Good evening Daniel

Those are Staghorn Sumac. Staghorn sumac can spread aggressively through root suckers, forming large colonies if not managed. This characteristic makes it useful for erosion control but can also make it invasive in some areas. 

Check out this helpful page from U of Minnesota Extension on how to manage woody plants. As discussed on this page, there are two main strategies -- physical control and chemical control. You may find it helpful to use both depending on the scale and location of your problem area.

Physical control is based on working the plant through all its stored energy by forcing it to push up new growth constantly. Repeated mowing and pruning out new growth from the biggest clusters will do this throughout the summer. 

Chemical controls utilize herbicides to kill the plant. However, if you spray, any plant will be affected by the herbicide, so good timing to avoid wind or potential rain is important, as well as proximity to waterways. In the case of larger plants, painting herbicide on the stump is preferable. If you decide to use herbicides, follow the instructions and precautions on the packaging and make sure animals, children, and other folks remain out of the area until the chemical has dispersed (listed on packaging). You may also want to consider the impact of herbicides on the Soil Food Web, and if use may impact other plantings in your lawn or garden.