Q.can i use preen on my garden ?
I had three operations last year and my garden got over run with johnson grass . i
I have preen and i was wondering if i can use it in my garden. if so when and how much to use. john t.

Preen is a brand of weed control products. There are different formulations. If you are referring to Preen Garden Weed Preventer, it is for control of broadleaf weeds and not grasses. It will not be effective for the Johnson grass.
This article outlines some strategy and products:
https://www.gardeningknowhow.com/plant-problems/weeds/controlling-johnson-grass.htm

A herbicide alone is not generally enough to control Johnson Grass.
Preen does list Johnson Grass as one of the targeted weeds.
Apply according to the packaging.
https://www.gardeningknowhow.com/plant-problems/weeds/controlling-johnson-grass.htm