It's a pre-emergent. Means it stops seeds from sprouting. Thus, won't work on existing plants.

That depends on what you consider okay. Read and follow all the label directions, wear long pants, long sleeved shirts, and eye protection when applying this product.

These restrictions would give me pause,

USE PRECAUTIONS
1. Do not graze or feed livestock forage cut from areas treated with HELM PRODIAMINE 65 WDG.
2. Follow all applicable directions, restrictions, and precautions on the labels of EPA-registered tank mix partners.
3. Do not blend HELM PRODIAMINE 65 WDG onto dry fertilizer or any other granular material.
4. Chemigation: Do not apply this product through any type of irrigation system unless instructed otherwise in this label.
5. Do not apply aerially.
6. Do not apply to golf course putting greens.

Why would the manufacturer prohibit its use on putting greens?

kimmq is kimmsr