Your first step is to confirm that indeed you are dealing with poison sumac, (Toxicodendron vernix). The information in the link below will help you make a positive ID:

https://www.poison-ivy.org/poison-sumact

Inspect the area to see how big of an outbreak you have of this plant. Poison sumac can grow anywhere but thrives in conditions where the soil is moist and is most commonly prevalent in swamps, marshes and along river and pond shorelines. If you have a large area, you might want to consider hiring a professional herbicide applicator. Poison sumac is poisonous all year round and at every phase of growth. All parts of the plant aside from the pollen contains urushiol, a toxin that is also found in poison oak and poison ivy. This toxin causes irritation and blistering of the skin. Urishiol can easily come off of the plant and come into contact with skin either by touching the plant or indirectly touching things which has come into contact with poison sumac (clothing, tools, animals, firewood).

Poison sumac can be managed any time of year, but control is most effective when the plant is in bloom in late spring through midsummer.

Natural poison sumac control: difficult but not impossible. You may be able control poison sumac by pulling or digging the plant, but be sure to get the entire root system or the plant will re-sprout. Be sure to dress appropriately – wear gloves, long, sturdy pants and long-sleeved shirts and avoid coming in direct contact with the plant. Wash your clothing immediately in hot, soapy water when you are finished working on the plants.

You can also cut the plant to ground level with pruning shears, but you’ll need to repeat the task every two weeks or so to keep up with new growth. If you are persistent, the plant will eventually die, but it may take a couple of years. Again, clean your tools in hot, soapy water.

Dispose of all plant parts in plastic bags.

A Note of Caution: Avoid burning poison sumac trees because heating the plant releases vapors that can cause serious allergic reactions. When inhaled, the vapors can even be fatal.

Chemical Control: Herbicides containing glyphosate are an effective means of control. Use the product strictly according to the directions on the label, and keep in mind that glyphosate is non-selective and will kill any plant it touches. RoundUp is one product that contains this chemical. You may have heard about it on the news as potentially causing cancers -- especially in those who use it frequently. So be advised. Always carefully read and follow all directions on the label.

Alternatively, you can cut the plants to a height of about 6 inches, then apply the weed killer to the cut stems. Use pruning shears, not a weed trimmer or mower, to avoid releasing irritating plant parts into the air.

Note: Chemical control should only be used as a last resort, as organic approaches are safer and much more environmentally friendly.