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Clear plastic, alone, when left on for several weeks during the heat of summer will kill grass. You also can use an herbicide containing glyphosate to eliminate grass. (Always read and follow label directions.) Then you can apply landscape fabric topped with rock to help keep out weeds.

This article will help: https://www.gardeningknowhow.com/garden-how-to/projects/how-to-solarize-garden-beds-to-eliminate-garden-pests-in-the-soil.htm

Unfortunately, your photos did not come through. I am unable to identify the plant in question.

What I can do, in the meantime, is point you to some articles that will help with weed control:

https://www.gardeningknowhow.com/plant-problems/weeds

Coming across a pipe is never good. I cannot speak for your local government's laws, but ours would require a permit to mess with any pipes underground. I would seek help from an official from your agricultural department.

Removing the root will be the solution, however, damaging the pipe may break laws or worse.

The article below says it is best applied in spring before the weeds start to grow. You can contact your local university extension agent for a more precise date in your area.

https://www.gardeningknowhow.com/special/organic/cornmeal-in-the-garden.htm

You can, but make sure that you know the type of weed so that you can use an herbicide that will work on the specific weed in question.

Here is an article that will help you with herbicide use:

https://www.gardeningknowhow.com/plant-problems/weeds/using-herbicide-in-gardens.htm

The best recourse is hand pulling along with some preemergent herbicides. But the bed needs to be weed free before using those. For grassy weeds, there are products that will kill grass but not ornamentals. But none of the products will eradicate all weeds.

You can lightly mulch your iris beds, but be careful not to get too close to the rhizomes or rot could set in.

If you get real desperate, you can always start a new iris bed, or remove all irises from existing bed, spray with a non-selective herbicide, wait recommended number of days to plant, then move all your irises back.

If you are very careful, you can "paint" an herbicide containing glyphosate on the weeds with a paintbrush.

Here is an extension publication that lists all the possibilities:

https://extension.uga.edu/publications/detail.html?number=C867-8&title=Weed%20Control%20in%20Iris