The best time to fertilize new grass seed is either before or while the seed is being sown. Additional fertilizer should be added about a month after planting when the grass has grown to between 1 and 1.5 inches tall.
Although starter fertilizers may vary in compensation, most products provide grass lawns with large quantities of essential nutrients such as nitrogen, potassium and phosphorus. Selecting a starter fertilizer rather than a product formulated for use on existing lawns can be an important concern when sowing new grass seeds. Grass seeds that have germinated do not require the higher concentrations of phosphorus contained in most starter fertilizers.