Pre-emergent weed control stops weeds before they get started. Fertilizer provides plants with the nutrients they need to grow. Synthetic products have been commercially available for both applications for decades. In the early 1990s, Iowa State University researchers discovered a completely organic approach to pre-emergent weed control in a product that also feeds plants.
Corn Gluten Meal History
Dr. Nick Christians patented the use of corn gluten meal as a pre-emergent herbicide in 1991. CGM, a by-product of milling corn, prevents weed seeds from developing roots. Without roots, the newly germinated weed seeds wither and die. In addition to retarding the growth of weeds, CGM adds nitrogen to the soil. Dr. Christians writes that the recommended pre-emergent application of 20 lbs. per 1,000 square feet also adds 1 lb. of nitrogen fertilizer to the soil.
Both CGM and synthetic pre-emergent herbicides have to be in place before the weeds germinate to have the desired effect. Crabgrass and other annual weeds start germinating in the spring once soil temperature stays in the 50s F through the night. Rather than sticking a thermometer in the soil every midnight, watch the forsythia in your area. When the forsythia blossoms start to swell, it's time to spread pre-emergent controls.(ref 4) CGM will retard all germinating seed for six to eight weeks, but won't do a thing to plants that have already started growing, so it is better to be a few weeks too early than to be a few days too late.
CGM Lawn Application
For lawns, spread 20 lbs. of CGM on every 1,000 square feet. If rain is not forecast within five days, water it in with approximately 1/4 inch of water. It will be most effective if several dry days follow the watering. CGM is non-selective. It retards the growth of any newly germinated seed, including the ones you want to grow. Don't use it on lawns where you plan to over-seed within six to eight weeks of application.
CGM in Flower and Vegetable Gardens
In the garden, don't use CGM in areas where you will direct sow seeds. However, where you are setting out bedding plants, it will keep weeds from sprouting while providing nitrogen for your plants. The rate of 20 lbs. per 1,000 square foot is sufficient in the garden, too, but it's almost impossible to over-apply, so don't worry too much about measuring, just spread liberally.
CGM as Fertilizer
CGM is an unbalanced fertilizer, meaning that it provides nitrogen, but no phosphorus or potassium. Nitrogen, phosphorus and potassium are the elements most essential to plant growth. Every fertilizer sold carries a label that indicates its percentage by weight of nitrogen (N) phosphorus (P) and potassium (K). That label is referred to as the fertilizer's "guaranteed analysis." CGM products generally carry a 9-0-0 or 10-0-0 label. Some companies are starting to blend CGM with other organic fertilizer sources to provide a more balanced product.