Answer: Weeds are never easy to control unless you stay on top of the problem and hand pull them the minute they appear. Anytime you leave bare soil, you leave a perfect home for weeds. I'd suggest digging, pulling or using an herbicide to get rid of the weeds once again, then laying down a thick mulch to keep weed seeds from germinating.
There are a couple things you can try. If the weeds you're dealing with are perennials that spread from runners or stolons along or below the ground (e.g., Bermuda grass or nut grass), you'll need to pull them out and apply a layer of several inches of mulch. Their roots can often go 2 feet down and live to sprout up another day, so the mulch helps keep them under control. If the weeds are coming up from seed, then you might want to try corn gluten. Corn gluten is a natural product that produces an enzyme that inhibits seed germination, and can be used as a preemergent herbicide. You can get it from Gardener's Supply Co.(www.gardeners.com; ph# 800/863-1700); they call it WeedzStop. You might want to try giving the area a slow soaking of water to soften the soil. Then the weeds will pull more easily. Of course, the water will germinate more weed seeds, but then you pull them ASAP. If you keep at it steadily for several waterings and several heavy-duty weeding sessions, you may be surprised at your success. Regardless, always pull weeds before they go to seed. One of the reasons they survive so well is that they produce jillions of seeds! You can solarize, in which you let the sun do most of the work to kill the weeds and seeds. You need to solarize during the hottest part of the summer, for up to 3 months. To solarize, moisten the soil, lay a 4 mm to 6 mm thick sheet of clear plastic over the grass, and seal the edges of the plastic with rocks or soil. This will naturally heat up to over 140 degrees F, in the top 4-8" of the soil, depending on soil type and temperatures. If you spread fresh manure on top of the grass before putting down the plastic, that will heat things up considerably. Or, you can use a product with glyphosate, such as Round Up. Glyphosate is a systemic. This means when it is sprayed on a plant, the plant absorbs and distributes it throughout its system. Eventually, it kills the entire plant, including the roots. The spray can drift to other plants, particularly if there is any breeze, and kill them also. With any product, be sure to read the label and make sure it applies to your situation. Then follow product instructions exactly.
Once you've gotten rid of the weeds you can plant your irises. Then use a thick layer of organic mulch to keep weed seeds from germinating.
Best wishes with your flower bed.
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