Sorry, but I tried it in the form of "Preen Organic" -- I thought it was absolutely worthless, didn't do a thing. Regular, old fashioned, non-organic Preen works really well, and could be applied right over the top of your onions as soon as they are rooted in well, about 2 weeks after planting.

You can use Preen (either organic or regular) on asparagus and potatoes, too -- anything but seeds or young transplants.

I haven't tried it personally, but have read where it was used under controlled conditions and it's effectiveness was variable. The main factor affecting it's effectiveness seemed to be the amount of irrigation.

In cases where it was applied and watered in and then irrigation was minimal thereafter it worked best, but areas where frequent irrigation/rainfall would be expected after initial application it's performance decreased.

I've halfheartedly used it. It didn't profoundly stop weeds in my case, but I didn't follow all the rules and I live in a climate where we get frequent showers for weeks at a time during the spring and fall.

It is a good source of nitrogen.

We use it in our back yard around my gardens and it has done a very good job...the weeds are down now to a managable state...My grass has never been greener.

I'm trying it this year. My front lawn was rife with annual weeks last year, so I thought I'd give it a go. I'll let you know in June how it went...

Cheers,

Tim