irises are so tough, you can remove nearly all soil from their roots if that's what it takes to get every last bit of weed root out of there. Once the iris plants are out ofthe bed, diligently dig out any weeds remaining in the beds until you have nothing but bare soil.
The next step is to mulch over the bare soil.
I prefer to use finished compost, mushroom soil or leaf compost on my perennial and annual beds, but triple-shredded hardwood mulch works too. Avoid single- and double-shredded products as their particles are large and slow to decompose (which is a great thing for tree and shrub beds but not for plants with more shallow root systems like perennials and annuals). Put down 3 or 4 inches of mulch.
Now it's time to replant your iris clumps.
Clear the mulch to the side of each planting site, dig your hole, and properly position the plant. Divide any clumps that have grown too large and are dying out in the center before replanting. Leave the top half of each rhizome exposed. Do not reapply the mulch over the rhizomes, just leave it spread around the plants, but never on top of them.
If a complete overhaul is not necessary, or it's physically impossible, you can hand dig the weeds or carefully apply an organic herbicide by soaking a rag in it and, wearing chemical resistant cloves, wiping the rag up the length of the grass blades. If you go this route, be careful to keep the rag from dripping on the foliage of any desirable plants.
After this process, you can mulch around the existing plants with the same materials discussed above, being careful to keep the mulch off the top of the rhizomes.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio. She is the author of several gardening books, including “Grow Organic” and “Good Bug, Bad Bug.” Her website is www.jessicawalliser.com.
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