On 10/6/2007 2:33 PM, Academia wrote:
If you use fertilizer containing weed killer they want you to put it on wet
grass and do not wash it off the grass.

But if you use fertilizer without weed killer they want it washed off of the
grass. Why? Is this fertilizer different chemically?

I used to use a "weed & feed" combination where the herbicide (monuron)
worked through the weed roots. It was for dichondra lawns (D.
micrantha, a broadleaf ground cover sometimes called "leaf lawn"). This
had to be rinsed off the foliage and into the soil. It was very
effective, killing grasses, oxalis, spurge, dandelions, and even
seedlings from my ash tree. Unfortunately, it's no longer available in
the U.S. The manufacturer declined to renew the EPA permits, claiming
the market was too small to bother with the permitting process. (D.
micrantha only grows in non-desert, mild-winter climates.) It's very
difficult to keep oxalis and spurge out of a dichondra lawn. Thus, when
monuron was no longer available, I had my garden redone with a lawn of
red fescue (Festuca rubra), an ornamental grass that needs mowing only
once or twice a year.

For a dry product, I can't imagine a "weed & feed" combination working
by contact.

On the other hand, dry fertilizer in the open will eventually absorb
moisture from the air and slowly dissolve (too slowly for an herbicide
to be effective). (Ever notice how dry fertilizer sometimes even cakes
while in the bag?) Some dry products (e.g., ammonium sulfate) will even
extract moisture from foliage that it contacts. In contact with
foliage, this will easily burn. Thus, you want to rinse the dry product
off the grass and into the soil. Not only does that prevent leaf-burn
but it starts moving the nutrients into the soil in a solution much more
dilute than what is created by absorbing moisture from the air.

--
David E. Ross
Climate: California Mediterranean
Sunset Zone: 21 -- interior Santa Monica Mountains with some ocean
influence (USDA 10a, very close to Sunset Zone 19)
Gardening pages at http://www.rossde.com/garden/