Question
Have a lawn filled with crabgrass (mostly) + also dandelion, clover, and chickweed weeds. I am putting down corn gluten for preemergence weed control.  Any suggestions for post -emergent organic weed control? clay soil with tall fescue grass.
Thanks in advance

Answer
Your Corn Gluten Meal won't work on perennial Weeds, unfortunately.  There are some solid rules to deal with these, the most important being:  (a) NEVER let those invading Weeds go to seed and (2) DIG them out, then sow with Clover or Grass Seed right away (Nature abhors a vacuum and will plant Weeds there if you don't plant something else).

One of my favorite articles on this subject appeared long ago on the Kansas State University Coop Extension website, 'Horticulturist Gives Top Reasons Weeds Invade Kansas Lawns':

http://www.oznet.ksu.edu/News/sty/2000/weedsinvade.htm).

Her list includes mowing, fertilizing and watering incorrectly; not dealing with thatch; insect damage; compacted soil; traffic; and hostile weather.  So happens they grow a lot of Tall Fescue in that state so this should be of interest.

Let's look at the Mowing factor.

April and May are when Cool Season Grasses do HALF their growing for the season.  If you Mow your Tall Fescue just right, you'll make them stronger -- while you'll weaken alien growth.

Purdue University's Cooperative Extension website posting, 'Lawn Care/Basics for Homeowners-Lawn Care/Mowing', explains why Mowing matters:

http://www.purdue.edu/dp/envirosoft/lawn/src/mowing2.htm#Height

'Each turf species has a characteristic mowing height tolerance range.  Mowing at heights above this range result in poor quality turf and where scalping occurs below the desired mowing height range.'

Some very scientific data says you should mow Kentucky Bluegrass, Perennial Ryegrass and Fine Fescue at 2 to 2 1/2 inches.  For Tall Fescue, mow 2 1/2 inches to 3 inches.

During Summer Drought and Heat spells, mow a half inch higher.

Purdue points out that ALL Grass benefits from the taller recommended mowings.  Mowing high keeps the blade longer.  That means more Photosynthesis, which means deep root growth and healthier, disease resistant, drought tolerant Grass.

Let's look at Fertilizing.

Nitrogen is actually one of your best weapons for whacking Weeds.

University of Illinois studied TALL FESCUE fed with different doses of Nitrogen for one published study:

www.turf.uiuc.edu/research/summaries/1994/94_3.1.pdf#search='university%20test%20mowing%20height'

The results:  Annual Bluegrass populations [Translation: WEEDS] SOARED when the Tall Fescue was mowed at only 1 or 2 inches.  The Researchers also observed: 'Crabgrass populations increased as mowing height decreased...'

Note that it did not matter HOW MUCH Nitrogen they fed the Fescue.  Any Nitrogen at all was better than none when it came to Weed suppression.  Putting down ZERO fertilizer had the worst results, they said. 'Tall Fescue that was not fertilized had significantly higher broadleaf Weed populations than turf fertilized with any Nitrogen.'

It just boils down to giving the Grass what it likes.  Especially when the Weeds dislike it.

By the way, the researchers in the study above also said: 'Even a low
annual rate of Nitrogen fertilization can decrease broadleaf Weed
populations and reduce or eliminate the need for herbicide control.'

Let's read that again:

'...REDUCE OR ELIMINATE THE NEED FOR HERBICIDE...'

Music to my ears.

Your Clay Soil will benefit from an increase in Earthworms and application of Organic matter.  A few Cover Crop Seeds wouldn't hurt, either -- Alfalfa, which you can grow as you see fit, maybe Clover, Vetch, you name it; they'll pour Nitrogen into your Soil and build it up.  Negotiate with the Earthworms; make concessions like 'no more chemical fertilizers' and 'no more aerating' if you have done anything like that in the past year or two (with the best of intentions, I understand).  Somehow I have a feeling you did not do anything like that, but just in case, I want to mention it.  Can't be too sure these days.

Strengthen your Tall Fescue and you will make your Lawn so thick, no Weed dare set foot there again.

Well, maybe a few.  Just stay on top of it.

Get your Soil tested -- the pH can be tilted to be hostile to Weeds, but only if you know which direction to tilt it.

Thanks for writing, and please feel free to followup.

L.I.G.