Pre emergent weed killer

Will do. I ended up putting down 2 kinds of Preen.
1 was corn gluten (I put this in areas where Preen recommeded based on the type of plant) and the other a poison.

Some good weed killers chemicals are also available in market. You could also contact any good weed specialist, because that would definite;y give a great solution for your problem.

Japanese Knotweed Specialist | Knotweed Eradication

But i think weed killers are mostly made up of harmful chemical. So i am looking for any good homemade weed killer solution. I have also heard that vinegar could be also used for this purpose..Is it really true.

TP Knotweed

If you are trying to eradicate knotweed you will need to use some kind of herbicide, it is a highly aggressive, invasive plant/weed . You will have to use something like Roundup or brush killer and possibly have to apply several times for complete removal.

Vinegar has been touted as a weed killer by some of the gardening "gurus". However store bought vinegar is a lower acidity than the kind that can harm weeds. You would need a solution closer to 10%, and that is not readily available to the home gardener, and if it was, it would be very expensive to obtain the amount needed.

When using herbicide, make sure to follow label instructions for the least toxic and safest method.

Weighing in, I would have to agree about needing to use an herbicide. Knotweed is highly aggressive, and the trouble with some home remedies like vinegar and salt (which is also suggested sometimes), is that vinegar will change the pH of the soil and salt can lead to salt toxicity, which will burn your other plants. In the end, surrounding plants are harmed and the weeds are still there.

Often, you can knock down a weed with one application enough to where you can hand weed afterwards if you don't want to reapply. For the most part, I garden organically, but in rare instances--and this would be one--I've resorted to an application of weed killer, and then stayed on top of it afterwards.