You will use the Dirty Beaker to mix the Herbicide and get a Herbicide Mixture item.
How to get Dirty Beaker
Go inside the Museum in the Main Street. Run to the right and you’ll see a big yellow machine. Click on the blue lever to raise the arm. Then jump up on the platforms and make your way to the upper-right corner. Watch out for the darker platforms which will crumble and fall when you step on them. Grab the dirty beaker out of the display case in top right.